Obelia hyalina Clarke, 1879

Calder, Dale R., 2013, Some shallow-water hydroids (Cnidaria: Hydrozoa) from the central east coast of Florida, USA, Zootaxa 3648 (1), pp. 1-72 : 58-59

Obelia hyalina Clarke, 1879

Fig. 16f

Obelia hyalina Clarke, 1879: 241 , pl. 4, fig. 21.— Fraser, 1944: 160.

Type locality. Mexico: “Ten miles (16 km) north of Zoblos Island” (= Isla Holbox ) ( Clarke 1879) .

Voucher material. Fort Pierce , Fort Pierce Inlet State Park, 27°28’29.5”N, 80°17’25.8”W, on stranded Sargassum fluitans , 14.vii.2012, 28° C, 35‰, collected manually, one colony, 7 mm high, without gonophores, coll. D.R. Calder, ROMIZ B3985 .

Remarks. The species originally regarded as Obelia hyalina Clarke, 1879 was generally considered valid for more than half of the 20 th century, as reflected in works such as those of Stechow (1912), Nutting (1915), Fraser (1944), Deevey (1950), Rees & Thursfield (1965), and Vervoort (1968, as O. congdoni ). Over the most recent 4–5 decades, however, it has been widely regarded as conspecific with O. dichotoma ( Linnaeus, 1758) . Evidence is now apparent from life cycle, nematocyst, and molecular studies that lumping in hydroid taxonomy during that period was excessive, including within the genus Obelia Péron & Lesueur, 1810 . This species has traditionally been distinguished from O. dichotoma in having hydrocauli that are monosiphonic and relatively little branched, and hydrothecal margins that are entire and not plicated ( Nutting 1915; Fraser 1944). Accordingly, Obelia hyalina is once again recognized as valid in this work. Colonies are usually much smaller in size (usually 15–20 mm) than those of O. dichotoma (as much as 50–350 mm) as described by Cornelius (1995b). It is also a species of tropical and warm-temperate rather than of boreal and mostly cool-temperate regions, where O. dichotoma was originally found.

Clarke (1879), in the original account of O. hyalina , made no mention of either substrate or depth of collection. Many subsequent records of the species have been based on specimens from Sargassum (e.g., Nutting 1895, 1915; Versluys 1899; Fraser 1912b, 1918, 1943, 1944; Broch 1913, as Laomedea sargassi ; Bennitt 1922; Leloup 1935, 1937, as L. sargassi ; Rees & Thursfield 1965), the substrate of colonies examined here. It appears that the common species of Obelia on pelagic Sargassum in the North Atlantic is O. hyalina , and gulfweed would be of common occurrence at its type locality in the southern Gulf of Mexico. Obelia hyalina is found on both Sargassum natans and S. fluitans , and it is one of the most frequent hydroid species on those holopelagic fucoids ( Calder 1995, as O. dichotoma ).

The name Laomedea sargassi Broch, 1913 has sometimes been applied to this species (e.g., Leloup, 1935, 1937). Broch (1913) had considered both Obelia hyalina Clarke 1879 and Gonothyraea hyalina Hincks, 1866 referable to genus Laomedea Lamouroux, 1812 , and proposed L. sargassi as a replacement name for the supposed junior homonym. No homonymy currently exists because the two species are again referred to different genera, and the binomen L. sargassi is an invalid junior objective synonym of O. hyalina . Somewhat less certain is the identity of Obelia congdoni Hargitt, 1909 . It was described from material found on Sargassum in the Woods Hole region, Massachusetts. Hargitt (1909) had considered it identical with material that Congdon (1907) had identified as O. hyalina from Bermuda, but he believed both his hydroids and those of Congdon were different from Clarke’s (1879) species. The origins of branches were thought to differ, gonothecae were said to be large with a terminal neck instead of small and rounded distally, and colonies were larger (20–30 mm instead of 12 mm high). Nutting (1915), Fraser (1944), and Vervoort (1968) are probably correct that O. congdoni and O. hyalina are conspecific.

According to Clarke (1879), Obelia hyalina was obtained north of “Zoblos Island,” which I have been unable to locate. Nutting (1900: 91) gave coordinates of “lat.N.24°8’, long.W.28°51’” for a station said to be 10 miles north of Zoblos Island, but that is in the eastern North Atlantic off the coast of Africa. In a narrative of cruises including the one during which Clarke’s material was collected, Alexander Agassiz made no mention of “Zoblos” Island. However, one of the transects surveyed during the work extended from “...the north side of the Yucatan Bank to Alacran Reef, and from there in a south-east direction into 20 fathoms off the Joblos Islands...” (A. Agassiz 1888: ix). Isla Holbox (roughly pronounced “hole-bosch”), at the northeastern tip of the Yucatan Peninsula, Mexico, corresponds with Agassiz’s account and is taken to be the site. The station off “Zoblos Island” (Isla Holbox) is the type locality of three other nominal species ( Eudendrium distichum , Campanularia coronata , Nematophorus grandis ) of hydroids described by Clarke (1879).

Reported distribution. Atlantic coast of Florida. Straits of Florida ( Fraser 1944).

Western Atlantic. Gulf Stream east of Nova Scotia, on Sargassum ( Fraser 1918) , to Brazil ( Vannucci 1949), and including the Sargasso Sea ( Leloup 1937, as Laomedea sargassi ), the Caribbean Sea ( Leloup 1935, as L. sargassi ), and the Gulf of Mexico ( Fraser 1944).

Elsewhere. Eastern Atlantic ( Rees & White 1966); questionable records exist from the tropical eastern Pacific ( Fraser 1948) and Indian Ocean ( Thornely 1904).

Figures

FIGURE 16. a, Clytia noliformis: hydrotheca and distal part of pedicel, ROMIZ B3984, scale equals 0.10 mm. b, Clytia noliformis: gonotheca, ROMIZ B3984, scale equals 0.10 mm. c, Clytia paulensis: part of colony with hydrotheca and pedicel, ROMIZ B1103, scale equals 0.20 mm. d, Clytia paulensis: hydrotheca and distal part of pedicel, ROMIZ B1103, scale equals 0.10 mm. e, Obelia geniculata: part of hydrocaulus with three hydrothecae, ROMIZ B1119, scale equals 0.25 mm. f, Obelia hyalina: part of colony with three hydrothecae, ROMIZ B3985, scale equals 0.25 mm. g, Obelia oxydentata: part of colony with three hydrothecae, ROMIZ B1112, scale equals 0.25 mm.