This chapter provides a preliminary account of the arachnids that occur on the islands of Bioko, Príncipe, São Tomé, and Annobón. We treat our assessment as preliminary because to comprehensively determine which arachnids inhabit a place, systematic, multi-year, seasonal surveys are critical. For many arachnid species, the annual lifecycle is temporal, often with adult males and females more common at particular times of the year (Cardoso et al. 2009). Standardized collecting methods have been proposed for arachnids, which allow for a more effective estimate of species richness when collecting efforts are limited (e.g., Malumbres-Olarte et al. 2016). To date, however, this type of surveying for arachnids has not been undertaken on any of the Gulf of Guinea islands. In fact, nearly all of the historical collectors of arachnids in the region, including West and Central Africa, collected them opportunistically while making general collections of flora and fauna or targeting other organisms. The lack of methodical collecting across much of the African continent and the monumental work needed for subsequent taxonomic investigation also hinders our ability to discern whether species recorded from the islands are endemic or are also present on the mainland. Consequently, our current knowledge of the arachnid fauna of the islands is far from complete given the few historical or contemporary surveys dedicated to documenting arachnid diversity and minimal taxonomic study of most major groups. Here we summarize what is (and likely is not) known about the arachnid diversity of the Gulf of Guinea archipelago in the hopes that it will inspire and guide future research on this understudied group.

Arachnids are a diverse and ancient group (>430 my old) of primarily terrestrial arthropods. There are 11 extant orders, all of which are known from mainland Africa, with 8 known from the Gulf of Guinea islands. While Acari, or ticks and mites, are present, we do not discuss them here. They are typically treated separately in arachnid faunal overviews and surveys because, although they are arachnids, they are extremely diverse and even more poorly known than other arachnids. Their life histories and habits differ greatly from the other arachnids in that many are plant or animal parasites or live in soil.

There are three orders of arachnids that have not yet been found on the Gulf of Guinea islands, although they are known from the adjacent mainland. One of these orders is the Palpigradi, or microwhip scorpions, which are very small, pale, eyeless animals that live interstitially in leaf litter, caves, or cracks deep in the ground. They are found worldwide, but due to their cryptic habits and small size, fewer than 100 species have been described (Harvey 2013a). The other two orders, however, are not minute. These are the Solifugae, or sun spiders, and the Thelyphonida, or whip scorpions. The former consists of about 1000 species that are primarily found in dry habitats, while the latter has only about 100 species described from a wide variety of tropical and sub-tropical ecosystems (Harvey 2013e; Murienne et al. 2013). Although they are not small, whip scorpions are burrowers, thus not easily detected, and there is only a single species known from Africa (Huff and Prendini 2009). Below, we discuss the seven other orders: Amblypygi or whip spiders, Opiliones or harvestmen, Pseudoscorpiones or pseudoscorpions, Ricinulei or hooded tickspiders, Schizomida or short-tailed whip scorpions, Scorpiones or scorpions, and include a focused discussion of the most diverse group, the Araneae or spiders.

A Brief History of Arachnological Research

In the 1700s and 1800s, most arachnid collecting in the Gulf of Guinea was done by naturalists making general collections of extant flora and fauna as well as fossils. The specimens were then sold or donated to European museums, where they were divvied up and given to experts on the various groups, who then described new species and/or published a species list. Ferdinand Karsch (1884) was likely the first arachnologist to focus specifically on arachnids from the Gulf of Guinea islands. He received the specimens for his study from Professor Richard Greeff of Marburg, who had lived on São Tomé and Príncipe islands for several months in 1879–1880. Greeff made some of the first maps of the islands and collected everything from crinoids (echinoderms) to sipunculids (peanut worms) to geckos and published his results in 1884. Other authors published smaller works on the island fauna, including Vieira (1893) and Pocock (1900), the latter of whom described some theraphosid spiders that had been purchased by trustees of the British Museum.

Leonardo Fea from the Museum of Natural History in Genoa, Italy made trips to Asia and Africa in the late 1800s to the early 1900s, spending 1900–1902 on São Tomé and Príncipe, Bioko, and Annobón, and 1902 in Cameroon and the French Congo (now the Republic of Congo, Gabon, and Central African Republic). Although primarily collecting specimens for his malacological and geological research, he also collected several arachnids. These specimens eventually made their way to perhaps the most prolific arachnologist of all time, Eugene Simon from the Natural History Museum in Paris. In the early 1900s, Simon began describing and cataloging a large collection that comprises the most comprehensive arachnid publications of the region (1907, 1909–1910), where he included a brief tribute to Fea who had died in 1904.

Following Simon’s work, there were a number of important publications in the 1900s. Hansen (1921) described harvestmen and other small arachnid groups primarily from the Fea collection. Roewer later described many of the harvestmen from Fea’s collection in a series of papers (1927, 1942, 1949). Additional research was conducted by Amélia Bacelar (1956), the first woman to formally study Gulf of Guinea arachnids, and also by Otto Kraus (1960). The latter was on an expedition to the “Gulf of Guinee” led by Herrn P. Viette on the Calypso (The Calypso was owned by Jacques Cousteau, purchased in 1950, so he also was likely on this trip, but there is no mention of him specifically). Prieto more recently published on the harvestmen of Bioko and Annobón (Prieto 1999). In 1998, arachnologist Darrell Ubick of the California Academy of Sciences (CAS) visited Bioko. Ubick’s 1998 collection from Bioko was the first targeted collection of arachnids in the region, resulting in over 5000 specimens that are deposited at CAS. A preliminary report published on the material identified 372 morphospecies, with 9 families and 5 genera otherwise undocumented from the region (Griswold et al. 1999). A series of trips to São Tomé and Príncipe have been made by other CAS arachnologists, including Charles Griswold and Joel Ledford in 2001, Tamas Szűts in 2013, and Lauren Esposito in 2016, and will likely yield undescribed species.

Arachnids of the Gulf of Guinea Islands

Here we have compiled a list of spider, scorpion, whip spider, harvestmen, hooded tickspider, pseudoscorpion, and short-tailed whipscorpion species from the Gulf of Guinea islands based on the published works mentioned above, community science observations documented using iNaturalist (iNaturalist 2021), the World Spider Catalog (WSC 2021), and the Western Australian Museum catalogs for the smaller arachnid groups (Harvey 2013a, b, c, d, e, f). The majority of specimens mentioned or described in publications are singletons—single individuals from unique localities. However, there are many caveats. For example, in Simon’s papers, specimens are listed as being from specific localities (i.e., a particular island), whereas in the WSC, the available information is provided at the country level without further specificity, with Bioko often included only as Guinea. Two of the species described from Fea’s material were not associated with any locality data beyond the region as a whole; consequently, due to the large geographic scope of his surveys, they could have originated from any of the islands and/or the mainland. Given the paucity of data from the region, we chose to err on the side of inclusion for compiling a list of arachnids from the Gulf of Guinea oceanic islands. In particular, we include taxa reported from the land-bridge island in the archipelago, Bioko, because species reported from this island may also occur on the oceanic islands. We have also analyzed publication information to assess the quantity of research conducted on arachnid fauna of the area as a way of beginning a conversation on the remaining work required before major evolutionary questions (i.e., biogeography) can be addressed within these diverse lineages.

Order Amblypygi

The amblypygids or whip spiders are a modestly diverse group of predatory arachnids with approximately 150 extant species found in tropical and sub-tropical regions around the world (Harvey 2013f). All amblypygids lack silk glands and are not venomous. Their first pair of legs are highly modified sensory organs that give the appearance of antennae or whips, and their chelicerae are modified into raptorial claw-like structures (Fig. 11.1). Amblypygids are often found in leaf litter and caves, and all species are nocturnal. Four species in two genera (Charinus, Damon) are known from the Gulf of Guinea islands, three of which (all Damon) are confirmed to also occur in continental Africa (Appendix) (Harms 2018).

Fig. 11.1
Eight images of different species of spiders and scorpions from the Gulf of Guinea islands. It includes Hysterocrates sp., Gasteracantha thomasinsulae, Assamiida,Damon medius, Nilus sp., pseudoscorpion,Pandinus imperator scorpion, and Ricinoides sp.

Representative Arachnids from the Gulf of Guinea islands: (1) Hysterocrates sp. (spider); (2) Gasteracantha thomasinsulae (spider); (3) Assamiidae (harvestman); (4) Damon medius (whip spider); (5) Nilus sp. (spider) predating on frog; (6) pseudoscorpion; (7) Pandinus imperator (scorpion); (8) Ricinoides sp. (hooded tickspider). Photo credits: (1) Guy Tansley, (2) Brian Simison, (3, 6) Gonzalo Giribet, (4) John Sullivan, (5) Andrew Stanbridge, (7) Nik Borrow, (8) Beat Akeret

Order Araneae

The Araneae or spiders are the most diverse arachnid group, with nearly 50,000 described species (WSC 2021). Spiders have chelicerae with fangs, and most species use these to inject venom into their prey, which range from insects to other spiders and small vertebrates (Fig. 11.1). Although the venom of some species can be dangerous to humans, most species do not pose a risk. Spiders also have spinnerets that extrude silk, which is used to build webs for prey capture, make retreats and egg sacs, for mating purposes, as well as dispersal via ballooning.

A total of 213 spider species have been recorded from the Gulf of Guinea islands, encompassing 48 families and 136 genera (Appendix). The most speciose spider families from the islands are Araneidae (40 species), Salticidae (33), Tetragnathidae (15), and Lycosidae (12). All other families have fewer than ten recorded species, which likely reflects poor collecting of several groups, including good dispersers or those with cryptic ecologies. Singleton families (families from which only a single species is known) make up 35% of the familial diversity on the islands. Several dozen species were described based on material collected from the oceanic islands in the archipelago and are not known to occur in continental Africa, suggesting they may be island endemics.

Order Opiliones

The Opiliones are commonly referred to as harvestmen and contain over 6500 described species distributed worldwide. Although they appear superficially similar to spiders (Araneae), they are not closely related and do not possess venom glands (Fig. 11.1). They also do not have silk glands and thus do not build webs. Collectively, 34 species in 7 families are known from the Gulf of Guinea islands, 10 of which are confirmed to also occur in continental Africa (Appendix).

Order Pseudoscorpionida

Pseudoscorpionida, commonly known as pseudoscorpions, are small arachnids, typically around 3 mm in length, with pincer-like pedipalps similar to those of scorpions (Fig. 11.1); however, unlike scorpions, some pseudoscorpions deliver venom with their pedipalps rather than a stinger on a tail, which they lack. This group includes over 3300 described species that occur in many kinds of environments, but they are often overlooked due to their size (Harvey 2013b). Pseudoscorpions spin silk from a specialized gland in their jaws to produce a cocoon. Thirteen species in four families are known from the Gulf of Guinea islands, eight of which are confirmed to also occur in continental Africa (Appendix).

Order Ricinulei

The Ricinulei are commonly known as hooded tickspiders but are not true spiders (Fig. 11.1). This group is not very diverse, with ~75 extant species described from tropical Africa and the Neotropics (Harvey 2013c). One species, Ricinoides crassipalpe, is documented from Bioko Island, and it is also found in continental Africa (Appendix). It is unclear whether any representatives of this enigmatic order occur on the oceanic islands in the Gulf of Guinea.

Order Scorpiones

The Scorpiones, commonly known as scorpions, are easily recognized by their grasping pincers and curved, segmented tail with a stinger (Fig. 11.1). This group includes over 2500 described species that can be found in a variety of habitats around the globe (Fet et al. 2000). Because their exoskeletons contain fluorescent compounds, scorpions glow under ultraviolet light, facilitating detection at night. Although all scorpions produce venom, most species do not pose a risk to humans. Three species in two families are known from the Gulf of Guinea islands (Appendix), all of which are also found in continental Africa. The only species known from an oceanic island in the archipelago is Isometrus maculatus, which is introduced on São Tomé (see below).

Order Schizomida

The Schizomida, commonly known as short-tailed whipscorpions, superficially resemble true scorpions but have a short tail that lacks a stinger, and their first pair of legs is antenniform. This group includes over 230 described species that can be found in tropical and sub-tropical habitats worldwide, with a few species occupying temperate habitats (Harvey 2013d). Only one species is known from the Gulf of Guinea islands, Schizomus parvus, which is documented from both São Tomé and Bioko islands (Appendix).

Diversity, Endemism, and Introduced Species

Based on the current literature, araneids are the most species-rich arachnid group in the Gulf of Guinea. Araneidae are the most diverse spider lineage on many tropical islands (e.g., Caribbean: Crews et al. 2015; Crews and Yang 2016), though some studies of tropical island spider diversity have found higher numbers of salticid species (Caribbean: Crews et al. 2019; Southeast Asia: Ponce et al. 2021). Salticids and some araneids (e.g., Gasteracantha, nephilines) are diurnal, the latter often with large aerial webs that would be more obvious to a non-spider specialist, whereas many of the other families have species that are nocturnal or that do not build aerial webs. Thus, for the Gulf of Guinea, it is difficult to know whether the large number of salticid and araneid species reported is due to a collecting artifact (i.e., non-targeted collecting) or if it is representative of the true diversity. For instance, sub-tropical island surveys employing standardized collecting have reported the highest species diversity of small and cryptic linyphiid spiders, underscoring the limitations of extrapolating from opportunistic sampling efforts (Macaronesia: Malumbres-Olarte et al. 2016, 2020). Likewise, although crab spiders (Thomisidae) and lynx spiders (Oxyopidae) are diurnal, they are generally cryptic or ground-living and easily overlooked. Targeted collecting methods to comprehensively survey arachnid diversity include vegetation beating, leaf litter sifting combined with malaise or Winkler traps, visual night searching for nocturnal arachnids using white and ultraviolet light, turning rocks and logs, and pitfall traps.

Of particular note in the Gulf of Guinea archipelago is the high diversity of tetragnathid species, likely owing to their relatively good dispersal ability and the extremely high humidity and abundance of freshwater on the islands, and mygalomorph families (Barychelidae, Cyrtaucheniidae, Ischnothelidae, Migidae, Theraphosidae). A preliminary report on a targeted spider collection on Bioko identified 81 theridiid morphospecies, 45 salticid morphospecies, 39 araneid morphospecies, and 32 linyphiid morphospecies. This far exceeds the 8, 14, 25, and 5 species, respectively, that have been formally documented from the island (Griswold et al. 1999).

The total number of spider species on each island is somewhat unclear because the literature and WSC often provide the country or general region rather than a specific island. What we do know is that São Tomé and Príncipe together have 130 recorded species, and that 33 (33%) of the 101 species that have specific locality data are from Príncipe and 51 (50%) from São Tomé. Ten species are known from Annobón and 91 from Bioko. Of the 211 species documented from the archipelago, 113 also occur on the mainland. The remaining ~100 species may be endemic to one or more of the islands, but more comprehensive sampling of continental diversity is needed to confirm their endemic status.

Based on the available records, however, Príncipe has 20 endemic species in 13 families (17 genera), and 61% of the families and 82% of the genera are represented from singletons. São Tomé has 23 endemic species (only slightly more than Príncipe based on the depauperate data) in 9 families and 13 genera, and 69% of the families and 82% of the genera are singletons. Bioko has 27 endemic species in 16 families and 27 genera, with 69% of families and 93% of genera represented by singletons. Annobón has two endemic species recorded: Thoriosa taurina (Simon 1909) (Ctenidae) and Hogna furva cingulipes (Simon 1909) (Lycosidae).

We can also examine the arachnid fauna of each island for instances of multiple closely related endemic species, which may point to within-island species radiations and provide further evidence that the taxa are indeed island and/or archipelago endemics. The only (non-introduced) genera shared between São Tomé, Príncipe, and Bioko are Castianeira (Corinnidae) (São Tomé and Bioko), Mallinella (Zodariidae) (Príncipe and Bioko), and Tetragnatha and Leucauge (Tetragnathidae) (São Tomé, Príncipe and Bioko). Genera with multiple species on a single island occur in the salticids Maltecora (2 in Príncipe, 1 on São Tomé) and Belippo (3 on São Tomé), and the theraphosid Hysterocrates (3 on São Tomé, 1 on Bioko). A number of (likely) introduced species are also documented in the literature, many of them cosmopolitan or cosmotropical, and almost all are associated with human construction or agriculture (indicated in Appendix). Most of the introduced species were collected from multiple localities, and some mentions date back to at least the 1800s, indicating that the introductions are not recent.

What We Know of the Arachnid Fauna, and Likely Do Not

To gauge the level of scientific activity on the Gulf of Guinea arachnid taxa through time, we used publication data and taxonomic changes as a proxy (i.e., the number of years from original description that a species has been idle). Because spiders are the most speciose arachnid group present on the islands, our conclusions are drawn from spider-specific taxonomic data. The average number of years since a species had been studied was 83.4, with the most recent being within the past year (Araneus apricus (Karsch 1884), Peplometus biscutellatus (Simon 1887)) (Dippenaar-Schoeman et al. 2020; Wesołowska et al. 2020). The two species with the longest period of inactivity (137 years) were both described by Karsch (1884): Singa concinna (Araneidae), described from an immature specimen but still considered valid, and Philodromus morsus (Philodromidae). At least 55 (of 210) species are only known from the original description (26.2%), with 22 (10.5%) having been mentioned in publications from the past 10 years, 70 (33.3%) in the past 50 years, and 88 (41.9%) in more than 50 years. The majority of species (58%) are known from both sexes, which is surprisingly high and could be explained by some of the most prolific collectors having spent long periods of time on the islands. The remaining species (41.3%) are known only from female (31.9%), immature (1.4%), or male specimens (8.1%). These differences are likely because males are often only active for part of the year.

Although our knowledge is incomplete, there are some interesting emerging patterns for arachnid diversity on the Gulf of Guinea islands. For one, three orders of arachnids appear to be entirely missing from the fauna, though all are present on the mainland: Thelyphonida (whip scorpions), Palpigradi (micro whip scorpions), and Solifugae (wind scorpions). Two additional orders are apparently absent from the oceanic islands: Ricinulei (hooded tickspiders) and Scorpiones (aside from an introduced species). The absence of some of these groups is unexpected for islands of this size and age (scorpions, whip scorpions), while the absence of others may be attributable to a gap in collecting effort (e.g., microwhip scorpions).

A thorough understanding of arachnids on the Gulf of Guinea islands is still severely lacking. The majority of species descriptions were made in the first half of the twentieth century, there have been very few collections made by arachnologists, and none made using standardized methods of collection that would allow for a better assessment of the proportion of described versus undescribed fauna (Cardoso et al. 2009). Additionally, nearly all of the arachnid research to date has focused on alpha taxonomy and has not included the use of any modern tools or technologies to expedite the rate of discovery and description (i.e., molecular methods). Taxonomic training programs for local naturalists or students and partnerships with global experts would likely go a long way in closing this knowledge gap.