Rhizogeton sterreri ( Calder, 1988 )

Calder, Dale R., 2019, On a collection of hydroids (Cnidaria, Hydrozoa) from the southwest coast of Florida, USA, Zootaxa 4689 (1), pp. 1-141 : 11-12

Rhizogeton sterreri ( Calder, 1988)

Figs. 1c, d

Rhizogeton fusiformis .— Joyce, 1961: 26, pl. 1, figs. 1, 2.— Shier, 1965: 8, pl. 1 [not Rhizogeton fusiformis L. Agassiz, 1862 ]. Rhizodendrium sterreri Calder, 1988: 10 , figs. 7, 8.

Type locality. Bermuda: Whalebone Bay, on pelagic Sargassum ( Calder 1988: 10, as Rhizodendrium sterreri ).

Material examined. Sanibel Island, beach at Lighthouse Point, 26°26’57”N, 82°01’06”W, on detached Thalassia at water’s edge, 13 March 2018, 20° C, 33.5‰, two colonies or colony fragments, up to 2 mm high, with male gonophores, coll. D. Calder, ROMIZ B4323.— Sanibel Island , beach at Lighthouse Point, 26°26’57”N, 82°01’06”W, on detached Thalassia at water’s edge, 13 March 2018, 20° C, 33.5‰, coll. D. Calder, ROMIZ B4324 [initially preserved in 70% ethanol (never in formalin)].— Sanibel Island , beach at Lighthouse Point, 26°27’00”N, 82°01’01”W, on detached Thalassia at water’s edge, 15 March 2018, 18° C, 34‰, two colonies, up to 2 mm high, without gonophores, coll. D. Calder, ROMIZ B4325 .— Fort Myers Beach , 26°27’27”N, 81°57’51”W, on detached Thalassia at water’s edge, 22 March 2018, 21° C, 33.5‰, one colony, 2 mm high, with male gonophores, coll. D. Calder, ROMIZ B4326 .

Remarks. This small and inconspicuous hydroid has been reported twice before from the Gulf coast of Florida, as Rhizogeton fusiformis L. Agassiz, 1862 , in the unpublished theses of Joyce (1961) and Shier (1965). With R. fusiformis being a species from cold waters in the boreal western North Atlantic (L. Agassiz 1862; Fraser 1944; Calder 2017), the binomen R. sterreri ( Calder, 1988) has been adopted here for the Florida population. The name R. sterreri was applied originally to a species found on pelagic Sargassum in warm waters of Bermuda ( Calder 1988, as Rhizodendrium sterreri ). Similar hydroids from the tropical islands of Guadeloupe, Les Saintes, and Martinique in the French Lesser Antilles, Caribbean Sea, have likewise been identified as R. sterreri ( Galea 2008, 2013). Moreover, records of Rhizodendrium sp. from the Caribbean coast of Panama by Calder & Kirkendale (2005) are considered here to have been based on the same species. As noted earlier ( Calder 2017), however, the various species of Rhizogeton L. Agassiz, 1862 have yet to be clearly differentiated on the basis of morphology, and all need to be compared by molecular methods to resolve relationships.

The reported geographic range of R. sterreri extends from Bermuda ( Calder 1988, as Rhizodendrium sterreri ) and Florida (this study) to the southern Caribbean Sea ( Bandel & Wedler 1987, as Rhizogeton fusiformis ), and from there to northern Bahía, Brazil ( Kelmo & Santa-Isabel 1998). Rhizogeton sterreri , like other species of the genus, is normally found in shallow to relatively shallow water (for the bathymetric range of R. nudus Broch, 1909 [1910], most frequently reported of the species, see Schuchert 2004). It is nevertheless an inhabitant also of the high seas, occurring as part of the epibenthos on pelagic Sargassum in the Gulf Stream and Sargasso Sea (see the Reported Distribution section below for records).

While originally described from pelagic Sargassum (Calder, 1998, as Rhizodendrium sterreri ), the hydroid appears to be a substrate generalist. Joyce (1961) reported it on Cuban shoal weed ( Halodule wrightii ), Shier (1965) on a chiton, and Galea (2008) on algae, sponges, mineral concretions, and in crevices on both scleractinians and hydrocorals. It was found here on the seagrass Thalassia testudinum .

Hydroids observed in this study were brownish to rusty red in colour when alive. As described by Galea (2008), the gastrodermis at the bases of the tentacles was bright white.

More details on the widespread but relatively obscure and inadequately known genus Rhizogeton and its putative species are given in works by Schuchert (2004, 2012), Galea (2008), and Calder (2010, 2017).

Reported distribution. Gulf coast of Florida. Seahorse Key ( Joyce 1961: 26, as Rhizogeton fusiformis ).— Cape San Blas area ( Shier 1965: 8, as Rhizogeton fusiformis ) .

Elsewhere in western North Atlantic. Atlantic Ocean: Gulf Stream, on pelagic Sargassum + Sargasso Sea, on pelagic Sargassum (Burkenroad, in Parr 1939: 23, as “…resembles Clava ”).— USA: Gulf Stream off New Jersey, 40°09’N, 70°56’W, on Sargassum nr. ramifolium ( Rackley 1974: 19, as Rhizogeton fusiformis ).— USA: Gulf Stream off North Carolina, 34°21’N, 75°36’W, on Sargassum nr. ramifolium ( Rackley 1974: 19, as Rhizogeton fusiformis ).—Sargasso Sea, E of North Carolina, 33°26’N, 71°56’W, on Sargassum nr. ramifolium ( Rackley 1974: 19, as Rhizogeton fusiformis ).— Colombia: Santa Marta area ( Bandel & Wedler 1987: 39, as Rhizogeton fusiformis ).— Bermuda: Whalebone Bay, on pelagic Sargassum ( Calder 1988: 10, as Rhizodendrium sterreri ).— Panama: Bocas del Toro area, Bastimentos (north), 09°20.898’N, 82°09.959’W, 1-4 m + Bocas del Toro area, Drago 2, 2- 4 m ( Calder & Kirkendale 2005: 477, as Rhizodendrium sp.).—French Lesser Antilles: Guadeloupe, Basse-Terre, Petite Anse, 16°05’47.00”N, 61°46’17.00”W, on algae, concretions + Basse-Terre, Anse à la Barque, 16°05’21”N, 61°46’00”W, on coral + Grande-Terre, Anse des Salines, 16°14’58.80”N, 61°10’50.45”W, on hydrocoral ( Galea 2008: 8).—French Lesser Antilles: Les Saintes, Terre-de-Haut, Pain de Sucre, 15°51’45”N, 61°35’60”W, on algae, concretions, sponge ( Galea 2008: 8).—French Lesser Antilles: Martinique ( Galea 2013: 49).—Caribbean Sea ( Wedler 2017b: 42, figs. 36A–D, as Rhizogeton fusiformis ).— Panama: Bocas del Toro area, Bocas del Drago ( Miglietta et al. 2018b: 108).

Figures

FIGURE 1. a, Pennaria disticha: part of branch, with pedicel and hydranth, Fort Myers Beach, ROMIZ B4347. Scale equals 0.5 mm. b, Corydendrium parasiticum: part of colony with a reduced hydranth, Southwest Florida Shelf, ROMIZ B449. Scale equals 0.5 mm. c, Rhizogeton sterreri: hydranth and stolon, Fort Myers Beach, ROMIZ B4326. Scale equals 0.2 mm. d, Rhizogeton sterreri: male gonophore and stolon, Fort Myers Beach, ROMIZ B4326. Scale equals 0.2 mm. e, Turritopsis nutricula: colony with two hydranths, Fort Myers Beach, ROMIZ B4327. Scale equals 0.2 mm. f, Cordylophora caspia: part of colony with hydranth and female gonophore, Caloosahatchee River at Fort Myers, ROMIZ B4329. Scale equals 0.2 mm.