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Phylum Porifera and Cnidaria

  • André C. MorandiniEmail author
  • Márcio R. Custódio
  • Antonio C. Marques
Living reference work entry

Abstract

The main features to recognize members of Porifera (sponges) and Cnidaria (corals, sea anemones, jellyfish) are presented. Some toxinological information is highlighted focusing on dangerous species like sea anemones and box jellyfish.

Keywords

Asexual Reproduction Stony Coral Oral Disc Calcareous Skeleton Radial Canal 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Introduction

The two phyla presented in this chapter are among the most ancient animal lineages that have appeared on Earth. After the paleontological record, they have appeared at least during the Ediacaran, over 600 million years ago, and molecular estimates indicate they could have appeared about one billion years ago, during the Cryogenian. Recent inferences on animal evolution also place them among the basal lineages of the animals, together with the phyla Ctenophora and Placozoa (Marques and Collins 2004; Erpenbeck and Wörheide 2007; Collins 2009; Hooper et al. 2011). However, different from ctenophores and placozoans, the Porifera (sponges) and Cnidaria (corals, sea anemones, jellyfish, hydroids) are well known to be highly toxic (Williamson et al. 1996). Therefore this chapter will be covering the dawn of the animal toxin, making them key-groups to understand the evolution of animal toxinology.

Here it is presented the basic features of the sponges and cnidarians in order to help to distinguish these peculiar animals in a context of their toxic nature, but the reader should refer to Zoology textbooks to get more general and biological characters of them (Brusca and Brusca 2003; Ruppert et al. 2003). Bibliography at the end of the chapter includes qualified literature for key information about the several groups of sponges and cnidarians (Bergquist 1978; Bouillon et al. 2006; Daly et al. 2007). The focus here are on the essential information needed by a Toxinology audience, providing the general morphological characters that define each phylum and their subgroups, a classification framework for both phyla, as wells as some ecological and toxinological information important to understand their natural history.

Phylum Porifera

This phylum comprises the so-called sponges. Members of the phylum have a variety of body forms (Fig. 1). Sponges have in common a distinctive type of cell, named choanocytes, which are flagellated and promote circulation of water through a unique system of canals (Maldonado 2004).
Fig. 1

Different sponges illustrating diversity of the group. (a) Aplysina aerophoba Piran, Slovenia. (b) Ircina felix Natal, Brazil. (c) Dysidea robusta Natal, Brazil. (d) Amphimedon viridis Guarujá, Brazil. (e) Desmapsamma anchorata Salvador, Brazil (Photo courtesy of Dr Eduardo Hajdu). (f) Polymastia janeirensis Guarujá, Brazil. (g) Mycale microsigmatosa Guarujá, Brazil

Poriferans are benthic sessile animals and can be found living in the marine or fresh water environments, occurring in all depths and latitudes. The body of a sponge is organized in a parazoan pattern and this means that they lack true embryological germ layering. There are no true tissues or organs, and although being a multicellular metazoan sponges function much like unicellular organisms in terms of their physiological aspects (Bergquist 1978).

The classification of the phylum Porifera in higher hierarchical categories is among dispute in recent times; mostly due to several findings based on molecular studies (Erpenbeck and Wörheide 2007; Hooper et al. 2011). There are about 8,700 described living species (data from late 2012) with a higher diversity in pristine tropical habitats. Traditionally the phylum is divided into three main classes based on the composition of the spicules and their main shape (number of living species in brackets): Calcarea [684 species], Hexactinellida [689 species], and Demospongiae [7,356 species].

Body Organization

The corporal pattern of sponges is intimately related to the cell types, because there are no organized tissues in their body. For understanding the poriferans it is important to have in mind two points: the aquiferous system and the totipotent nature of the cells.

The body of a sponge is composed of two layers: the external is called ectosome and the internal is called choanosome. The canals and all external surfaces are covered by the pinacoderm, composed by pinacocytes and presenting small holes called dermal pores or ostia. The water is pumped through these apertures to the water channels due to the flagella beating of the choanocytes, which are organized in choanocyte chambers that occupy most of the choanosome. The skeleton and various types of cells are embedded in an extracellular matrix (mesohyl) mostly composed of collagen fibers. It may vary in thickness depending on the species but is extremely important for the sponges because several physiological processes occur there (digestion, skeletogenesis, transport, gamete production).

A sponge is completely dependent of the water flow promoted by the flagella of the choanocytes through the aquiferous system. The larger the area of the choanocyte chambers the more food is captured by the animal. To increase the area for filtration it is possible to find different body arrangements in the poriferans, which are mainly folding of the surface. There are classically three types of organization (which have no phylogenetic meaning): asconoid, a single central cavity covered by choanocytes, with no folding; syconoid with some folds, and leuconoid with discrete flagellated chambers. The asconoid condition is found only in small calcareous species (up to 10 cm in height) presenting a vase-shaped form, which are clearly radially symmetrical. The central cavity of the vase-shaped body is called atrium or spongocoel, which is connected to the outside by a single aperture called osculum. In some species the entrance of water occurs via specialized cells of the pinacoderm named porocytes (tube-like). The water flow in such sponges is: ostium (aperture of the porocyte) → atrium (spongocoel) → osculum. The syconoid condition is achieved by simple folding of the body wall forming lateral projections, and the choanocytes are restricted to the inner side of such projections (choanocyte chambers). The aperture through the water comes into the choanocyte chambers is called prosopyle and the exit is the apopyle. Such sponges also can be radially symmetrical and the water flow is: pore (ostium) → prosopyle → choanocyte chamber → apopyle → atrium → osculum. The leuconoid condition is due to further folding of the body and formation of choanocyte chambers of different shapes. The organization is basically the same as in the syconoid sponges, but in leuconoid forms the atrium is reduced and a well developed system of inhalant and exhalant canals can be observed; due to that there are also some more oscula; these sponges are asymmetrical. In leuconoid sponges the water flow is: dermal pore → inhalant canal → prosopyle → choanocyte chamber → apopyle → exhalant canal → osculum. There are at least two more types of arrangements of the aquiferous system (sylleibid and solenoid), but there might be some intermediate forms (Cavalcanti et al. 2011). In general, the organization of hexactinellid sponges is different from the other two groups, and this led some researchers to propose a different phylum for them (Symplasma).

Cellular Organization

Because there are no organized tissues with specialized functions, different cell lineages play important roles in the life of a sponge. As mentioned before the pinacocytes are the cell type responsible for the layering of external surfaces and in some internal areas (inhalant and exhalant canals). Although the pinacoderm function as an epithelial tissue, it cannot be defined as such due to the apparent absence of a well-developed basal membrane in most species. The shape of the pinacocytes can vary depending on the area they occur, and they can engulf small particles. The porocytes are tube-like cells of the pinacoderm forming ostia (water entrance) and can have some contractile capacity that helps in regulating the diameter of the aperture. Choanocytes are one of the most characteristic cell type in sponges: due to the presence of a beating flagellum they create a negative pressure that drives water into the aquiferous system. The flagellum is surrounded by a collar of cytoplasmic microvilli in which food particles are collected and engulfed through phagocytosis or pinocytosis. Several different types of cells are found in the mesohyl, mostly ameboid. Some of these are able to secrete collagen (collencytes, lophocytes and spongocytes); others produce calcareous or siliceous spicules (sclerocytes) that form the skeleton. Some cells have a limited contractile capacity, the actinocytes. Among the ameboid cells of the mesohyl there are the archaeocytes that are highly mobile and can differentiate into any other cell type. They play an important role in digestion, transport of nutrients and excretory products. Sponges have a great capacity of regeneration, being able to form a functional individual from single cell suspension, and this is a prominent line of research.

Skeleton

Mainly two cell types produce the skeleton of sponges: spongocytes secrete collagen fibers and sclerocytes form the spicules. The collagen, which is a feature present in all metazoans, can appear as fibrils or organized in a matrix called spongin. The spicules (silica or calcium) are produced in an organized way by the sclerocytes. Depending on the type of the spicule more than one cell can be involved in the production. These mineral skeletal structures are classified according to their relative size (as microscleres and megascleres), shape, number of axis and ornamentation and have an important role in the systematics of sponges. The organization of the skeleton may involve distinct types of spicules embedded or connected by collagen fibers or a matrix in different degrees and can include inorganic accumulated material such as sand grains. Some lack spicules and its skeleton is composed only by dispersed collagen fibrils or organized in fibers, a group that includes those species used as bath sponges.

Physiological Aspects

Digestion in sponges occurs intracellularly, mainly on choanocytes and archaeocytes due to phagocytosis and pinocytosis. The openings of the body (ostia) will affect the size of particles that gets into the aquiferous system, normally smaller than 50 μm. Depending on the size of a food particle it will be captured on the way to the choanoderm surface and the mobility of the mesohyl cells will ensure the adequate distribution of nutrients to all body areas. Dissolved organic matter (DOM) is also incorporated by sponges. Excretion products are released by the cells into the matrix and then diffuse to the water system, although the formation of fecal pellets was observed in some species. Sponges are filter-feeding animals, but there is a family (Cladorhizidae) in which the members are carnivores and predators. Most of these species are deep-water forms, and they capture prey with tentacle-like projections and hook-shaped spicules. Simple diffusion takes also place for gas exchange and freshwater sponges present contractile vacuoles for osmoregulation. There are neither nerve cells nor sense organs in sponges, but they can respond to environmental stimuli by body contraction, closing ostia and oscula, stopping or inverting the water flow.

Life Cycle and Reproduction

Due to the totipotency of the archaeocytes, sponges have a high regeneration capacity through several asexual reproducing processes. Pieces or fragments can regenerate new individuals, and this is widely used by “farmers” in bath sponge cultures (Pronzato et al. 2008). Freshwater species (and a few marine species) produce small spherical dormant structures named gemmules. These structures are composed by aggregated archaeocytes enclosed by a thick collagenous layer and siliceous spicules, which confers the capacity to withstand lower temperatures and/or desiccation. When conditions are favorable a small aperture opens (micropyle) and the archaeocytes migrate to the exterior and begin to form a new sponge individual. Regarding sexual reproduction, sponges are in the majority hermaphroditic animals but producing eggs and sperm in different periods of time. Eggs are formed by differentiation of choanocytes and archaeocytes while sperm formation seems to be derived primarily from choanocytes. Gametes are released in the water and fertilization is external, but in some species it is internal (inside the aquiferous system). There are roughly three types of larval forms: coeloblastula, parenchymula, and amphiblastula; which are ciliated outside. When these larvae are released they seek for an adequate substrate for settling, and the outer layer looses the flagella and flagellated choanocytes start to differentiate internally. There are young forms (just after settling and choanocyte formation) called rhagon and olynthus depending on the group.

Many other animals use sponges as a substrate or hiding place like crustaceans, ophiuroids, annelid worms and fishes. But some cnidarians (hydrozoans and scyphozoans), mollusks and crustaceans take advantage of the water current to have a continuous supply of food. It has been found that the sponges harbor a diverse and important microbiota, which can be species-specific and compose more than half of the dry weight of the animal. Freshwater forms have association with symbiont green algae, zoochlorellae. Some sponges are capable of bioerosion, excavating calcareous substrates (corals, mollusks shells) and promoting a cycling of the available Calcium Carbonate in the sea. The process involves mechanical and chemical action.

Historical Aspects

In the past, due to the benthic sessile habit and the asymmetrical growth, the sponges were considered among the Zoophyta, later together with cnidarians they were treated as Coelenterates and Radiates until the elevation as a phylum in 1826 by Grant.

Sponges are the earliest lineage that diverged in the animal tree of life. The lack of organized tissues and the physiology based in distinct cell types, especially the flagellated choanocytes, suggests the view of a flagellated protist origin, most probably among the Choanoflagellata (Maldonado 2004).

Classification

Class Calcarea. Sponges with spicules composed of Calcium Carbonate. The group is divided into two subclasses (Calcinea and Calcaronea) based on the type of larvae and the position of the nucleus in the choanocytes. Examples are the genera: Chlatrina, Leucascus, Sycon, Leucetta, Amphoriscus.

Class Hexactinellida. Sponges with spicules composed of silica usually six-rayed, and a syncytium of somatic cells. The group is divided into two subclasses (Amphidiscophora and Hexasterophora) based on the type of substrate that it is attached, and fusion of spicules in a network or not. Examples are the genera: Hexactinella, Hyalonema, Dactylocalyx, Euplectella, Lophocalyx.

Class Demospongiae. Sponges with the skeleton composed by spicules of silica (usually monaxonic or tetraxonic); by collagen fibers, or by combinations of both in various degrees. The group is divided into many orders that are not yet clearly resolved in terms of phylogenetic relationships. Examples are the genera: Amphimedon, Cliona, Thetya, Mycale, Tedania, Geodia.

Toxinological Aspects

Sponges are sessile benthic animals that cannot move around to avoid predators; thus these animals developed a wide spectrum of chemical substances (see Blunt et al. 2013, and previous reviews of this series). Several of these substances are being studied focusing on their pharmacological potential and it is known that several metabolites with interesting properties are in fact produced by the associated microbiota (Thomas et al. 2010). Research in biochemical compounds derived from sponges revealed a widespread occurrence of antimicrobial agents; but antitumor, cytotoxic and neurotoxic extracts are common. The spicules can provoke some skin irritation but the chemical compounds are most important to cause dermatitis and also allergic reactions (Fig. 2). Species of the genera Tedania, Haliclona, Neofibularia among others were reported causing accidents.
Fig. 2

Erythema and edema in the finger tips of a sponge collector. Contact with animal occurred 12 h before image was taken (Photo courtesy of Dr Vidal Haddad Jr)

Phylum Cnidaria

This phylum includes a huge variety of beautiful and deadly animals (Fig. 3), such as the feared jellyfish , the Portuguese man-o-war and its highly complex organized relatives, the colorful and varied stony corals, sea anemones, gorgonians, the ubiquitous, small and highly abundant hydroids, the delicate and gorgeous stalked jellyfish. Ecologically, cnidarians may act as “producers” like the corals and their symbiotic algae in the coral reef ecosystem, or be related to massive biomass production such as in jellyfish blooms, or even occupy high levels in trophic chains like the box-jellyfish. Despite the huge biological diversity of the members of the phylum, there is a single feature that unites all cnidarians: the presence of cnidae. The cnida (Fig. 4) is a secretion product of the Golgi apparatus of a single cell (called cnidocyte), which is used for different functions: capturing prey, defense, fixation/adhesion, building tubes, etc. They are the most ancient specialized structure to inoculate toxins in animal evolution (Özbek et al. 2009).
Fig. 3

Different cnidarians showing diversity of body forms in the group. (a) Scyphozoan jellyfish Rhizostoma pulmo Piran, Slovenia. (b) Anthozoan hard coral Mussismilia hispida São Sebastião, Brazil (Photo courtesy of Dr Sergio N. Stampar). (c) Hydrozoan hydroid Hydrocoryne iemanja Brazil. (d) Anthozoan sea anemone Anemonia sulcata Piran, Slovenia. (e) Staurozoan stalked jellyfish Haliclystus antarcticus King George Island, Antarctica. (f) Hydrozoan siphonophore Physalia physalis Guarujá, Brazil

Fig. 4

Nematocyst of the hydroid Hydrocoryne iemanja. (a) Tip of medusa tentacle, magnification 400×. (b) Undischarged stenotele nematocyst, magnification 1000×. (c) Discharged stenotele nematocyst, magnification 600×

The majority of cnidarians are marine animals, with only a few species living in freshwater, like the well-known and model organism Hydra. They can be found in all places from the intertidal zone down to abyssal depths, from tropical areas to polar regions, and colonizing the benthos (sea bed) and the plankton (water column) and, although cnidarian species are not considered to be part of the nekton, some of them are very good swimmers. They exhibit an astonishing array of life histories, including most of the embryological pathways of development (Byrum and Martindale 2004). Their developmental programs are varied – many times the same genome may be found in completely different forms, either in the plankton or in the benthos, reproducing sexually or asexually, etc. These different expressions of forms exhibiting quite different ecologies evidently reflect into the morphological diversity of the group. A major feature of many cnidarians is the presence of two major phases in their so-called metagenetic life cycles, in which different stages are expressed along their life span: polyp and medusa. They have essentially the same body architecture, cytology and histology, but the polyps generally live on hard substrate and the medusae generally have a free-swimming habit.

The classification of the phylum Cnidaria in higher hierarchical categories was highly improved recently, based both on morphology, life history and DNA characters (Bridge et al. 1995; Won et al. 2001; Daly et al. 2003; Marques and Collins 2004; Collins 2009). The cnidarians comprise around 11,300 described species (Daly et al. 2007) that can be divided into two main groups based on the life cycle pattern: those with polyp and medusae in their life cycle and those with only the polyp stage. The classes Cubozoa (36 species), Hydrozoa (~3,500 species), Scyphozoa (~200 species), and Staurozoa (~50 species) compose the subphylum Medusozoa (sometimes called Tesserazoa or Metagenetica) – cnidarians with metagenetic life cycles. The subphylum Anthozoaria (sometimes called Ametagenetica), with its single class Anthozoa (7,500 species), presents cnidarians with only the polyp stage in the life cycle. Although classification systems are liable to change and updating the recent advances in phylogeny and classifications for a given group is mandatory, an updated view of classification of the cnidarians is presented here.

Body Organization

The general body plan of the cnidarians is simple. They have only two tissue layers, the epidermis (derived from the ectoderm) and the gastrodermis (derived from the endoderm), and animals with only these two epithelia are called diploblastic, considered to be an intermediary grade of the evolution the animals. The epidermis and gastrodermis are separated by the mesoglea – a layer of variable thickness mostly composed of collagen fibers, occasionally with some scattered cells, but not related to the mesoderm of the triploblastic animals. The general shape looks like a sac: one end opens and connects with the outside world through the mouth (defining the oral end) and the other end (defined as aboral end) is closed, like a cul-de-sac. The oral-aboral axis establishes the body symmetry of the cnidarians: it is elongated in the polypoid forms (cylindrical or columnar in general shape), and not so elongated in the medusoid forms (defined as bell, umbrella, dish or cubic-shape). Considering only the external morphology, the majority of the cnidarians are radially symmetrical, with a mouth located on a small elevation of the oral region and encircled by tentacles; the opposite (aboral) end forms the basal or pedal disc and works as the attaching point to the substrate. The internal space is named gastrovascular cavity because it works for digestion and distribution of nutrients, O2, etc. to different body parts. This cavity was also known as coelenteron and it was used in the past to define the Coelenterata, joining cnidarians and ctenophores into a single phylum, what is currently not accepted.

Polyps of cnidarians have a huge morphological diversity derived from the many varieties of asexual reproduction, ultimately enabling a colonial (or modular) organization. The basic pattern (or module) is characterized by a simple gastrovascular cavity in small forms. However, in larger animals, such as sea anemones, this cavity may be subdivided by mesenteries which vary in number and arrangement depending on the taxa. External symmetry is essentially radial, but there might be biradial or tetraradial forms when the internal symmetry is observed. In anthozoans, most of the polyps are solitary forms fixed to the substrate by their pedal discs; but in forms living on soft bottom this disc can become rounded (called physa in sea anemones) and used for burrowing and anchoring the animal to the sediment. The mouth can be on a somewhat elevated part of the oral disc (oral cone for scyphozoans; hypostome for hydrozoans) or on a wide oral disc like in sea anemones. In Anthozoa the mouth is slit-shaped and opens into an epithelio-muscular actinopharynx of ectodermal origin that connects to the gastrovascular cavity. This actinopharynx might possess one or two ciliated grooves called siphonoglyphs that helps to move water in and out of the cavity. The presence and the number of such siphonoglyphs define the anthozoan symmetry. Anthozoans with a single siphonoglyph have a single plan of symmetry internally, and are considered to be “radio-bilateral”, in which the side defined by the ciliated groove is named sulcal (or dorsal) and the opposite is named asulcal (or ventral). Anthozoans with two siphonoglyphs have two plans of symmetry and, therefore, are called “biradial”. In scyphozoan polyps (scyphistomae) the gastrovascular cavity possesses four mesenteries, and therefore these animals have a “tetramerous” symmetry, likewise the staurozoan polyps (stauropolyps). Finally, hydrozoan and cubozoan polyps (hydropolyps and cubopolyps, respectively) have no internal features – therefore they have infinite plans of symmetry and are considered to be “radial”.

The number of mesenteries varies greatly among the groups, from no mesenteries in cubopolyps and hydropolyps to hundreds in some sea anemones. Anthozoan octocorallian polyps have a fixed number of eight mesenteries, but anthozoan hexacorallians have a varied number, depending on the group, what makes this a relevant character for taxonomy. In anthozoans these mesenteries are named “complete” or “perfect” if they reach the pharynx, otherwise they are named “incomplete” and “imperfect”; but all mesenteries have a free region aborally in relation to the pharynx, and the rim of that, named mesenteric filament, have cilia, glandular cells and cnidae.

Colonial or modular cnidarians are formed by asexual reproduction. In Anthozoa the zooids of the same colony are usually connected by their gastrovascular cavities, but in some octocorals there are canals called solenia interconnecting the internal spaces of polyps. The hydrozoan colonies have diverse organization and varied shapes, from reptant to erect colonies. The reptant colonies are formed by the growth of the stolon (or hydrorhiza) over the substrate, occasionally budding off polyps or stems. The erect colonies may also have a stolonal growth, but their shape is characterized by the development of the main supporting axis of the colony in the form of a hydrocaulus, resulting in stems and branches. Erect colonies may have a monopodial growth in which the primary polyp elongates itself at the end of the hydrocaulus forming a stem, from which secondary and tertiary polyps/hydrocaulus buds off, forming the lateral branches that may be arranged alternate/opposite/randomly, at a single/multiple plane(s), etc. Alternatively, it is also possible to observe sympodial growth in which the primary polyp stops its growth at a certain point and the colony develops by the budding of new polyps. Colonial organization is highly developed in hydroids presenting an exoskeleton called perisarc. This exoskeleton protects underneath living tissues, collectively named coenosarc. The perisarc has different names related to its regional specialization, like hydrotheca (perisarc surrounding the hydranth), gonotheca (surrounding the gonozooid), nematotheca (surrounding the nematophores/nematozooids), etc. The presence of the perisarc defines the two main groups of hydrozoans, collectively called hydroids (a non-monophyletic taxon): thecates (those with hydrotheca) and athecates (without hydrotheca). The polymorphism of the polyps of Hydrozoa is usual in thecates and athecates, and the specialized modules play different roles in the colony: gastrozooids or hydranths are in charge for capturing and digesting prey, dactylozooids or nematozooids for defense, gonozooids or gonangia for sexual reproduction, etc. Evidently, gastrozooids, gonozooids, etc. have particular morphologies in each different group, and these peculiarities are majorly used in the taxonomy of the hydroids. The gonozooids have a tissue axis called blastostyle from where the gonophores bud off. Gonophores are the structures bearing the gonadal elements, and may be a medusa or medusoid (ultimately released to the water column) or a fixed gonophore (ultimately retained by the gonozooid). The hydroids have several levels of reduction of the fixed gonophores.

However, the highest developed polymorphism can be observed in the Siphonophorae (hydrozoans like the Portuguese man-o-war) and in the Pennatulacea (anthozoans like sea-pen and sea-pansies). The siphonophores form colonies of highly differentiated polymorphic zooids with special functions, like for swimming (nectophores), floating (pneumatophores), defense/attack (dactylozooids), feeding (gastrozooids), and sexual reproduction (gonozooids). The zooids are organized in highly complex groups named cormidia that may be protected by bracteae. In the pennatulaceans the colony develops from the primary polyp that differentiates into a stalk and a frond (raquis). The stalk is in charge to attach the colony to the substrate, and the raquis develops secondary zooids, that can be autozooids (feeding) or siphonozooids (promoting water circulation). The raquis can be elongate and cylindrical or flat and wide depending on the taxon. Sessile colonial cnidarians in general have their morphology (size and shape) modulated by the habitat hydrodynamics caused by the water flow.

The medusa stage does not form colonies. Medusae are free-swimming stages, although there are a few benthic and sessile forms. The medusae usually have a thicker mesoglea than the polyps, and the body is in the form of a bell, umbrella or somewhat flattened like a dish. The aboral surface is called exumbrella and the oral surface is called subumbrella. The subumbrella has a tubular projection in the central part named manubrium, with the mouth at its end. The gastrovascular cavity occupies the center and, it can be divided marginally by septae or canals (radial and circular ones). The cubomedusae and hydromedusae usually have four radial canals or extensions of the gastrovascular cavity, and in hydromedusae there is also a marginal circular (or ring) canal connecting them. The scyphomedusae have the central part of the cavity divided into four gastric pouches. The internal anatomy of cubomedusae, hydromedusae and scyphomedusae reflect their tetramerous symmetry. The hydromedusae also possess a thin membrane at the umbrella margin, the velum; analogous to a membrane at the margin of the cubomedusae called velarium, in which canals of the gastrovascular cavity are present.

Cellular Organization

The two epithelia of the cnidarians have a limited portfolio of cells. The epithelio-muscular cell has its most apical portion with typical epithelial lining functions but its most basal portion has contractive capacity. Intracellular fibers of neighboring cells can connect to each other forming areas capable of true muscle contractions. Besides those cells, the epidermis has also cnidocytes, glandular, sensory, and interstitial cells. The undifferentiated interstitial cells can develop into any cell type, thus explaining the high regeneration and morphogenetic capacity of the cnidarians. The cnidarians are the first metazoans to present sensory structures, and also have an elaborated repertoire of types of cell junctions (gap junctions, septate junctions, and desmosomes).

The cnidocytes are the cells that bear the cnidae, organelles secreted by the Golgi apparatus. Cnidae are capsules with a coiled thread immersed in a mixture of toxins. They may be triggered through different types of stimuli (chemical, electrical, and/or mechanical) over the prey or predator serving for attack or defense (Özbek et al. 2009). In medusozoans (except by Staurozoa) the cnidae also possess a cilium, the cnidocil, that works as a mechanical trigger for the discharge. By triggering, the operculum (present in the Hydrozoa and Scyphozoa) opens and the thread disentangles penetrating the tissues of the prey/aggressor and injecting the toxin of the capsule. In anthozoans there is no cnidocil and the operculum is tripartite. The reactions of prey/predator are highly differentiated and depend on the type of cnida and chemical composition of the toxin. Some studies report the discharge of cnidae (a kind of exocitosis) as the fastest living mechanism reaching 2 ms with an acceleration of 40,000 g. What could promote such mechanism? There are three hypotheses to explain the discharge of the cnidae: (1) osmotic – a fast entrance of water; (2) tension – tension generated by the formation of the cnida; and (3) contractile – in which contractile units press the capsule. The developing cnidae are formed by the cnidoblasts, cells differentiated from interstitial cells located in the epidermis and gastrodermis, depending on the taxon. The evolutionary origin of the cnidae is speculative – there is a hypothesis that suggests the organelle is a kind of symbiosis with specific unicellular organisms (protists) at the origin of the cnidarians. There are three major types of cnidae: (1) nematocysts possess a double capsule with toxins composed by phenols and proteins, threads with spines and with an apical pore; (2) spirocysts have a single capsule and the inside fluids are composed of mucous and glycoproteins (more adhesive) and there is no apical pore; (3) ptychocysts have a folded thread, no spines, and no apical pore, being exclusively adhesive. Nematocysts, the most common and ubiquitous form of cnida, have a diversified morphology of the thread and capsule, being of taxonomical importance (Mariscal 1974). They appear very early in the life cycle, being already present in the planula (the first larval type of cnidarians), and are present all over the living tissues (although they are not present in the gastrodermis of hydrozoans).

The body of the cnidarians is internally lined by gastrodermis, which is responsible for the digestion that occurs both in the gastrovascular cavity and inside the cells. The gastrodermis can be more developed in some areas forming mesenteries or septae that divides the cavity into smaller parts, thus increasing the digestive area and providing further support to the soft body.

Skeleton

Like many other marine invertebrates, cnidarians developed a hydrostatic skeleton that works basically with the entrance of water into the gastrovascular cavity, and with the closing of body aperture (mouth). The tension of the water kept in the gastrovascular space is provided by the epithelio-muscular cells arranged longitudinally and circularly, making it possible to support the body structure. When the mouth opens and the water comes out, the animal loses rigidity and its size is reduced. This dynamic process is essential for physiological demands, like excretion, respiration, circulation, etc.

Skeletons are more evident in colonial cnidarians, but they also exist in solitary ones. In all cnidarians the skeleton is exclusively produced by the epidermal cells and is external. In hydroids, a usually thin perisarc layer made of chitin protects the hydranths, hydrocaulus, stolon and reproductive structures. This type of exoskeleton also protects scyphopolyps, in which they are named periderm. The octocorals developed other types of skeleton in the form of free sclerites and/or a horny axis (solid or with spaces) composed of proteinaceous fibers (gorgonin), within the group it is possible to follow the complexity increase of the skeleton. In stony corals and some hydroids (informally named hydrocorals) the exoskeleton is impregnated by the secretion of Calcium creating a massive structure that, by overgrowth and accumulation, originates the famous coral reefs known from tropical regions.

Skeletal structures are widely used to identify species and define higher taxa. In stony corals the individual skeleton of each polyp is called corallite, with an external wall (theca) and the floor (plate). A holding axis arises form the basal plate there, the collumela, as well as internal septae that hold the soft body of the polyp. Corals grow slowly, but the symbiosis with photosynthetic dinoflagellates (generally referred as zooxanthellae) speeds up calcareous deposition. During colony growth the corallites are sealed by tabulae that work as support for continuous and overgrowth developing of the several linings of polyps. The two non-related hydrocoral families (Milleporidae and Stylasteridae) have the skeleton with pores from which gastrozooids and dactylozooids emerge.

Motion

Motion of cnidarians occurs by different manners depending on the life cycle stage and, habitat. Larval forms such as planulae usually displace by ciliary beating, although some planulae without cilia creep and crawl over the substrate (e.g., staurozoans). In polyps, the action of epithelio-muscular cells promotes contractions reducing the height and increasing the diameter of the body, making some species able to move over the substrate. The ability to stretch the body column is an important mechanism to capture prey and is also related to the hydrostatic skeleton, body musculature, tentacles and oral disc. In sea anemones the body wall musculature of the column is more developed, and they also have longitudinal bundles on the sides of the mesenteries, called retractor muscles. In some species a set of ring musculature of the column forms a sphincter, usually located between the column and the oral disc. Also, a circular fold, forming a collar, can contract and protect the oral disc and the mouth. The ceriantharians, however, do not have a sphincter nor retractor muscles in the mesenteries, and the epidermal longitudinal musculature is continuous and allow a fast retraction inside the tube, partially dug on soft substrate.

Interstitial species have a different organization of the muscle fibers, allowing vermiform movements inside the substrate. Epifaunal non-sessile individuals, like Hydra, use body contractions and tentacle adhesion to tumble over the substrate – to a certain extent this may be also observed in staurozoans. Stauromedusae motion can be compared to polyps because they have an adhesive pedal disc besides sticky tentacles and rhopalioids. On the other hand, some sea anemones are able to swim by detaching from the substrate, using alternate contractions of the body column and shaking tentacles; other species can attach to the surface tension film and thus move around. This drift strategy is evidently also common in neustonic/pleustonic colonies, such as the Portuguese man-o-war (Physalia physalis), Blue Button Jellyfish (Porpita porpita) and the by-the-wind sailor (Velella velella).

In medusoid forms, the epithelio-muscular cells are organized in specific areas of the body, on the epidermis or just underneath that. These cells are generally organized in rings and near the margin of the bell, forming a coronal or ring musculature, but in some species these cells can have radial arrangements. The contraction of these specialized structures together with the antagonistic action of the gelatinous mesoglea promotes the swimming of the medusae. When they are relaxed (i.e., muscles not contracting and with mesoglea not tensioned) the subumbrellar cavity is filled with water. When the ring muscles contract the subumbrellar space is reduced and water is expelled; thus the animal displaces towards the aboral region– depending on the morphology and biology of the animal, this water flow may work as a jet (the so called jet-propulsion of jellyfish) or a row (a rowing movement). The radial muscles can help to return to the relaxed condition, determined by the shape of the dense mesoglea. The presence of a velum (in the hydromedusae) and a velarium (in the cubomedusae) promotes a more efficient and stronger water exhaling flow by reducing the aperture of the bell. This strategy of using tissue membranes is not present in the scyphomedusae, but they are also highly efficient in swimming. Different medusae may have distinct swimming modes: while some swim continuously towards the water surface and then stop moving sinking slowly in the water column, others have the ability to change direction or are attracted to light sources (Costello et al. 2008). The presence of environmental discontinuities like changes in temperature and salinity may concentrate medusae in some areas. A few jellyfish species have benthic habits, and these animals move using their tentacles or body structures like adhesive warts. The swimming strategies are intimately related to food capture and can vary a lot depending on the group.

Physiological Aspects

The nervous system of cnidarians has many physiological aspects similar to bilateral animals, as well as ctenophores. Nerve cells are arranged in a net-like pattern, densely concentrated at the oral region, tentacles and pedal disc – thus forming a non-centralized and diffuse system. There are two nerve nets underneath the two epithelial layers (epidermis and gastrodermis), between those layers and the mesoglea. The subepidermal net is more developed than the gastrodermal one. The majority of the nerve cells are non-polarized, and electric impulse spreads to all directions.

Together with ctenophores (with uncertain affinities), cnidarians are the most basal animal group presenting sense cells; organized as pigment spots, statocysts, motion receptors, and sense pits. There is no central processing area like a brain or cerebral ganglion, thus the information has to be processed equally in the nerve net all over the body of the animal. In polyps the motion receptors are distributed over the body surface, but more concentrated in the tentacles in order to help in perception of prey and predators. In hydromedusae the nerve cells are arranged in two circular rings near the margin and innervate tentacles, muscles, and sense organs. Ocelli, when present, are located in areas with pigmented and photoreceptor cells forming spots or pits. Statocysts are also located in a pit on the body wall or in closed vesicles containing a statolith and sensory cilia. Bell movement tilts the body and cilia of the statocyst are stimulated but the statolith, inhibiting muscle contraction on that side of the body. Many cnidarians live in illuminated areas to favor the development of zooxanthellae in their tissues. In cubomedusae and scyphomedusae the sense cells are grouped in a unique sensory structure, the rhopalia. These structures are located in clefts between the marginal lappets of the scyphomedusae or in niches on the exumbrella of the cubomedusae. Characteristically the rhopalium of cubomedusae is highly developed, including photoreceptor structures forming complex eyes with cornea, lens and retina, plus a statocyst and ocelli (Fig. 5). Thus, cubomedusae may chase their prey actively, moving away from obstacles, and may even use environmental landmarks to guide their displacement.
Fig. 5

(a) The box jellyfish Chiropsalmus quadrumanus (Cubozoa) from the Brazilian coast (Photo courtesy of Dr Alvaro E. Migotto, http://cifonauta.cebimar.usp.br). (b) Rhopalium of the same species in higher magnification

Many cnidarians are bioluminescent, especially deep water jellyfish , but also some polyps can emit light. Bioluminescence is used to attracting prey, reproductive partners, and to avoid predators. The conservative process is the reaction luciferin-luciferase, but there are some other proteins involved (aequorin) and bioprospection of these compounds is an important field of research.

Food capture occurs with the tentacles surrounding the mouth of the polyp. The cnidae of the epidermis of these tentacles are stimulated and then discharged. Cnidae work as small harpoons injecting toxins and holding the prey. In some species the tentacles wrap the prey and direct them to the mouth, in others the mouth and hypostome extend towards and “swallow” the prey. In the medusae the swimming activity creates water flows that direct the prey into distinct parts of the animal (tentacles, oral arms, umbrella) and subsequently transferred to the mouth.

Digestion starts inside the gastrovascular cavity right after ingestion of prey. Gastrodermal cells secret digestive enzymes starting an extracellular process. Ciliary beating and body contractions help to move and mix the content. Smaller particles are engulfed through phagocytosis and pinocytosis by the gastrodermal cells starting the intracellular digestion. Nutrients are distributed all over the body by contractions and ciliary movement. In anthozoans the mesenteric filaments present a free margin with a tri-lobed end: the median lobe (cnidoglandular lobe) kills the prey with the cnidae and produces digestive enzymes for digestion; the lateral lobes are ciliated and help moving food inside the cavity. In a group of sea anemones this median lobe is much developed and full of cnidae, called acontium, and helps killing the prey (internally) or as a defense when exteriorized through the mouth or small side apertures named cinclids. Species with reduced tentacles, like some corallimorpharians and sea anemones, use contractions of the oral disc to capture prey. Many cnidarians may also use mucous to capture small organisms. Food rests (like prey carapaces and undigested pieces) are expelled through the mouth by body contraction and ciliary beating.

The cnidarians are essentially carnivorous, but several species may be associated with unicellular symbionts that photosynthesize (zoochlorellae in freshwater and zooxanthellae in sea water animals). This kind of association is well known for stony corals that use the additional nutrients produced by the zooxanthellae to enhance the secretion of calcium carbonate. Besides corals, some species of sea anemones, hydroids, scyphozoans, and one species of cubopolyp also present symbiosis with zooxanthellae. The symbionts are usually located in the gastrodermis, and are responsible for most of the coloration of these cnidarians (Fig. 6). Zooxanthellae provide mainly sugar and amino acids for the hosts and these provide back nitrogen and phosphorus for their metabolism. This association is so important for some corals that many species can only build coral reefs when the symbionts are in good health, and this is the reason they are restricted to warm, clear and shallow waters. Adverse conditions may cause corals bleaching, a process in which the corals loose their zooxanthellae (Fig. 7). Such reduction in the concentration of the symbionts can be caused by many environmental factors, which culminate with the expulsion or consumption of the zooxanthellae by the coral tissues, frequently causing the death of the coral as well.
Fig. 6

Zooxanthellae symbiotic of the scyphozoan jellyfish Cassiopea. (a) Tip of medusa oral arm showing zooxanthellae and digitata, magnification 100×. (b) Squash preparation of zooxanthellae, magnification 1000×

Fig. 7

Bleached small colony of the stony coral Mussismilia hispida

Gas exchange occurs by diffusion through the whole body surface that is in contact with the environment. The proportional increase of the exchangeable area by the presence of the septa in the gastrovascular cavity is very important to promote a more efficient diffusion. Occasionally, some species may change to anaerobic respiration under certain specific conditions.

Cnidarians do not have a single and specialized organized circulatory system, and this function is provided by the gastrovascular cavity, besides digestive and exchanging functions. Jellyfish have a more developed canal system promoting a better distribution of the nutrients and a way to move out the wastes.

Excretes produced by cellular metabolism (mainly ammonia) are released by diffusion through the body wall or eventually released in the cavity and from there to the external environment. The majority of the cnidarians is marine, thus body concentration is the same or similar to the marine environment, making osmoregulation negligible. In freshwater species, like Hydra, there is a periodical discharge of fluids through the gastrovascular cavity that is kept as hyposmotic.

Life Cycle and Reproduction

Cnidarians usually have separate sexes (i.e., they are gonochoristic). There are only a few hermaphroditic species that can be simultaneous or sequential (Tardent 1985). Male and female usually release gametes directly to the water and fertilization is by chance (Campbell 1974). However, several species developed different strategies to maximize gamete fertilization, for instance, synchronizing the gamete release to short periods of time; having aggregation behavior of the mates in large populations to reproduce; developing internal fertilization, sometimes even grouping gametes in packages or spermatophores; and even evolving mating behavior in some species. Cnidarians are certainly the most basal clade of animals to have mating behavior with courtship and indirect transference of spermatophores.

There is no general pattern of embryonic development for the whole group of cnidarians – the different groups have several different types of cleavage, sometimes more than one in the same group. The early cleavages have a radial and holoblastic pattern, but later there are many distinct pathways of development. The gastrulation process, forming the epidermis and gastrodermis, is extremely variable in the group and some species even have a combination of patterns during the embryogenesis (Byrum and Martindale 2004). At the end of gastrulation the result may be a hollow (i.e., coelogastrula) or a solid (stereogastrula) gastrula, demanding or not feeding for its development. The presence of this first cavity in the larvae (planulae) is important to understand its mechanisms for dispersal and settlement/recruitment. The free-swimming or crawling planulae seek for a suitable substrate to settle and metamorphose to the initial polyp of a given species. The substrate may be specific (i.e., the planulae may have a strict association with a given substrata that triggers the metamorphosis) or the planulae may be generalist concerning the type of substrata. Ecologically this is quite important because specialized planulae cause the range distribution of their species constrained to the range distribution of their substrata; conversely, generalist planulae may have a broad geographical distribution, and are more prone to be dispersed. This latter case is the phenomena involving invasive species – they usually have a generalist behavior concerning substrata making it possible to colonize different regions of the world.

Asexual reproduction is widespread in the phylum Cnidaria. The main process of asexual reproduction in the polyp stage of hydrozoans is by budding, increasing the polyp population and/or forming colonies. Medusae may also bud by forming new individuals from the body wall (either from the exumbrella or from the manubrium), but this is more rare than in polyps. Some species may present other types of asexual reproduction like fission, either longitudinal or transversal to the oral-aboral axis of the polyps. The presence of large numbers of interstitial (undifferentiated) cells provides the cnidarians a higher regeneration capacity. This may be periodically adopted by certain species – many hydrozoan species periodically dedifferentiate their polyps and use interstitial cells to reorganize their tissues again, like in a rejuvenation process.

As mentioned before, in some hydrozoans the medusa stage is degenerated to a certain degree or even suppressed (like in Hydra) and the polyps assume the sexual reproduction. Combining the many possibilities result in a variety of different life cycles. Therefore, in some species the polyps may reproduce both asexually by budding and sexually by producing gametes. There is also a direct life cycle without a polyp stage, in which the planula differentiates directly into another larva called actinula and from this to a medusa. In all hydromedusae the gametes develop in a specific area of the epidermis near the radial canals or the manubrium. Differently, the gametes develop in the gastrodermis in all other groups of cnidarians (Tardent 1985).

In scyphozoans with metagenesis the polyps undergo strobilation, basically a process of asexual reproduction in which the polyp body is apically cut into transversal successive discs, and each one of these discs differentiates into a small larval jellyfish called ephyra. Developing polyps (named strobilae) can produce different numbers of ephyrae and then are considered to have a polydisc (when producing many ephyrae) or monodisc (a single ephyra). After releasing all ephyrae the scyphistoma (scyphozoan polyp) usually regenerates and, after a period of growth and reserve accumulation, it strobilates again. After reaching sexual maturity, scyphomedusae either release gametes in the water or the fertilization occur internally – brooding of planulae can be observed in the latter process. Only two scyphozoan species have holopelagic life cycles (i.e., without a polyp stage during their life cycle), Pelagia noctiluca and Periphylla periphylla. Curiously in Pelagia the planula larva itself differentiate into an ephyra before reach the adult medusa stage. In some members of the order Coronatae a reduction of the medusa stage is also observed, in a convergent process with the Hydrozoa.

In cubozoans the polyps are reduced, but they have mechanisms of budding, increasing the number of individuals (i.e., cubopolyps). Medusa formation occurs by metamorphosis, in which a single polyp transforms into a single medusa. However, in a few species it was observed the existence of remnant polypoid tissue after the metamorphosis is complete, and these remnants can regenerate a new small polyp. For species in which the sexual reproduction is known, they tend to group and some may exhibit mating behavior, including courtship and the male medusae transfer sperm packages, sometimes in the form of spermatophores, to the females, or may release them in the water – in any case, fertilization is internal.

For staurozoans it is known that some species have a polyp (named stauropolyp), which differentiates into a stauromedusa, and this latter phase produces and releases gametes. The planula larva of this group is creeping and has no cilia.

For anthozoans, the unique polyp stage is evidently the sexual phase, and the gametes are in the mesenteries. They have distinct mechanisms of asexual reproduction like longitudinal and transversal fission, budding, and pedal laceration, but transversal fission and budding from tentacles are rare. Hermaphroditism is commoner in anthozoans than in medusozoans, although both exist depending on the species. Many species brood their planulae inside the gastrovascular cavity, and these planulae are released only on late stages of development, when they are ready to settle. Stony corals have a synchronous and simultaneous gamete release – such mechanism of mass spawning is interpreted as a mechanism to maximize the likelihood of fertilization as well as reduce predation over the eggs. As already mentioned, anthozoans do not present a medusa stage, so the dispersive stage of this group is the planula larva. Many larvae have short lives, but in Ceriantharia the planktotrophic cerinula larva can live for longer periods in the pelagial. Coral larvae can also live for weeks or months in the plankton, thus favoring dispersal. Also, many species use different asexual modes of reproduction while others develop coloniality, including some groups with modular polymorphism.

Few cnidarian species are parasites. The only intracellular parasite of the group is Polypodium, an enigmatic parasite of eggs of sturgeons. There is some doubt if the myxozoans, previously classified as protists, should be considered as highly derived cnidarian fish parasites. Extracellular parasites are commoner, including sea anemones and narcomedusae parasitizing other species of medusae. Mutualism between cnidarians and other animals is also common. Many hydroids are associated with crustaceans, gastropods, bivalves, worms and even other hydroids. Some sea anemones are symbiotic of hermit crabs, in some cases even producing a somewhat rigid covering for the crustacean (carcinoecium). Such associations provide easier access to food source and mobility to the cnidarian, and the crustacean usually benefits from the cnidae as a further source of defense. But there are some animals that feed on cnidarians but do not digest the cnidae, subsequently using them for defense (cleptocnidae), like flatworms (Microstoma), comb jellies (Haeckelia) and many nudibranch species. Several fish species also benefit from the stinging ability of the cnidarians as a defense, like the clownfishes (Amphiprion) and sea anemones, and shepherd fish (Nomeus) and the Portuguese man-of-war. But some associations do not represent advantages for the cnidarians, like with some hyperid amphipods, majid crabs, and palaeomonid shrimps, that use to feed on the cnidarian.

Understanding the different levels of metagenesis expression is essential to have a picture of the evolution and ecology of the cnidarians. The study of the biology of the species reveals that there are many instances for medusa or polyp reduction, and the interpretation of such observations (life cycle patterns) together with environmental characteristics can be important tools to understand the diversification, abundance and evolutionary pathways of cnidarians.

Historical Aspects

Cnidarians are known since the ancient Greek times (Aristotle, ~380–320 BC) when they were classified as polyps (knide) and medusae (akalephe). Later on the group was treated by Linnaeus (1758) as Zoophyta, a category between animals and plants that included animals with radial symmetry or totally asymmetric like sponges , bryozoans, and flatworms. Cuvier (1812) used the name Radiata to group medusoid cnidarians and echinoderms. Only in 1847 Leuckart divided radiate animals into Echinodermata and Coelenterata (which included sponges, cnidarians, and ctenophores). Finally, the name Cnidaria was proposed by Verrill in 1865 for the polypoid forms, and the status of phylum was proposed in 1888 by Hatscheck, by splitting that from the former Coelenterata.

The precise chronological origin of the cnidarians is uncertain because there are several fossils that are not clearly defined as cnidarians, but present some similarities with polyps or jellyfish. Based on the most recent studies the oldest cnidarian fossils would have occurred during the Ediacaran period (~630–540 million years ago).

Cnidarians are a key group in the traditional hypotheses for the origins of the animals, like the colonial theory (flagellate colonial protists forming a hollow ancestral organism – the blastaea), and the syncytial theory (ancestral cnidarians similar to turbellarian flatworms). However these are nowadays considered as alternative scenarios independent of the general phylogenetic pattern of the origin of the metazoans. But to understand the evolutionary origin of cnidarians, it is important to recognize which body pattern was presented in the ancestral: polypoid or medusoid. These two possibilities were intensely discussed in the literature by the middle of the last century. The idea of a medusoid cnidarian ancestor has few morphological arguments. Currently the idea of a polypoid ancestor is widespread and supported by morphological data as well several different molecular markers.

Classification

Subphylum Anthozoaria, Class Anthozoa. Members of this group are sea anemones, corals, sea fans, and gorgonians. They are solitary or colonial, but exclusively marine. All members studied present a circular mitochondrial DNA, and never present a medusoid stage in the life cycle. They have cnidae in both epithelia (epidermis and gastrodermis); eight hollow tentacles or multiple of six tentacles, with cavities contiguous to the wide gastrovascular space (divided by longitudinal mesenteries with a free rim aborally forming the mesenteric filament). An actinopharynx is present and may have 0, 1, or 2 opposite ciliated grooves (siphonoglyphs). Germ cells differentiate and develop in the gastrodermis.

Subclass Octocorallia. Members of this group are called octocorals and they are colonial forms, their polyps have 8 pinnate tentacles around the mouth opening, eight complete or perfect mesenteries, a single siphonoglyph, and free or fused calcareous skeleton sclerites. Order Alcyonacea includes soft corals (without a support skeletal axis) and gorgonaceans (with a support skeletal axis composed of a horny substance – gorgonin – or calcareous); Order Helioporacea (blue corals) characteristically present a calcareous skeleton composed by aragonite crystals; Order Pennatulacea (pennatulaceans) include polymorphic colonies with the primary polyp working as an anchoring structure (peduncle) from which a wide supporting raquis with secondary polymorphic polyps emerge (autozooids and siphonozooids), and there is also calcareous sclerites. Examples are the genera Carijoa, Muricea, Leptogorgia, Renilla, Alcyonum.

Subclass Hexacorallia. Members of this group are stony corals, sea anemones, corallimorpharians, black corals, and ceriantharians. They can be either solitary or colonial, usually have a hexamerous symmetry and characteristically present spirocysts. The monophyletic status of the group is dubious because of their wide morphological variability. Order Actiniaria (sea anemones) never form colonies or calcareous skeleton, the column has specialized structures (warts, papillae, vesicles), and typically bear two siphonoglyphs. Order Antipatharia (black corals) have a horny spiny axial skeleton, polyps with six tentacles, six complete mesenteries and up to six secondary mesenteries. Order Ceriantharia (tube anemones) have marginal and oral crown of tentacles, live inside the substrate within a tube produced by mucous secretion and discharged cnidae (ptychocysts). Order Corallimorpharia (corallimorpharians) can be solitary or clonal, without a calcareous skeleton, without siphonoglyphs or a ciliated part of the mesenteric filament; they are usually confounded with sea anemones but with many similarities to stony corals. Order Scleractinia (stony corals) is mostly composed by colonial forms, do not have siphonoglyphs or ciliated parts of the mesenteric filaments, and they produce a calcareous skeleton with septae. Order Zoanthidea (zoanthids) have two circles of marginal tentacles, polyps with a single siphonoglyph, colonial forms have a stolon-like structure with gastrodermal canals, and most species incorporate sand grains in the body wall. Examples are the genera: Bunodosoma, Anemonia, Anthopleura, Antipathes, Ceriantheomorphe, Pachycerianthus, Discosoma, Corynactis, Siderastrea, Mussismilia, Acropora, Fungia, Palythoa, Zoanthus.

Subphylum Medusozoa. Cnidarians with life cycle alternating from polyp to medusa. Solitary or colonial forms, almost exclusively marine with a few species in freshwater. Characteristically presenting linear mitochondrial DNA, sometimes segmented like in Cubozoa. Divided into four classes.

Class Cubozoa. Cubomedusae, sea wasps, box jellyfish; the name is derived from the cubic shape of the bell; 36 marine species, some tolerating estuarine waters. Characteristically with four rhopalia with complex eyes, a velarium of subumbrellar origin, pedalia holding tentacles, gonads developing in the gastrodermis and metagenetic life cycle with solitary polyps that metamorphose into medusae. Carybdeida are the medusae with a single tentacle per pedalium and without gastric saccules. Chirodropida are the medusae with ramified pedalia holding several tentacles and gastric saccules on the subumbrella. Examples are the genera: Alatina, Carybdea, Chironex, Chiropsalmus, Chiropsella, Copula, Tamoya, Tripedalia.

Class Hydrozoa. Hydromedusae and hydroids, ca. 3,500 species, mostly marine, some estuarine and a few living in freshwater. Characteristically they have a gastrovascular cavity without mesenteries, cnidae only in the epidermis, colonial forms with variable degrees of polymorphism, gametes stored in the epidermis, medusae originating from lateral buds and with a velum. The group can be divided into two subclasses.

Subclass Hydroidolina. When present, members of this group have statocysts of ectodermal origin; colonial forms are polymorphic. “Anthoathecates” (a non-monophyletic taxon) are represented by hydroids without a skeleton around the hydranth, medusae without statocysts and gonads restricted to the manubrium. Order Leptothecata is represented by hydroids with skeleton around the hydranth (theca) and gonophores (gonotheca), medusae with gonads along radial canals. Order Siphonophorae is represented by polymorphic polypoid colonies with zooids for swimming and floating, with holopelagic life cycle. Examples are the genera: Aglaophenia, Apolemia, Clytia, Corymorpha, Eudendrium, Obelia, Pennaria, Physalia, Sertularia.

Subclass Trachylina. When present, members of this group have statocysts of ecto-endodermal origin. Order Limnomedusae have minute polyps without theca (when present), medusae with gonads over radial canals. Order Narcomedusae are jellyfish with lobate margin and tentacles from the exumbrella. Order Trachymedusae is mostly composed by species with holopelagic life cycle, medusae with gonads usually on radial canals, but one group of trachymedusans have interstitial habit with direct life cycle from a stage similar to an actinula larva. Examples are the genera: Aglaura, Cunina, Gonionemus, Liriope, Olindias, Solmaris.

Class Scyphozoa. “True” jellyfish , scyphomedusae, around 200 species, exclusively marine, some living in estuarine waters. The medusoid stage is the most conspicuous of the life cycle, but they also do have polyps. Characteristically the medusae have the margin with clefts and projections (marginal lappets), usually eight rhopalia in the clefts, gonads differentiating in the gastrodermis. Polyps with four mesenteries with septal funnels with muscular strains. Medusae are formed by transverse apical fissions (strobilation) originating ephyrae. Order Coronatae with medusae presenting a coronal furrow on the exumbrella and polyps with a periderm tube (exoskeleton). Order Semaeostomeae with medusae presenting four long oral arms, gastrovascular cavity divided by septae or canals, usually tentacles on umbrella margin. Order Rhizostomeae with medusae presenting eight oral arms with minute mouth openings, central mouth and tentacles absent, gastrovascular cavity divided by a network of canals. Examples are the genera: Aurelia, Chrysaora, Cyanea, Linuche, Lychnorhiza, Nausithoe, Periphylla, Rhizostoma, Rhopilema.

Class Staurozoa. Sessile medusae, stauromedusae, with ca. 50 marine species. Members of the class have capitate tentacles in eight groups, metamorphosis of polyp into stauromedusa, absence of strobilation and ephyrae, creeping planula larva without cilia. Taxonomy of the groups needs thorough revision due to several inconsistencies of the present classification. Examples are species of the genera: Craterolophus, Haliclystus, Kishinouyea, Lucernaria, Manania, Stenoscyphus.

Toxinological Aspects

The cnidae of the cnidarians is a stinging structure potentially harmful to man. Although cnidae are microscopic structures, some of them can inflict painful reactions and even cause death (Mariottini and Pane 2014). Envenomation caused by cnidarians can occur occasionally during professional (fishing) or recreational (bathing, diving) activities.

There are reports of accidents involving all groups of cnidarians and all life cycle stages, polyp, medusa and even the planula larvae. Polypoid forms like sea anemones, plume-like hydroids, soft corals, fire corals or hydrocorals, and siphonophores can sting due to the presence of different types of cnidae (and also different types of venom) in their tentacles, warts, or any specialized structure of the body column. Medusoid forms like scyphomedusae, cubomedusae and hydromedusae can also sting, but mostly due to the presence of the cnidae on their tentacles and mouthparts.

Envenomation by cnidarians may be quite variable, depending on two variables: the person and the cnidarian. Concerning the person, the severity of the envenomation may be related to the age of the person, the extension of the area of the body affected, health condition of the person, and potential allergic reaction. Concerning the cnidarian, it is related to the species involved, dimensions of the animal, condition (healthy or moribund), and types of cnidae. There might be local reactions (inflammation, dermatitis, pain) (Fig. 8), long-term reactions (keloids, pigmentation, scars) (Fig. 9), systemic reactions (pulmonary edema, blurred vision, vomiting, convulsions), and fatal reactions caused by cardiac and respiratory arrest, renal failure, and anaphylaxis. A list of the most dangerous cnidarians includes several species of jellyfish: the cubozoan Chironex fleckeri; the hydrozoan Gonionemus vertens; the scyphomedusae Pelagia noctiluca and Cyanea capillata. But there are also some dangerous polyps: sea anemones of the genera Alicia, Actinia, Urticina and Anemonia; and the hydrozoans Physalia physalis (a polypoid colony), Aglaophenia, Macrorhynchia.
Fig. 8

Recent acute sting caused on the back of a bather by a Portuguese man-of-war (Physalia physalis). Photograph taken 15 min after contact (Photo courtesy of Dr Vidal Haddad Jr)

Fig. 9

Late sting (1 week after contact) caused on the foot of a bather compatible with Portuguese man-of-war or cubomedusae. Note linear markings with superficial necrosis (Photo courtesy of Dr Vidal Haddad Jr)

Cnidarians, as well as sponges, are long known as a source of potential natural bioactive compounds that, combined with pharmacological techniques, may be the basis to develop new drugs or biomedical/biotechnological materials/processes. Several molecules, including anticancer and antioxidant compounds, were isolated from cnidarians (Zaharenko et al. 2008). Prostaglandins, local anesthetics, and vasoconstrictors have already been developed based on isolated bioactive substances from anthozoans, for instance, the most investigated group.

Many cnidarians were studied focusing on the hemolytic action of their venoms on vertebrates, and they present different levels of activity: the scyphomedusae Cyanea, Pelagia, and Rhizostoma; the cubomedusae Carybdea and Chironex; the octocorals Litophyton and Sarcophyton; the anemones Metridium and Urticina; and the hydromedusae Crossota and Pandea. Octocorals (Clavularia, Xenia), hexacorals (Tubastrea, Palythoa, Actinia, Anemonia), scyphozoans (Chrysaora, Cyanea), cubozoans (Chiropsalmus, Chironex) and hydrozoans (Physalia, Aequorea) were studied concerning the antiproliferative and cytotoxic activities (neurotoxic, hepatotoxic, and cardiotoxic), and also have effects over cell division. In summary, according to several studies, the action of the cnidarian venoms cause pore formation in membranes or oxidative stress.

Conclusions and Future Directions

Although the morphology of sponges and cnidarians is well known there are still classification issues to be solved. Novel methodologies on molecular techniques will certainly provide further advances into taxonomy combined with detailed morphological approaches. Sponges have been a source of a myriad of bioactive compounds and much more can be discovered. Cnidarians possess intracellular organelles that inject toxins (cnidae); those structures and toxins are underexplored in terms of pharmacological aspects.

Cross-References

Notes

Acknowledgements

The authors were supported by grants 2010/50174-7, 2010/5234-6, 2011/50242-5, 2013/04084-4 São Paulo Research Foundation (FAPESP), and CNPq (557333/2005-9, 490158/2009-9; 301039/2013-5, 305805/2013-4); this is a contribution of NP-BioMar, USP.

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Copyright information

© Springer Science+Business Media Dordrecht 2014

Authors and Affiliations

  • André C. Morandini
    • 1
    Email author
  • Márcio R. Custódio
    • 2
  • Antonio C. Marques
    • 1
    • 3
  1. 1.Departamento de Zoologia, Instituto de BiociênciasUniversidade de São PauloSão PauloBrazil
  2. 2.Departamento de Fisiologia, Instituto de BiociênciasUniversidade de São PauloSão PauloBrazil
  3. 3.Centro de Biologia MarinhaUniversidade de São PauloSão SebastiãoBrazil

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