Sertularia arcta Dalyell, 1847: 224 –226, pl. 42, figs. 1–15.
Campanularia serpens Hassall, 1848: 2223; Hassall and Coppin, 1852: 163, pl. 21, fig. 4.
Capsularia serpens: Gray 1848: 151.
Coppinia mirabilis Hassall, 1848: 2223.
Reticularia immersa Thomson, 1853: 443, pl. 16 a, figs. 2–3.
Filellum serpens: Hincks 1868: 214 –215, pl. 41, figs. 4 a–b (synonym of Reticularia immersa Thomson, 1853); Fraser 1944: 215 – 216, pl. 44, figs. 198 a–d; Naumov 1960: 281, figs. 47, 170, 171; Naumov 1969: 303, figs. 47, 170, 171; Vervoort 1972: 49 – 50; Cornelius 1975: 378 –381, fig. 2; Peña Cantero et al. 1998: 302 –304.
? Filellum tubiforme Schydlowsky, 1902: 168, pl. 3, fig. 29.
Lafoea serpens: Vanhöffen 1910: 311.
Grammaria serpens: Vervoort 1946: 194 –196, fig. 82.
Filellum serpens serpens: Naumov 1969: 303, figs. 170–171.
? Filellum serpens tubiforme: Naumov 1969: 303, fig. 172.
Reticularia serpens: Rees and Thursfield 1965: 87 –88.
[More complete synonymy in Peña Cantero et al. 1998: 302–303]
Type series. Holotype—“Dublin, 1842, infertile colony on herbarium specimen of Abietinaria abietina (Linnaeus, 1758), coll. A.H. Hassall, 1973.10.8.4; mentioned Gray, 1848: 151 ” (Cornelius 1975: 379).
Type locality. From the holotype, Dublin, Ireland (Cornelius 1975: 379).
Description after literature data. (Trophosome after Peña Cantero & García Carrascosa 1995: 20, translated; coppinia after Hassall 1848: 2223): Colony with barely visible stolonal hydrorhiza. Hydrothecae arising from hydrorhiza; about half of hydrotheca adnate to substratum. Adnate portion of hydrothecae 0.26–0.28 mm long; free part 0.10–0.22 mm long. Hydrotheca bearing circular aperture, 0,08– 0.10 mm wide, rim even, slightly flared, without renovations. Coppinia—“Polypidom parasitic, massive, hirsute; polype cells elongated, tubular, often curved, arising at irregular distances (and generally at the angles of junction) out of a cellular basis, the apertures of the cells or spaces of which are often themselves covered in by a lid perforated by a small tubular orifice”.
Distribution. Eurybathic species, living as an epibiont mainly other species of hydroids (Peña Cantero et al. 1998). It has been recorded around the world, but records are mainly based on infertile material. According to records based on fertile material, the species seems to have a temperate-cold boreal distribution (cf. Peña Cantero et al. 1998).
Remarks. Following the general decision concerning the generic name Filellum after application of case by Cornelius and Calder (1986), the ICZN (Opinion 1485, ICZN 1988) also ruled in favor of the conservation of the specific name serpens Hassall, 1848, as published in the binomen Campanularia serpens Hassall, 1848, the type species of the genus Filellum.
The identity of the species Campanularia intertexta Couch, 1844 was questioned by Cornelius and Calder (1986), because the specimens analysed by Couch (1844) were growing on Lafoea dumosa and Sertularella polyzonias, the latter being a typical substrate for Filellum serpens. The authors consider that it is possible that the type material of Campanularia intertexta Couch, 1844, includes F. serpens, besides L. dumosa and Orthopyxis integra, as proposed by Cornelius (1982).
Hassall (1848) described the trophosome and gonosome of F. s e r p e n s as two different species, Campanularia serpens and Coppinia mirabilis, respectively. The first author who united them was Hincks (1868), and the name “coppiniae” since then refers to the peculiar reproductive structures found in some genera of the family Lafoeidae. Coppinia mirabilis Hassall, 1848 is considered a junior synonym of Sertularia arcta Dalyell, 1847, as evidenced by Hincks (1868) and reviewed by Cornelius and Calder (1986).
Reticularia immersa Thomson, 1853 is considered a synonym of Filellum serpens (Hassall, 1848) and its conspecificity was corroborated by Cornelius (1975) through the analyses of its type material.
Although data on the coppinia of the species is sketchy (e.g., “Coppinia of typical structure”, Naumov 1969: 303; “…coppiniae typical of the sub-family”, Cornelius 1975: 380) the trophosome of F. s e r p e n s may be differentiated from that of most species of the genus (viz. F. antarcticum, F. magnificum, F. nitidum, and F. serratum), but it is similar to those of F. adhaerens and F. disaggregatum, with more than half of the hydrothecae adnate to the substrate. However, the hydrothecae and hydrorhiza of F. adhaerens form a mesh laying on the substrate, which is also found in F. parasiticum.
Concerning the sexual structures, F. disaggregatum has a completely different coppinia, whereas F. serpens, F. parasiticum and F. adhaerens have similar coppiniae, with gonothecae having juxtaposed walls and a distal neck, and curved protective tubes arising among the gonothecae. However, there are some differences in the gonothecal morphology, polygonal in F. adhaerens (Nutting 1901: 178, pl. 21, fig. 4) and F. parasiticum, and fusiform in F. serpens (Dalyell 1847, pl. 42, fig. 6, also in Peña Cantero et al. 1998: 304).
The taxonomic status of F. s e r p e n s is dubious, because of the imprecise morphological descriptions associated with the species. It is likely that a group of different species may be confused and incorrectly assigned to the species under discussion. However, since F. serpens is the type species of the genus, it is by definition the senior synonym, although other congeners appear to be much more clearly described. Therefore, dismissing the name serpens would demand major nomenclatural acts at the genus level that we would prefer to avoid at this time.