Racemoramus, Calder, 2012

Calder, Dale R., 2012, On a collection of hydroids (Cnidaria, Hydrozoa, Hydroidolina) from the west coast of Sweden, with a checklist of species from the region 3171, Zootaxa 3171 (1), pp. 1-77 : 25

Racemoramus , gen. nov.

Diagnosis. Colonies erect; growth monopodial with terminal hydranths encased in hydrothecae. Hydrocaulus monosiphonic, usually unbranched, straight or nearly so, relatively rigid, not divided into internodes; gradually tapering from proximal to distal end; annulations, if present, only at origin from slender hydrorhiza. Hydrothecae pedicellate, quite fragile, arising from hydrocaulus singly or in comparatively short monopodial and grape-like clusters (“panicles”) at irregular intervals and often in different planes, directed obliquely upwards; radially symmetrical, deep cone-shaped, tapering into pedicel; diaphragm present basally; desmocytes small, forming a ring just above diaphragm; margin with conical operculum consisting of a folded continuation of the hydrothecal wall, opercular facets distinct, enclosing hydrothecal cavity when closed, not clearly demarcated from hydrothecal rim. Hydrothecal pedicels of varied length, with a few annulations basally. Nematophores absent.

Gonophores possibly free medusae; gonothecae very elongate, club-shaped, sometimes slightly curved or twisted, arising from hydrocaulus or from hydrothecal pedicels; aperture terminal, pedicel short, annulated.

Type species. Campanulina panicula G.O. Sars, 1874 , designated herein.

Etymology. From Latin racemus (a cluster) and ramus (branch) in reference to branches with grape-like clusters of hydrothecae in the species. The gender of the name is masculine.

Remarks. Although the name Racemoramus , gen. nov., is masculine, the specific name of its type species, panicula G.O. Sars, 1874 , as published in the binomen Campanulina panicula , is taken to be a noun in apposition and its suffix does not change.

The monopodial growth form of Racemoramus panicula (G.O. Sars, 1874) is unusual among leptothecate hydroids, and especially so amongst so-called “campanulinids,” as noted previously ( Kramp 1941; Cornelius 1995a). Its distinctive colony form distinguishes Racemoramus from these and all other genera of the suborder Campanulinida Hincks, 1868 .

The difference in colony form is taken as sufficiently significant taxonomically to warrant establishment of a new genus. While superficially similar to Plicatotheca Calder & Vervoort, 1986 , its colonies are fundamentally very different in being racemose instead of sympodial. While hydrothecae of the two are similar, the operculum does not fully enclose the hydrothecal cavity in Plicatotheca and its opercular facets are linguiform instead of pointed. Desmocytes forming a ring immediately above the diaphragm are large in Plicatotheca , but small and inconspicuous in Racemoramus .

Included in Racemoramus here are Campanulina panicula , C. denticulata Clarke, 1907 , and C. indivisa Fraser, 1948 . These three have generally been considered conspecific in recent work, although their combined geographic and bathymetric ranges are extremely wide, as discussed below. On zoogeographic grounds I prefer to maintain C. denticulata from the Pacific and Indian oceans as distinct from the Atlantic C. panicula . However, Racemoramus indivisus , comb. nov., from bathyal waters off California ( Fraser 1948) is here considered conspecific with R. denticulatus , comb. nov., originally described from abyssal bottoms off Peru ( Clarke 1907). Vervoort (1966) was the first to consider them as synonyms.

Calder et al. (2009) assigned Fraser’s (1948) Campanulina indivisa to the genus Plicatotheca Calder & Vervoort, 1986 , but that is incorrect because its colonies are racemose rather than sympodial.

Neither the life cycle of R. panicula , type species of the genus, nor that of R. denticulatus (including R. indivisus ) is known. It remains to be established whether a medusa stage exists in species of the genus, and if so whether it is referable to a known medusa genus. Kramp (1941) observed gonothecae in material from the Trondheimfjord, and Christiansen (1972) reported finding them in material from Nygrunn in the Oslofjord, but neither was able to establish whether gonophores were fixed sporosacs or free medusae. Vervoort (1966) found only empty gonothecae in Opercularella denticulata from the Indian Ocean, but thought it likely that a medusa was produced in the species. Rees & Rowe (1969) found fertile material from Sweden with what appeared to be a developing medusa.

The familial affinities of Racemoramus remain obscure because of a lack of information on its life cycle. Its unique colony form and hydrothecal morphology exclude it from Campanulinidae Hincks, 1868 , as defined by the type genus Campanulina Van Beneden, 1847 . It is provisionally assigned here to Phialellidae Russell, 1953 based on the resemblance of its hydrothecae, including presence of a diaphragm, to those of phialellid genera including Phialella Browne, 1902 , Opercularella Hincks, 1868 , and Plicatotheca .