Introduction

Sea anemones of the genus Metridium de Blainville, 1824, are common and abundant in habitats accessed by humans, including docks and piers, shipwrecks, and rocky habitats in the intertidal and subtidal. Their relatively large body size and their fringe of many short tentacles on an undulating oral disc make them both conspicuous and distinctive (Fig. 1). The traits that makes them recognizable in their native habitat have been an advantage in tracking the occurrence of Metridium in new habitats (Riemann-Zürneck 1975, 1986; Griffiths et al. 1996; Häussermann and Försterra 2005; Carlton 2011; Martin et al. 2015; Laird and Griffiths 2016; Glon et al. 2020; Teng et al. 2021; Yan et al. 2021; Häussermann et al. 2022; Molinet et al. 2023; Gimenez et al. 2023).

Fig. 1
figure 1

A clonal group of Metridium senile in the public aquarium, Newport, OR. Note the thin, undulating oral disc of the largest specimen and the numerous, strongly tapering tentacles

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The current synthesis of taxonomy for Metridium identifies five species within the genus (Rodríguez et al. 2023): Metridium canum Stuckey, 1914; Metridium dianthus (Ellis, 1767); Metridium exile Hand, 1955; Metridium farcimen (Brandt, 1835); and Metridium senile (Linnaeus, 1761). Two species previously listed within the genus (see Fautin 2016), Metridium huanghaiense Pei, 1998 and Metridium sinensis Pei, 1998, have been synonymized with M. senile (see Li and Xu 2020 and below). All species of Metridium have a smooth, muscular column and a wide oral disc fringed with numerous, short tentacles. Within species, there may be diversity in color, tentacle length, and polyp size (Hand 1955; Fautin et al. 1989). Differences among species include overall body size and reproductive biology, but high intraspecific variation and plasticity causes confusion about species boundaries.

Confusion in the systematics of Metridium is remarkably long standing and durable. These problems have two components. One is identity: how many species are there, and what are the boundaries and diagnoses of those species? The second is nomenclature. Because of the taxonomic confusion and geographic range of this group of anemones, names have proliferated. A genome-scale phylogeny for Metridium (Glon et al. 2023) has clarified circumscription of the lineages within the genus, finding two well-supported lineages (Fig. 2). While convincingly answering the question of identity for the Atlantic and Eastern Pacific representatives of Metridium, this does not resolve the question of nomenclature, or remedy the long-standing issues surrounding usage and circumscription. Here, we explicate the rationale for considering the valid species of Metridium to be M. senile and M. farcimen, taking a historical and geographic approach in discussing the taxonomy of the group. Our goal is to acknowledge and address the evidence and context for prior taxonomies and to make clear how these relate to our current understanding of diversity in the genus. We highlight key studies that use Metridium and connect these to the systematic framework to support the application and interpretation of those works.

Fig. 2
figure 2

Simplified phylogenetic tree for Metridium, based on target enrichment sites from across the genome (see Glon et al. 2023 for original trees and methods). For Metridium senile, names at the end of branches refer to clade names from Glon et al. 2023 and refer broadly to the geographic location from which the samples were collected

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How many species are there in Metridium?

Because members of Metridium show high variation in size and color and lack discrete diagnostic features, genetics were hailed for their potential for evaluating diversity within this group. Following Hoffmann’s (1976, 1981) pioneering work on genetics of Northeastern Atlantic populations of M. senile, Bucklin and Hedgecock (1982) evaluated differences between large-bodied forms from the Pacific, smaller bodied forms from the Pacific, and smaller bodied forms from the Northwest Atlantic and recommended recognizing those as three species. Bucklin (1985) followed that work with a closer look at the smaller bodied, clonal forms in the Atlantic. The enzyme loci she used failed to distinguish between populations in the British Isles that were identified as M. senile dianthus and M. s. pallidum but were able to distinguish populations from the Eastern (British Isles) and Western (MA, USA) North Atlantic (Bucklin 1985).

Contemporary sequence–based analyses of genomic data (Glon et al. 2021, 2023) detect a distinct population in the Northwest Pacific, consistent with the finding of Bucklin and Hedgecock (1982). Contrary to early allozyme studies, genomic analyses find a single, wide-spread population in the North Atlantic, with shared haplotypes between the Eastern (Ireland, Norway) and Western (New Brunswick, Canada, and ME, USA) North Atlantic (Fig. 2). These findings are robust in terms of method and type of genomic data and affirm the conclusion of McMurrich (1911) and Stephenson (1935), who argued for a single species in the Atlantic. Local differences within Atlantic populations noted by Bucklin (1985) and Hoffmann (1976) are not incompatible with the broader interpretation of gene flow and connectivity between Atlantic populations, suggesting some local differentiation, either through sexual or asexual means.

More unexpectedly, genome-scale genetic analysis also finds that the Atlantic populations of Metridium are not segregated from some of the Pacific populations of Metridium. Aquaculture practices may explain the connection between, e.g., French and Japanese populations, since oyster farming in France in the mid-twentieth century included culture material from Japan (Grizel and Heral 1991). Less recent exchanges through the Arctic are also possible (see Glon et al. 2023). Although Atlantic populations are not genetically segregated from all Pacific populations, some Pacific populations are distinct from the mixed Atlantic and Pacific population. Members in the mixed Pacific and Atlantic group are highly variable in size and color and often occur in aggregations of multiple individuals. Individuals in the distinct Pacific group are typically larger bodied and solitary.

The large, solitary Metridium

The distinctiveness of the large-bodied species within Metridium has been recognized since at least 1934, when Carlgren (1934) noted that a large specimen from Alaska of what was called Metridium fimbriatum Verrill, 1865 differed in the size and relative occurrence of cnidae, compared to M. dianthus from Europe. Carlgren (1934) mentioned that these differences were not consistent across specimens, which he interpreted as a difference in development, with specimens of M. dianthus losing some types of cnidae as they matured.

Using electrophoretic data, Bucklin and Hedgecock (1982) distinguished large bodied, solitary individuals within Pacific populations of what they called M. senile, and subsequently described these large-bodied forms as M. giganteum (Fautin, Bucklin, & Hand, 1989). The new name was justified by the imprecision of the usage of the name M. fimbriatum, which Verrill (1865) erected for Metridium in the Northeast Pacific but which had not applied exclusively to large and solitary polyps. Nomenclatural and archival research by Fautin subsequently uncovered older names for the large, solitary polyps: Tilesius (1809), following an epithet used by locals in Kamchatka, called large polyps “Actinia priapus” and Brandt (1835) described large, solitary polyps from Kamchatka as Actinia Polystephanus farcimen, but neither of those authors provided illustrations and type specimens were neither mentioned in the original description nor subsequently found (see Fautin and Hand 2000; Fautin 2016). The connection between Actinia priapus Tilesius 1809 and Actinia farcimen of Brandt (1835) was suggested by McMurrich, although he was uncertain (McMurrich 1901: 13 “may also be a synonym”). There is greater certainty for the description given by Brandt (1835), which can only be this species, given the large size, smooth column, and numerous tentacles, leading Fautin and Hand (2000) to synonymize M. giganteum with Actinia farcimen, calling the species Metridium farcimen (Brandt, 1835). They also include specimens originally described as Isometridium rickettsi Carlgren 1951 within M. farcimen (see Fautin and Hand 2000).

Undisputedly, the largest bodied species in the genus, M. farcimen reaches heights of up to a meter, and is distributed from at least the tip of Kamchatka, Russia, through the Aleutian Islands to the Gulf of California, occurring at depths of greater depth at lower latitudes (Fautin and Hand 2000). Type specimens are unknown for M. farcimen but have been deposited for its junior synonyms Metridium giganteum Fautin, Bucklin & Hand, 1990 and Isometridium rickettsi (see Fautin and Hand 2000).

The smaller, often clonal Metridium

The very broad range and phenotypic variability of the smaller bodied, often clonal species within Metridium has led to confusion about its circumscription and to an abundance of names. At present, the clonally reproducing members of Metridium in North America and Europe are generally called Metridium senile or Metridium dianthus. The pattern of usage is difficult to parse, but in general, Metridium senile is applied to populations in North America and to smaller and shallower populations in Europe. Metridium dianthus is used primarily for European populations, and typically applied to larger animals with more numerous tentacles found in deeper water. The European names seem to align with what Bucklin (1985) and Stephenson (1935) before her referred to as varieties dianthus (larger body, more numerous tentacles) and pallidum (smaller body, fewer tentacles, typically shallower), respectively, but the distinctions are not clear, and usage is inconsistent (Manuel 1988). Genetic analysis does not find distinction between the Eastern and Western Atlantic populations, or between these and the smaller-bodied form of Metridium in the Pacific.

The history of names and their use is complex and warrants thorough discussion to eliminate the confusion that has hampered assessment of diversity in the genus. Andres (1883: 139) encapsulates the problem very well (translation ours): “the synonymy is complicated because the species was described independently by various authors and because the compilers [of biodiversity and taxonomy] gradually accepted all names without noticing their identical characters.”

We recognize Metridium senile (Linnaeus, 1761) as the appropriate valid name for the single species of Metridium in the North Atlantic and for some populations of Metridium from the Pacific. The epithet “senile” is the oldest species epithet for this taxon (Daly et al. 2007), in continuous use since its description, with the combination Metridium senile first published by Haddon (1889: 323). Haddon’s (1889) use of this combination is made within the discussion of Paraphellia expansa Haddon, 1886, rather than as part of an entry focused on Metridium and has not been previously credited in key treatments of this species (e.g., Stephenson 1935; Fautin 2016). Although it is the junior subjective synonym of M. senile, Metridium dianthus remains the type species of Metridium by monotypy (Dunn and Hulseman 1979; Fautin 2016, and references therein).

Priapus senilis was described by Linnaeus (1761) from Sweden and was included in the twelfth edition of Systema Naturae as Actinia senilis, reflecting Linnaeus’s creation of the genus Actinia for what are now recognized as actiniarians (Linnaeus 1767). Like many early descriptions, neither of Linnaeus’ (1761, 1767) written treatments contained much diagnostic power. Contemporary with this generic re-assignment by Linnaeus (1767), Ellis described Actinia dianthus Ellis, 1768 with a written description that is brief but accompanied by a diagram that shows an anemone with a stout column and a strongly lobed disc fringed with numerous, short tentacles (Fig. 3a). Notably (and contra the locality information given in Fautin 2016), the name Actinia dianthus is used explicitly in reference to animals found in Sussex (Ellis 1768: 436), although the manuscript in which it is described refers in its title to “newly ceded islands” and includes accounts of diversity in the West Indies. Perhaps because the image so effectively captured the distinctive features of this anemone, the name Actinia dianthus was quickly adopted and widely used for the European member of Metridium, albeit under multiple generic assignments (Ellis and Solander 1786; Oken 1815; de Blainville 1830; Johnston 1838; Couch 1844; Dalyell 1848; Gosse 1855, 1860; Tugwell 1856; Milne Edwards 1857; Stephenson Wright 1859; Andres 1883; Pennington 1885; Carlgren 1893, 1934; Nordgaard 1905; Walton 1908; Nafilyan 1912; Gemmill 1920; Elmhirst 1925).

Fig. 3
figure 3

Key depictions of Metridium senile in historical literature. a Drawing included by Ellis (1768) in his description of Actinia dianthus; b Drawing of Actinia dianthus included by Rapp (1829) in his monograph of sea anemones; c Drawing of Actinia dianthus included in Baster (1759). This figure was referenced by Linnaeus (1761) in his re-classification of Priapus senilis as Actinia senile. All images provided through Biodiversity Heritage Library

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An additional name used for this species comes from a mention by Müller (1776) of “Actinia plumosa” in his monograph of Danish and Norwegian animals. This was interpreted as a proposal of a new name, Actinia plumosa Muller, 1776 by some workers (e.g., Gmelin 1788; Rapp 1829; de Blainville 1830; Ehrenberg 1834; see Fig. 3b). However, because “Actinia” was also used as a common name for sea anemones, it is plausible that Müller (1776) was using this phrase descriptively to mean “the plumose anemone,” a common name that remains applied to Metridium senile and an interpretation underscored by the absence of a description or diagnosis along with the name. de Blainville (1824: 470) used the phrase in this way, noting that the M. dianthus of Oken (1815) was “l’actinia plumosa” of Linnaeus, with use of an article and the lack of capitalization differentiating the common French name “l’actinia” from the genus “Actinia.” Gmelin (1788) treated Actinia plumosa as a name and considered it a synonym of Actinia senilis, citing “Syst nat XII. 2, p. 1088 n.3” and the Latin description Linnaeus gave in that reference under his entry for Actinia plumosa; despite making this connection, Gmelin elected to use the junior epithet “plumosa” rather than the senior “senilis.” Despite these early citations of the synonymy between A. plumosa and P. senile and the association of these two with Actinia dianthus by de Blainville (1824), after the mid-nineteenth century, most accounts listed this species as Metridium dianthus, (e.g., Milne Edwards 1857; Wright 1859; Hertwig 1882; Andres 1883; Pennington 1885; Carlgren 1893, 1933; Dixon and Dixon 1891; Duerden 1898; Nordgaard 1905; Walton 1908; Nafilyan 1912; Gemmill 1920; Gravier 1920; Pax 1920, 1928; Stephenson 1920; Verrill 1922; Elmhirst 1925; Stephenson 1925).

The precedence of A. dianthus over P. senilis was not universally recognized. Adams (1800) attributed specimens from Wales to Actinia senilis, noting the “soft hairy appearance arising from the slender form and number of tentacles.” Although Adams’ (1800) concept aligns with that figured by Ellis (1768) and is from the British Isles rather than Scandinavia, he applied Linnaeus’ name P. senilis to the specimens rather than Ellis’ (1768) name A. dianthus. Andres (1883: 138) indicated that the Actinia senilis of Adams (1800) and of Pennant (1812) was a synonym of Actinia dianthus, but also indicated that he did not consider these wholly identical with Linneaus’ (1761) concept of P. senilis. Haddon (1889: 323), in a discussion of problems with Sagartia Gosse, 1860, noted that the appropriate name for Actinia dianthus (at that time called Actinoloba dianthus) was Metridium senile. McMurrich (1911, 1914) explicitly synonymized Actinia dianthus with Priapus senile because populations identified under those names overlap in size, color, and internal anatomy. He further argued that, despite the imprecision of Linnaeus’ (1761) original description, subsequent treatment by Linnaeus and others makes the name P. senilis appliable, which reinforced Haddon’s (1889) argument that P. senile has priority over A. dianthus (see McMurrich 1911). Even Verrill’s (1914) argument for use of the epithet dianthus over senilis acknowledged identity between the concepts of the names and the priority of senilis; Verrill preferred dianthus on the basis of quality of the description and the “obscenity” of the Linnaean name, not precision or priority. Despite McMurrich’s (1914) reiteration of the importance of priority and the clarity of intent with this specific Linnean name, given its connection to local vernacular names and the citation by Linnaeus of images from e.g., Baster (1759: Tab XII, see Fig. 3c) that provide diagnostic features, prominent experts like Verrill (1922), Pax (1922, 1928), and Carlgren (1928, 1933), continued to use Metridium dianthus over Metridium senile. This promoted confusion because it implied that there were differences between M. dianthus and M. senile.

The confusion over names was intensified by the discovery of similar animals in the Northwestern Atlantic. Le Sueur (1817) proposed the name A. marginata Le Sueur, 1817, for specimens from Boston Bay with a wide, lobed oral disc and numerous short tentacles. This name was used a handful of times (e.g., Milne Edwards 1857; Verrill 1864; Parker 1900), but not exclusively: Couthouy (1839) identified specimens from Massachusetts under the European names A. plumosa and A. senilis and Stimpson (1853) reported A. dianthus and A. marginata from Grand Manan, citing A. dianthus as a questionable identification and giving Johnston, 1838, as its authority (Johnston cites Ellis as authority for the species). Andres (1883) synonymized Metridium marginata with M. dianthus in his catalog of Actiniaria without much explanation, but McMurrich (1901) elaborated on this, noting that the anatomical attributes of M. dianthus reported by Carlgren (1893) from the Eastern North Atlantic were identical to what he observed in Metridium from the Western North Atlantic.

Stephenson (1935) synthesized life history, anatomy, and cnidae from various European populations and found no consistent anatomical or life history features to differentiate them, concurring with McMurrich (1901, 1911), and citing M. dianthus and M. marginatum among the junior synonyms of M. senile. To account for the extensive variation in this species, Stephenson (1935) recognized varieties, distinguishing variety M. senile dianthus as the larger-bodied forms and variety as M. senile pallidum the smaller bodied forms but noting that there are no easily quantifiable features differentiating these and that “it is extremely difficult to say where pallidum ends and dianthus begins” (Stephenson 1935: 221).

In his revision, Stephenson listed five confirmed synonyms and four questionable synonyms (Table 1), with the ambiguity lack of detail in the descriptions of those taxa. The otherwise comprehensive list provided by Stephenson (1935) includes a few errors and points for clarification. Stephenson (1935: 214) lists McMurrich (1911) as the first use of the combination Metridium senile, omitting the earlier combination by Haddon (1889). The name Actinia pallida, created by Holdsworth (1855), has been interpreted as M. senile (see Stephenson 1935), but the description is broad enough to include other species (e.g., Diadumene cincta). Stephenson (1935) cites Dalyell (1848) as coining the name Actinia dianthus-plumosa, but this seems to be a misinterpretation by Stephenson of the formatting convention of that section of Dalyell’s book: although joined by a dash in the header, the epithets are treated as distinct names in the text, rather than as a single, hyphenated epithet. Two of the names recognized as synonyms of M. senile by Stephenson (1935), Actinia aurantiaca Jordan, 1855 and Actinia pellucida Alder, 1858, are homonyms of Actinia aurantiaca Delle Chiaje, 1825 (now Condylactis aurantiaca) and Actinia pellucida Hollard, 1848 (now Aiptasiogeton hyalinus (Delle Chiaje, 1822) (see Fautin 2016).

Table 1 Nominal species that are in full, partial, or questionable synonymy with Metridium senile. Species are listed in order of their original description
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Stephenson’s (1935) discussion of anatomy and variation emphasizes North Atlantic populations, but he includes Metridium fimbriatum Verrill, 1865 among the junior synonyms of M. senile. Verrill (1865) described Metridium fimbriatum from San Francisco, noting its high similarity to M. marginatum of the Northwest Atlantic. Verrill claimed that M. fimbriatum had more numerous and slender tentacles than M. marginatum but provided no specific information about tentacle number or length. In his description of M. fimbriatum, Verrill (1865) hypothesized that M. marginatum, M. fimbriatum, and M. dianthus will “eventually be found to belong to one variable and widely diffused species.” Torrey (1898) acknowledged that the primary difference between these was geographic. McMurrich (1901) explicitly regarded M. fimbriatum as a synonym of the Atlantic species M. marginatum and M. dianthus, noting that the broad range of variation in color and size within each of the recognized species overlapped, and that anatomical differences related largely to size and irregularities of asexual reproduction. Torrey’s (1902) landmark treatment of variation in Metridium underscores McMurrich’s (1901) conclusion.

Although Metridium fimbriatum was initially used for small, bodied, clonal forms, it came to apply also to the large-bodied polyps from Alaska and the Pacific Northwest of the USA and Canada. McMurrrich (1901: 13) was equivocal on the connection between smaller bodied, highly variable forms and the larger bodied forms described by Tilesius (1809). Carlgren (1933) identified differences that corresponded to size and geography among M. fimbriatum, presaging the differentiation between M. farcimen and M. senile.

Hand (1955) re-evaluated the variation among Pacific populations of Metridium and compared these explicitly to samples and accounts of Atlantic populations. Based on anatomy and cnidom, Hand (1955) recognized two forms in the Pacific. He did not recognize M. senile as an appropriate name for small bodied Metridium occurring in the eastern Pacific. Hand (1955) called the large bodied, generally northern form Metridium senile fimbriatum, noting that this combination was suggested in a footnote by Carlgren (1934) and used by Uchida (1938) for a population in Japan, but failing to acknowledge that the name was created for small-bodied forms. Hand described Metridium exilis Hand, 1955 (correctly rendered as M. exile: see Fautin 2016), for small-bodied specimens from the intertidal of open coast and bays of California. Contra the synonymies by McMurrich (1901) and Stephenson (1935), Hand recommended that the name M. senile be reserved for Atlantic populations, citing the perceived absence of catch tentacles in populations in the Pacific as a distinguishing feature; Hand (1955) was incorrect in his interpretation of these as absent in the smaller bodied, asexually reproducing Metridium of the Pacific (see Purcell 1977) or in the Metridium of the North Atlantic (see Williams 1975).

Despite Hand’s (1955) recommendation that the small-bodied, clonal Metridium of the Northwestern Pacific be identified as M. exile rather than M. senile, most authors continued to use the name M. senile (Purcell 1977; Sebens 1981; Walsh 1981; Walsh and Somero 1981; Bucklin 1982; Greene and Schoener 1982; Purcell and Kitting 1982; Chadwick 1987; Francis 1988; Harris 1991; Lissner et al. 1991; Parker and Tunicliffe 1994; Masuda and Stone 2003; Kramer and Francis 2004; Goddard and Love 2010; Nelson and Craig 2011; but see Kostina 1988; Harris 1991). The lack of use in reference to specimens or living populations and a proposal of synonymy by Riemann-Zürneck (1975) notwithstanding, M. exile was treated as a valid name (e.g., Östman et al. 2010; den Hartog and Ates 2011). Allozyme (Bucklin and Hedgecock 1982), and life history (Bucklin 1980, 1987) data were interpreted to support distinction between the Atlantic M. senile and the Pacific M. exile, but this has not been replicated with sequence-based genetic analyses (see Glon et al. 2021, 2023) or any detailed and repeatable analyses of behavior or anatomy. We reject Hand’s (1955) distinction between Atlantic and Pacific populations of the smaller-bodied, asexually reproducing Metridium and affirm Stephenson’s (1935) contention that both Atlantic and Pacific populations belong to Metridium senile (Linnaeus, 1761). This makes M. exile a junior subjective synonym of M. senile, as initially proposed by Riemann-Zürneck (1975).

Two additional names have been applied to populations of Metridium in the Pacific Ocean. Metridium huanghaiense Pei, 1998 (originally huanghaiensis: see Fautin 2016) and Metridium sinense (previously rendered as sinensis) were first described in Pei (1998) from the Yellow Sea. Although a key accompanies the description that purports to differentiate these species from M. senile fimbriatum, the text of the description does not distinguish them (Fautin and Hand 2000) and does not provide comparable or descriptive information about the new species, describing M. sinense as not undergoing asexual reproduction and M. huanghaiense as having an opaque body wall and tentacles that are not in a ruffle. The citation of Verrill (1865) and Carlgren (1934) as the references for M. senile fimbriatum furthers the confusion: Verrill’s (1865) description was clearly what is now M. senile, as it is small-bodied, clonal, and abundant in the midcoast of California, whereas Carlgren (1934) applied the name to animals now considered M. farcimen. The key in Pei (1998) cites the body of M. senile fimbriatum as large but also allows for asexual reproduction, a trait further elaborated upon in the text (Pei 1998: 134), which suggests that Verrill’s (1865) sense of the taxon is intended, rather than Carlgren’s (1934). Based on an examination of type and other specimens from Pei’s (1998) descriptions, Li (2013) and Li and Xu (2020) considered M. huanghuaense and M. sinense both junior synonyms of Metridium senile.

Sea anemones described as similar to M. senile have been documented from South America under two names. Riemann-Zürneck (1975) identified a population of large-bodied animals with an undulating oral disc and numerous short tentacles as Metridium senile lobatum (Carlgren, 1899). Zamponi (1978) erected a new genus and species, Paraisometridium pehuensis Zamponi, 1978 (correctly rendered as P. pehuense: see Fautin 2016) for a population of anemones from Pehuenco, Buenos Aires, Argentina. Zamponi’s (1978) description underscored the similarity of P. pehuense with M. senile and assigned the new genus to Metriididae. Subsequently, populations inferred to represent introduced M. senile have been documented in Argentina (reviewed in Gimenez et al. 2023), Chile (reviewed in Häussermann and Försterra 2005; Häussermann et al. 2022), and South Africa (Griffiths et al. 1996). Recognizing the anatomical similarity between P. pehuense and M. senile and the potential of M. senile to be introduced, den Hartog and Ates (2011) synonymized P. pehuense with M. senile; Gimenez et al. (2023) concurred with this interpretation, but noted that the type material for P. pehuense is poorly preserved making the synonymy somewhat difficult to affirm. Likewise, based on anatomical study, ecological modeling, and biogeographic evidence, Häussermann et al. (2022) and Gimenez et al. (2023) argue that Riemann-Zürneck’s “M. senile lobatum” are more appropriately identified as M. senile and likely represent an introduction to South America, an interpretation that seems to align with Riemann-Zürneck’s interpretation of the material (see e.g., Riemann-Zürneck 1986). However, Fautin and colleagues (1989) argued that M. senile lobatum was a synonym of Actinothoe lobata (Carlgren, 1899). Genetic evaluation of populations of putative M. senile from Argentina and Chile confirmed identity of these with Northern Hemisphere populations (see Gimenez et al. 2023; Glon et al. 2023). At present, P. pehuense seems appropriately categorized as a junior synonym of M. senile, and M. senile lobatum is a pro parte synonym of both A. lobata and of M. senile. Records and accounts of Actinothoe lobata likely encompass some records more appropriately designated M. senile, but disentangling these and revising that species is outside of the scope of the present work.

The concept of Metridium senile proposed here encompasses populations that span the temperate North Atlantic and Pacific Oceans. The original description of Priapus senile was based on specimens from the Baltic Sea but did not designate a type specimen; type material also is not known for Actinia dianthus (see Fautin 2016). Based on records from the literature and museum specimens, we consider M. senile to be generally sub-Arctic, but populations may occur at or near the Arctic Circle (Rodríguez et al. 2023). Populations in the Southern Hemisphere are inferred to be anthropogenically introduced, but cold-water connections via the Pacific Ocean are possible (Glon et al. 2021, 2023). In temperate regions, M. senile ranges from the intertidal to approximately 200 m; at lower latitudes, they may occur only subtidally. Metridium senile is highly variable in color and size, with the largest specimens approximating M. farcimen in size and the smallest being orders of magnitude smaller. Generally capable of asexual reproduction, clonal aggregations of M. senile may dominate in suitable habitat.

A dubious species of Metridium

Metridium canum is a composite species for which the identity of the constituent species is unclear. Hand (1961) re-described specimens of M. canum from Otago Harbor (New Zealand) as Mimetridium cryptum Hand, 1961, a member of family Actinophoridae Carlgren 1938. Hand (1961: 83) equivocated on whether the Otago specimens were the same as the Kermadec (now Rangitāhua) Island material originally described as M. canum, given the differences in habitat between temperate Otago and subtropical Rangitāhua, difference in reported size (Otago material is at least an order of magnitude larger in body size), and differences in natural history (under stones in Rangitāhua vs. in sediment in Otago Harbor). He left it as a partial synonymy, with the detailed accounts by Parry (1951) and Batham (1956) of Metridium canum interpreted to be Mi. cryptum (see Hand 1961; Batham 1965). Thus, the sole report of specimens of M. canum other than those attributable to Mi. cryptum is the original description by Stuckey (1914), which was not accompanied by the deposition of specimens, included no figure of the material observed, and failed to describe attributes that would substantiate its inclusion in Metridium. The appropriateness of the genus designation of M. canum was questioned by Stephenson (1920: 528), who questioned whether it might be a juvenile, or misattributed to the genus. The small size and location reported for M. canum in Rangitāhua is out of line with what is known of Metridium, whose members are generally larger than 1 cm in diameter as adults, and which are temperate rather than subtropical. The animals described by Stuckey (1914) have approximately two times as many tentacles as mesenteries: with 200 tentacles, the smaller than 1 cm diameter column would bear five cycles of mesenteries; given that Stuckey (1914) reports 45 pairs mesenteries (inferred to be four cycles, with the last incomplete) the final cycle is likely to be microcnemic. These attributes are consistent with multiple taxa that occur in the region, including members of Diadumenidae Stephenson, 1920, Haliactinidae Carlgren, 1949, and Sagartiidae Gosse 1858 in addition to Metridium. The information provided by Stuckey (1914) is insufficient to identify new specimens or affiliate his description with a described taxon. Thus, we consider Metridium canum a nomen dubium.

Differentiating species of Metridium in the field

Although our concept for M. senile simplifies identification of the smaller bodied form, difficulty remains in differentiating a large specimen of M. senile from a member of M. farcimen. Their ranges overlap in the Northwestern Pacific Ocean, and, for specimens of comparable size, no diagnostic anatomical differences have been reported. Although length of nematocysts capsules differentiate M. farcimen from M. senile, the range of capsule sizes overlap and only the size of capsules in the acontia are significantly different in animals of equivalent size (Kramer and Francis 2004), with ecological differences (flow regime, depth: see Anthony 1997) important in the attainment of very large size in M. senile. Genetic differences are likewise not absolute, with some individuals of apparent hybrid origin being detected through genome sequencing (see Glon et al. 2023). In the shared range of M. farcimen and M. senile, large animals can only be identified definitively through microscopic or genetic analysis.

Family-level taxonomic implications

Three genera have been assigned to family Metrididae: Metridium de Blainville, 1824, Isometridium Carlgren, 1951, and Parisometridium Zamponi, 1978. Both Isometridium and Paraisometridium are monotypic and are now recognized to be junior synonyms of Metridium: Isometridium ricketsi is a junior subjective synonym of M. farcimen, and P. pehuense is a junior subjective synonym of M. senile. Thus, in our view, Metrididae has only one valid, accepted genus, Metridium, and two valid accepted species, Metridium senile (Linnaeus, 1761) and Metridium farcimen (Brandt, 1835).