Rhizorhagium roseum M. Sars, in G.O. Sars 1874

Rhizorhagium roseum M. Sars, in G.O. Sars, 1874

Figs. 11, 12

Rhizorhagium roseum M. Sars, in G.O. Sars, 1874: 129.

Type locality. Norway: Mangerfjord, “ Bongnestrømmen ” (Bognestrømmen), 20 fm (37 m) on Tubularia indivisa (Rees 1956: 114).

Material examined. NS: Bay of Fundy, 8 km NE of Centreville, 27.v.1936, on Halecium muricatum, five colonies or fragments of a colony or colonies, all with female gonophores, coll. J. Stevenson, ARC 8650054.– NS: Bay of Fundy, 8 km NE of Centreville, 27.v.1936, on Halecium muricatum, one fragment of a colony or colonies, with female gonophores, coll. J. Stevenson, ROMIZ B4220.

Description. Hydroid colonies stolonal, arising from a hydrorhiza of creeping stolons. Hydranth pedicels unbranched, of varied length but mostly long, approaching 1 cm high, somewhat curved or twisted in preserved specimens, of uniform diameter or widening only slightly towards distal end; perisarc of pedicel mostly wrinkled, some parts more distinctly so than others, but not regularly annulated, thickest basally, gradually thinning out distally, extending as a filmy covering over base of hydranth, forming a large, loose, goblet-shaped, somewhat rugose pseudohydrotheca, perisarc not extending as tubes over bases of tentacles or hypostome. Hydranths quite large, vasiform to sac-shaped; tentacles filiform, tapering from slender tip to broad base, about 10-12 in number, arranged around distal end of hydranth; hypostome nipple-shaped, exceptionally large relative to size of hydranth.

Gonophores fixed sporosacs. Female sporosacs borne on short, slender, tapered pedicels arising from stolons of hydrorhiza, sometimes occurring in a dense cluster; spadix enveloped by a large, balloon-shaped, transparent membrane covered with thin perisarc; several planulae developing from sides of spadix within spacious cavity of gonophore capsule. Male gonophores not seen.

Cnidome (Fig. 12)

desmonemes (n = 10): 3.9–4.2 µm long × 2.4–2.8 µm wide (undischarged)

small microbasic euryteles (n = 10): 5.9–6.8 µm long × 2.9–3.5 µm wide (undischarged)

large microbasic euryteles (n = 10): 8.0–8.8 µm long × 4.0–4.5 µm wide (undischarged)

unidentified nematocyst (n = 1): 12.0 µm long × 6.4 µm wide (undischarged)

Remarks. Although the generic name Rhizorhagium and the species name R. roseum were made available in a publication by G.O. Sars (1874), authorship of both names was attributed therein solely to M. Sars. That original account, a preliminary one without illustrations, included a diagnosis of the genus (in Latin) and comments on occurrences in Norwegian waters of its only included species. Michael Sars had encountered the hydroid near Bergen, and Georg Ossian Sars reported finding it at Lofoten. An exhaustive description, accompanied by illustrations, was provided later (M. Sars 1877). In that work, the generic name was spelled Rhizoragium, which must be taken to be an incorrect subsequent spelling (ICZN Art. 33.3). Both Broch (1916, as Perigonimus roseus) and Rees (1938) added to taxonomic knowledge of the species, and Rees (1956) determined that the type material was from Bognestrømmen in the Mangerfjord, Norway. A recent synonymy list and overview is given by Schuchert (2007). Noteworthy amongst the synonyms is Garveia groenlandica Levinsen, 1893, and it was under that name that the species was recorded from the east and west coasts of the North American continent during the first half of the twentieth century (Fraser 1937, 1944, 1947a).

This, the first record of Rhizorhagium roseum from the Bay of Fundy, is based on material (ARC 8650054) from Minas Channel, Nova Scotia. The specimens, collected in 1936, were somewhat deteriorated and the colour of the colony or colonies had faded, but they appear to have been robust and thriving at the time of collection. Numerous female gonophores were present. The specimens were found on Halecium muricatum, and other hydroids appear from literature records to be frequent substrates of the species (e.g., Schuchert 2007; Calder 2012). Although scant collection data were available with the sample, water temperatures in the bay at the time of collection (May) would have been cold (<10°C).

Rhizorhagium roseum has usually been assigned to the family Bougainvilliidae Lütken, 1850 (e.g., Rees 1956; Calder 1972, 2012; Bouillon et al. 2006; Schuchert 2007, 2012; Antsulevich 2015; WoRMS). In an earlier work (Calder 1988), the genera Rhizorhagium M. Sars, in G.O. Sars, 1874 and Parawrightia Warren, 1907 were grouped within Bougainvilliidae as a new subfamily, Rhizorhagiinae Calder, 1988. Hydroids assigned to the subfamily were distinguished from other bougainvilliids in having hydranths that were vasiform and enveloped by distinct pseudohydrothecae, hypostomes that were unusually prominent and nipple-shaped, tentacles that were arranged in two or more close whorls, and gonophores (where known) that were fixed sporosacs (Calder 1988). Their hydroids morphologically resemble those of some genera (e.g., Leuckartiara Hartlaub, 1914, Neoturris Hartlaub, 1914, Halitholus Hartlaub, 1914) assigned to Pandeidae Haeckel, 1879. Moreover, a recent phylogram in Prudkovsky et al. (2016) indicates that R. roseum has a much closer affinity with Pandeidae Haeckel, 1879 than with Bougainvilliidae. Included in a clade with R. roseum were Leuckartiara octona (Fleming, 1823), L. nobilis Hartlaub, 1914, Neoturris breviconis (Murbach & Shearer, 1902), Hydrichthys boycei Warren, 1916, Catablema vesicarium (A. Agassiz, 1862), and Garveia nutans Wright, 1859. All are regarded as pandeids except G. nutans, currently and likely erroneously taken be a bougainvilliid. Considerable divergence was apparent between this clade and others that included various species of Bougainvillia Lesson, 1830. As for G. nutans, it strongly resembles R. roseum morphologically in having a vasiform hydranth enveloped by a pseudohydrotheca, an exceptionally large hypostome, and fixed sporosacs of similar morphology. Based on morphological characters, and especially on the molecular evidence of Prudkovsky et al. noted above, R. roseum and the subfamily Rhizorhagiinae are transferred here from Bougainvilliidae to Pandeidae. Also assigned herewith to the subfamily, along with Rhizorhagium and Parawrightia, is Garveia Wright, 1859. Meanwhile, the possible taxonomic makeup and nomenclatural status of Rhizorhagiinae needs further consideration, as its scope may encompass one or more genera already assigned to Pandeidae. Recent diagnoses of Pandeidae include those of Schuchert (2007) and Calder (2010), but the family as presently conceived is likely polyphyletic (Daly et al. 2007).

Garveia, Rhizorhagium, and Parawrightia are obviously much alike, and characters that have been highlighted in differentiating them need to be reassessed. Schuchert (2007) earlier noted the inadequacy of contemporary diagnoses in distinguishing Garveia and Rhizorhagium, and suggested that molecular phylogenies were needed to sort out their relationships. Based on current evidence it seems almost certain that those two genera will prove to be identical because of the close resemblance of their type species, G. nutans and R. roseum. If so, then Rhizorhagium would become a junior subjective synonym of Garveia, and R. roseum would be assigned the binomen Garveia rosea. Certain other species now assigned to both Rhizorhagium and Garveia are probably also misclassified. For example, Rhizorhagium arenosum (Alder, 1862) has been shown to have affinities with Bougainvilliidae rather than with R. roseum, the genus Rhizorhagium, and the family Pandeidae (Prudkovsky 2016). Similarly, Garveia franciscana resembles bougainvilliids more closely than Garveia and its type species G. nutans. It may warrant removal to Calyptospadix Clarke, 1882, an obscure bougainvilliid genus. As noted by Schuchert (2007) earlier, however, it seems best to maintain current usage until more information is available on these genera.

In terms of identification, hydroids of Rhizorhagium roseum are distinctive in the prominence and shape of their hypostomes, their stolonal or mostly stolonal colonies, the considerable length of their pedicels, the pronounced development of their pseudohydrothecae, their balloon-shaped sporosacs that arise only from the hydrorhiza, and in life by the rosy to reddish colour of their colonies (e.g., Sars 1877; Rees 1938; Schuchert 2007, 2012). As with hydroids of species of the pandeid genus Amphinema Haeckel, 1879, and of Garveia nutans, hydranths bend over sharply on being disturbed (Wright 1859; Schuchert 2007; Calder 2012).

The cnidome of Rhizorhagium roseum in material examined here appears to comprise desmonemes, small and large microbasic euryteles, and a rare category that could not be identified (Fig. 12). Three of the latter kind were seen discharged, along with one that was undischarged, but it was impossible to determine the character of the thread in either case. The other three kinds of nematocysts were common to abundant.

Distributionally, Rhizorhagium roseum has been considered a circumpolar Arctic-boreal species (Schuchert 2012). Although recorded on this coast as far south as Vineyard Sound, Massachusetts, by Fraser (1944, as Garveia groenlandica), that record warrants verification. In waters of eastern Canada, it is known from Bay of Islands, Newfoundland (Fraser 1918, as G. groenlandica), Sugluk Creek (Sugluk Inlet), Hudson Strait (Fraser 1931, as G. groenlandica), SE Hudson Bay (Calder 1972), and the Saguenay River, Quebec (Calder 1972). The reported bathymetric distribution of the species is from 10–200 m (Schuchert 2007; Antsulevich 2015). It was recorded by Brinckmann-Voss (1996) from an intertidal tidepool at Race Rocks, British Columbia, and from infralittoral and subtidal areas nearby.

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

Eastern North America: Foxe Basin and Greenland to southern Massachusetts (Fraser 1944, 1947a, as Garveia groenlandica; Calder 1970; Schuchert 2007).

Worldwide: Northeast Atlantic from the White and Barents seas to western Scotland; Iceland; Faroes; Svalbard; Northeast Pacific from Alaska to southern Vancouver Island (Brinckmann-Voss 1996; Schuchert 2001a, 2007; Calder 2012; Ronowicz et al. 2013; Antsulevich 2015).