Rhizogeton fusiformis L. Agassiz 1862

Rhizogeton fusiformis L. Agassiz, 1862

Figs. 7 c–e, 9

Rhizogeton fusiformis L. Agassiz, 1862: 224, pl. 20, figs. 17–23.— Linkletter et al. 1977: 6.— Calder, 2010: 13.

Type locality. USA: Massachusetts, Nahant, in tide pools (L. Agassiz 1862: 224).

Material examined. NB: St. Andrews, Biological Station wharf, pontoon, 10.vii.1972, <1 m, one colony, with male gonophores, coll. A. Brinckmann-Voss, ARC 8650057.—NB: Richardson, Deer Island, 44°59’42”N, 66°56’45”W, wharf, pontoon slip, just below surface, on Mytilus edulis, 19.xii.1988, 5°C, 34‰, one colony, without gonophores, coll. D. Calder, ROMIZ B1022.—NB: Stuart Town, Deer Island, 45°01’07”N, 66°56’16”W, <1 m, on Ascophyllum nodosum, 24.v.1999, 34‰, 9° C, one colony, without gonophores, coll. D. Calder, ROMIZ B3091.—NB: Letete, Tucker’s Brook, 45°02’52”N, 66°53’36”W, intertidal, in brook, on Ascophyllum nodosum, 24.v.1999, 23‰, 12° C, two colonies with female gonophores, coll. D. Calder, ROMIZ B3101.—NB: Letete, Tucker’s Brook, 45°02’52”N, 66°53’36”W, intertidal, in brook, on Ascophyllum nodosum, 30.v.1999, 23‰, 16° C, two colonies, with male gonophores, coll. D. Calder, ROMIZ B3102.

Description. Athecate hydroids forming stolonal, monomorphic colonies, arising from a creeping and often reticular hydrorhiza. Stolons of hydrorhiza covered by thin perisarc, sometimes forming a complex network of tubes. Hydranths sessile, elongate, club-shaped to cylindrical to tapering from end to end, up to about 1 cm high, naked except for a short collar of thin perisarc at proximal end, while sometimes widely separated, under optimal conditions forming dense aggregations of polyps up to several cm long; tentacles as many as 10-30 or more, filiform, tapering from broader base to slender tip, appearing scattered but tending to occur in several irregular whorls over distal half to 2/3 of hydranth, with those of one whorl alternating with those of adjacent whorl or whorls, distal tentacles longer than proximal ones; hypostome elongate, dome-shaped. Hydrorhizal spines absent; nematophores absent. Colour orange to ochre-red.

Gonophores fixed sporosacs, arising on peduncles of varying length from hydrorhizal stolons. Male gonophores initially almost round, on short peduncle, becoming elongate-oval to fusiform with longer peduncle, about 1 mm high, entire structure covered with thin, filmy perisarc; white to deep orange in colour. Female gonophores spherical, on short peduncle, later becoming sac-shaped and borne on a long peduncle; perisarc thin, filmy, eventually shed over distal end to expose developing embryos on spadix.

Cnidome (Fig. 9)

desmonemes (n = 10): 5.0–6.3 µm long × 3.2–3.8 µm wide (undischarged)

microbasic euryteles (n = 10): 7.9–8.2 µm long × 2.9–3.3 µm wide (undischarged)

Remarks. Rhizogeton fusiformis L. Agassiz, 1862 was originally described from hydroids growing on rocks and stones in intertidal pools at Nahant, Massachusetts (L. Agassiz 1862). Type specimens (MCZ 52) from that location were collected during the month of July (A. Agassiz 1865). Only two other reports provide original observations on the species in the western North Atlantic. Notes and an illustration of the trophosome of R. fusiformis were provided in a widely overlooked work by H.J. Clark (1865b). He also complemented a description of male gonophores of the species by L. Agassiz (1862) with the first account of female gonophores and their development (H.J. Clark 1865a, 1865b). Subsequent references to the species by Verrill (1879), Hargitt (1901), Kingsley (1910), and Fraser (1944, 1947a) appear to have been based on the original description of R. fusiformis by L. Agassiz (1862). Other than a listing of the species in a Bay of Fundy checklist (Linkletter et al. 1977), and a brief note of its occurrence in Passamaquoddy Bay, Canada (Calder 2010), this is the first published record of R. fusiformis from the northeast coast of North America since the accounts of Louis Agassiz, Alexander Agassiz, and Henry James Clark in the 1860s. Macroscopically, hydroids of R. fusiformis generally resemble those of Clava multicornis (Forsskål, 1775), and the two species were found together during this study in the Bay of Fundy region, even occurring at times on the same thallus of the fucoid alga Ascophyllum nodosum.

The description of R. fusiformis given here is based in part on the original account of L. Agassiz (1862), and in part on new observations of specimens from the Bay of Fundy. Type material (MCZ 52) from Nahant, Massachusetts, examined previously (Calder 1988), is in poor condition and of little use in characterizing the species. The finest specimens examined here were found on thalli of A. nodosum in an intertidal brook running into Passamaquoddy Bay from a brackish pond near Letete, NB (ROMIZ B3101, ROMIZ B3102). These colonies, collected during late spring (24 May 1999, 30 May 1999), bore gonophores in the form of fixed sporosacs. Development of embryos and release of planula larvae were described by H.J. Clark (1865b) from specimens collected during May farther south in Massachusetts. A statement by L. Agassiz (1862: 226) that spent gonophores of R. fusiformis metamorphose into hydranths was disputed by H.J. Clark (1865a: 342, 1865b: 76, 77). He believed that spent gonophores likely wither and are resorbed, and that hydranths may then develop from the base of the old gonophore pedicel.

Rhizogeton fusiformis is the type species, by monotypy, of Rhizogeton L. Agassiz, 1862. Five nominal species are currently assigned to the genus (WoRMS), including R. nudus Broch, 1910, R. ezoense Yamada, 1964, R. sterreri (Calder, 1988), and R. conicus Schuchert, 1996, together with R. fusiformis. According to Schuchert (2012), an unnamed species of Rhizogeton, similar to R. nudus but distinct genetically, occurs in the English Channel and the Mediterranean Sea. He also noted that Arctic populations differ from R. nudus in the number of eggs borne by female gonophores, and might consititute a distinct species. In addition, the identity of a species of Rhizogeton occurring across the tropical Indo-Pacific region remains uncertain (Calder, 2010). Rhizogeton nematophorus Antsulevich, in Antsulevich & Polteva, 1986 is distinctive in having nematophores, and has been assigned instead to Similomerona Schuchert, 2004, as S. nematophora.

Although poorly known, Rhizogeton is a genus having an essentially worldwide distribution, with species reported from the Arctic (Schuchert 2001a), North Atlantic (Schuchert 2004), South Atlantic (Genzano et al. 2009), North Pacific (Antsulevich 2015), South Pacific (Schuchert 1996), and Indian oceans (Millard 1975). It extends across the tropical Indo-Pacific in neritic waters from Mozambique to Hawaii (Calder 2010), and is represented by a species comprising part of the pelagic Sargassum fauna in the open North Atlantic (Calder 1988).

Morphological characters seem generally inadequate as a means of resolving current questions about validity of the various putative species of Rhizogeton. Nematocysts of R. conicus appear to be larger than in other known species (Cooke 1975; Kubota 1976; Calder 1988; Schuchert 1996, 2004), but the complement of categories (desmonemes; microbasic euryteles) is the same throughout. Molecular studies are needed to resolve taxonomic issues within the genus. Rhizogeton ezoense Yamada, 1964, best known from the northwest North Pacific (Hirohito 1988) but also reported from the northeast North Pacific (Brinckmann-Voss 1996), has been regarded by some (Antsulevich & Polteva 1986; Antsulevich 1987, 2015; Park 1997) as conspecific or likely conspecific with R. fusiformis. Rhizogeton fusiformis and R. nudus have long been maintained as distinct species (e.g., Kramp 1911; Dons 1012; Calder 1972; Antsulevich & Polteva 1986; Stepanjants 1989; Schuchert 2001a, 2004, 2012; Ronowicz et al. 2013; Antsulevich 2015) even though there is little to separate them morphologically. If the conclusion of Park (1997) is correct that they are conspecific, the range of the species would be extended well beyond the western North Atlantic. Meanwhile, a record of R. fusiformis in European waters by Browne (1897) was thought by Schuchert (2004) to have been based on R. nudus instead. Schuchert (2012) included R. nudus, but not R. fusiformis, in his guidebook on north-west European athecate hydroids. As for the specific name of the former species, it has been spelled in the literature as both nudum and nudus. Dons (1912) and Schuchert (2004) noted that the latter is nomenclaturally correct because this Latin adjective must agree in gender with the masculine generic name Rhizogeton (ICZN Art. 31.2).

Advances in the classification of hydrozoans are currently being made through application of molecular methods. One early hypothesis (Cartwright et al. 2008) has included the genus Rhizogeton within Gonoproxima Cartwright et al. 2008, a group of Filifera in which gonophores arise from hydrocauli, pedicels, or stolons rather than from hydranths. That clade was said to comprise Dicoryne Allman, 1859b together with the families Bougainvilliidae Lütken, 1850, Oceaniidae Eschscholtz, 1829, Pandeidae Haeckel, 1879, and Rathkeidae Russell, 1953. Rhizogeton nudus was included amongst a total of 110 taxa of Hydrozoa in the analysis. Molecular work has yet to be done on R. fusiformis.

Recorded distribution. Bay of Fundy, mentioned without data: Bay of Fundy (Linkletter et al. 1977); Passamaquoddy Bay, NB (Calder 2010).

Eastern North America: Newfoundland (reported herein) to Nahant, Massachusetts (L. Agassiz 1862).

Worldwide: As above, and possibly extending to the northwestern North Pacific.