Zanclea implexa Alder 1856

Zanclea implexa (Alder, 1856b)

Figs. 4 e, f, 6

Tubularia implexa Alder, 1856b: 439.

? Zanclea costata.— Trott 2004a: 271 [not Zanclea costata Gegenbaur, 1856].

Type locality. UK: England, Northumberland, North Sea, 30 miles east of Holy Island (Alder 1856b).

Material examined. NS: Sandy Cove, Digby Neck, 22.vii.1970, intertidal, on mussel shell (Mytilus edulis), one hydranth with a medusa bud, coll. K.W. Petersen, SNM HYD- 001058.— NS: Petit Passage, south of East Ferry, 14.x.1970, water’s edge at extreme low tide, on mussel shells (Mytilus edulis) and adhering algae, several colonies, without gonophores, coll. K.W. Petersen, SNM HYD- 000493.— Petit Passage, NS, south of East Ferry, datum + 6 cm, 14.x.1970, coll. K.W. Petersen, SNM HYD- 001050.

Description. Hydroid colonies stolonal, rarely with a pedicel having a single small branch, arising from a branched and anastomosing hydrorhiza overgrowing mussel shells and algae. Hydrorhizal stolons mostly creeping but occasionally detached from substrate to form a tangle of hydrorhizae and hydranth pedicels. Pedicels varying from short to very long for the genus, as much as 4 mm high, 0.2 mm wide, narrowest at insertion with hydrorhiza, usually widest at or near distal end, each bearing a terminal hydranth; perisarc moderately thin, straw-coloured, occurring in present specimens as a double tube, with a wavy inner layer and a less-wrinkled outer layer, constricted at pedicel base, terminating distally at base of hydranth. Hydranths elongate, slender, cylindrical to capitate, reaching 1.75 mm long, 0.4 mm wide; hypostome dome-shaped. Tentacles solid, capitate, slender, fairly short, as many as 50 or more, scattered over nearly all of hydranth and with a whorl of 5 to 7 around manubrium; acrosphere nearly spherical except for slightly flattened proximal side.

One developing gonophore observed, presumably a medusa bud, borne on a distinct pedicel, arising amongst tentacles on proximal half of hydranth.

Cnidome (Fig. 6)

Hydranths—

small stenoteles (n =10): 7.0–7.9 µm long × 4.8–5.2 µm wide (undischarged)

large stenoteles (n =10): 12.2–13.1 µm long × 10.3–11.1 µm wide (undischarged)

macrobasic euryteles (n =10): 21.0–22.1 µm long × 8.2–9.4 µm wide (undischarged)

Remarks. Alder (1856b) briefly described Zanclea implexa (as Tubularia implexa) from hydroids growing on an old anchor taken at a depth of 40 fathoms (73 m) off Holy Island, northeast England. No illustrations of it were provided until the following year (Alder 1857). Allman (1864) assigned the species to Zanclea Gegenbaur, 1856, and Hincks (1868) included an account of it in his book on British hydroid zoophytes. Detailed information also appeared in the monograph on gymnoblastic hydroids by Allman (1872: 223–226 and 290–292, as Gemmaria implexa). A contemporary synonymy of Z. implexa is given in Schuchert (2010), and the nomenclatural status of the genus name Zanclea has been discussed earlier (Calder 1988, 1992, 2013). According to Cornelius & Garfath (1980), syntype material of the species exists at the Hancock Museum (now the Great North Museum: Hancock), Newcastle-upon-Tyne, England. The life cycle of the species is still incompletely known, with the medusa stage not having yet been reared to maturity (Schuchert 2010).

Confusion over species limits in Zanclea deepened following studies by Russell and Rees (1936) on the life cycle of specimens identified as Z. implexa. They determined, correctly, that some of the characters employed earlier in distinguishing zancleid species and genera varied depending on the stage of development, and were of little or no taxonomic value. Several binomena that had been applied to both hydroid and medusa stages were thereupon referred by them to the synonymy of Z. implexa. Russell (1953), Kramp (1961), Brinckmann-Voss (1970), and others followed by including Z. implexa as a synonym of Z. costata Gegenbaur, 1856, and both of those names were assigned in turn to the synonymy of Z. alba (Meyen, 1834) by Calder (1988). Petersen (1990) observed that identification of species assigned to Zanclea was extremely difficult, and that their delimitation at the time was in a state of chaos. As the 20th century came to a close, several new or seldom-used characters were adopted to better distinguish zancleid taxa. Meanwhile, the species studied by Russell and Rees (1936) is now believed by Schuchert (2010) to have been Z. sessilis (Gosse, 1853) rather than Z. implexa.

Species diversity within the genus Zanclea had been greatly underestimated until the work of Gravili et al. (1996) and Boero et al. (2000) on adaptive radiation in zancleids from the Mediterranean Sea, Papua New Guinea, and California. Particular emphasis was given in their studies to differences in cnidome and to symbiotic relationships between zancleids and their substrates, particularly Bryozoa. The validity of several named species was confirmed, and eight new ones were described. More than 30 species of Zanclea are currently recognized as valid in the WoRMS (Schuchert 2017) list, with one of them being Z. implexa. In a recent review of the group by Schuchert (2010), Z. costata is regarded as a species of the Mediterranean, a resurrected Z. implexa is found in the North Sea, and Z. alba is a species of the warm North Atlantic and Gulf Stream, common on pelagic Sargassum.

Hydroids referable to Zanclea from the Bay of Fundy (SNM HYD-000493, SNM HYD-001058) have been assigned in this study to Z. implexa. Specimens were collected by K.W. Petersen at low tide in Petit Passage and Sandy Cove, NS. No way was found to differentiate them from North Sea specimens of the species as described by Schuchert (2010), and Z. implexa is therefore believed here to be amphi-Atlantic in distribution. As with the account of Z. implexa by Schuchert, specimens examined here from the Fundy region were distinguished by the following: (1) hydranth pedicels varied from short to long (up to 4 mm high) and were covered with perisarc; (2) hydranths were small (up to 1.75 mm long); (3) macrobasic eurytele nematocysts were present, with their capsules forming a dense ring around the hypostome; (4) no microbasic mastigophores were observed; (5) hydroids occurred on various substrates (both mussel shells and algae), but not on bryozoans. Also of note, pedicels consisted of two tubes, with an inner wavy layer connected to an outer smoother layer by “regularly disposed, radiating offsets” (Allman 1877: 224) of perisarc (Figs. 4 e, f).

Morphologically, the species is much like Z. giancarloi Boero, Bouillon & Gravili, 2000 from the Mediterranean Sea, but it apparently differs in having double-layered perisarc surrounding the pedicels. Zanclea giancarloi is also likely a species of warmer waters than the decidedly boreal Z. implexa. Molecular studies are needed to confirm whether Fundy specimens are indeed conspecific with Z. implexa, or whether they represent a new species.

Specimens reported as Zanclea costata by Trott (2004a) from Cobscook Bay, ME, in the Fundy region (see Appendix 1), were likely based on Z. implexa as well. His record was based on an entry in an unpublished checklist by Prof. Norman Meinkoth. As noted above, Z. costata is now thought to be restricted in distribution to the Mediterranean Sea. That species differs from Z. implex a in having larger hydranths (2–5 mm vs. <2 mm high), a different substrate (infaunal bivalves), and microbasic mastigophore nematocysts (Schuchert 2010).

Fraser (1944) included accounts of hydroids assigned to two species, Zanclea costata and Z. gemmosa McCrady, 1859, from the Atlantic coast of North America. All records were from south of Cape Cod, Massachusetts, and none is likely to have been based on the cold water Z. implexa. Reports of Z. costata from the region, a species regarded as endemic to the Mediterranean Sea (Schuchert (2010), likely constitute misidentifications. Meanwhile, the original description of Z. gemmosa by McCrady (1859) was based solely on medusae from Charleston Harbor, South Carolina. Given the absence of life cycle studies on that species, reports of hydroids identified as Z. gemmosa remain doubtful. At least some records of both Z. costata and Z. gemmosa from the western North Atlantic, especially those from pelagic Sargassum, were probably based instead on Z. alba. In light of studies on Zanclea from Europe, cited above, it is likely that diversity within the genus along the east coast of North America has been underestimated.

As noted above, the skeleton surrounding pedicels of specimens examined here consisted of a pair of perisarcal tubes of different diameter, described early on in the species by authors including Alder (1856b: 440; 1857: 108, pl. 9, figs. 5, 6), Wright (1859: 107–108), and Hincks (1868: 60, pl. 9, figs. 3a, b). Allman (1872) concluded that this was a misinterpretation, believing that the supposed inner layer was simply the outer wall of the coenosarcal tube. Russell & Rees (1936) confirmed the existence of two layers of perisarc in hydroids identified as Z. implexa (but regarded as Z. sessilis by Schuchert 2010), and suggested that the inner layer was formed during renovation of the hydranth as a new tube developed around it within the old tube. This explanation accords at least in part with observations made here, with the inner tube often extending beyond the orifice of the outer and older tube to reach the base of the hydranth (Fig. 4 e).

Macrobasic euryteles in present material were scattered throughout the hydranth body and in the coenosarcal tubes, but were especially concentrated in a band around the hypostome. A hypostomal band of euryteles was also noted in hydroids of Z. implexa from Europe by Schuchert (2010).

Recorded distribution. Bay of Fundy: recorded for the first time.

Eastern North America: recorded for the first time.

Worldwide: Northwest Europe, from Norway to northeastern England (Schuchert 2010); Atlantic Canada (this study).