Hebellopsis sibogae Billard, 1942

Calder, Dale R. & Faucci, Anuschka, 2021, Shallow water hydroids (Cnidaria, Hydrozoa) from the 2002 NOWRAMP cruise to the Northwestern Hawaiian Islands, Zootaxa 5085 (1), pp. 1-73 : 15-16

Hebellopsis sibogae Billard, 1942

Fig. 4d, e

Hebellopsis sibogae Billard, 1942: 70 , fig. 8.

Type locality. Indonesia: Siboga Expedition Sta. 129, Sulawesi, Karkaralong group, anchorage off Kawio and Kamboling, 04°40ʹ19″N, 125°24ʹ05″E ( Billard 1942; van Soest 1976).

Voucher material. Pearl & Hermes Atoll, on Macrorhynchia philippina , 19.ix.2002, two hydrothecae and pedicels, to 0.55 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5410.—Pearl & Hermes Atoll, on calcareous rubble, 28.ix.2002, one colony, 0.55 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5411.—Kure Atoll, on sponge, 25.ix.2002, one colony, 0.6 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5412.—Kure Atoll, 25.ix.2002, one colony, 1 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5482. Gardner Pinnacles, on calcareous rubble, 14.ix.2002, one colony, 3.5 mm high, without gonophores, ROMIZ B5494.

Remarks. Hydroids examined here closely resemble those of Lafoea gigas Pieper, 1884 (type locality:Adriatic Sea), Laomedea michaelsarsi Leloup, 1935 (type locality: Tortugas, Florida, USA), and Hebellopsis sibogae Billard, 1942 (type locality: Indonesia). Morphological differences between the three are presently unclear. The first of these, well-known in the Mediterranean Sea and vicinity, has been reported infrequently elsewhere ( Peña Cantero & García Carrascosa 2002; Galea 2008). The second has been reported from the western and eastern North Atlantic ( Leloup 1935; Vervoort 1959). The only record of either one in the Pacific Ocean is that of L. michaelsarsi from Darwin Island, Galapagos Islands ( Calder et al. 2003, as Hebellopsis michaelsarsi ). Reports of Hebella pocillum ( Hincks, 1869) from the west coast of North America by Fraser (1937a, 1947) have occasionally been included with question in the synonymy of L. gigas (e.g., Peña Cantero & García Carrascosa 2002, as Scandia gigas ), but such inclusion seems erroneous. The third species is known only from Indonesia, based on the original account of Billard (1942). As for the first two, they may be conspecific ( Galea 2008).

With reference to the generic assignment of these species, the well-known L. gigas has been referred at various times to Lafoea Lamouroux, 1821 , Hebella Allman, 1888 , Scandia Fraser, 1912 , Hebellopsis Hadži, 1913 , and Croatella Hadži, 1915 . Its characters clearly differentiate it from Lafoea in having stolonal rather than erect colonies, and isolated rather than aggregated gonophores. Unlike the hebellid genera Hebella , Scandia , and Anthohebella Boero, Bouillon & Kubota, 1997 , a well-developed hydrothecal diaphragm rather than a rounded annular thickening is present at the base of the hydrotheca. This species has usually been misassigned in current literature to Scandia , as S. gigas , but its gonosomal characters clearly differ from those of S. mutabilis ( Ritchie, 1907) , type species of that genus. Female gonothecae are pumpkin-shaped ( Peña Cantero & García Carrascosa 2002) to spherical ( Gravili et al. 2015) rather than sac-shaped, somewhat corrugated, and distally truncate ( Fraser 1944), while those of the male are fusiform ( Peña Cantero & García Carrascosa 2002) rather than resembling hydrothecae in shape and size ( Fraser 1944). The species was assigned to Hebellopsis by Calder (2013) based on the character of its hydrothecal diaphragm, but that too is taken here to be incorrect given its unusual gonothecae. It diverges from the putative hebellid genera Bedotella Stechow, 1913a in lacking nematophores and nematothecae, and from Halisiphonia Allman, 1888 in lacking laterally flattened and fan-shaped gonothecae. Hydroids of Staurodiscus Haeckel, 1879 are as yet scarcely known.

In a molecular study by Maronna et al. (2016), “ Scandia gigas ” was considered a “rogue taxon” and included as “ Leptothecata incertae sedis ” because of its unexpected and isolated placement in phylograms. Excluded from Hebellidae in their analyses, it has been provisionally retained in the family here pending further study. In appearing to be genetically remote from other known hebellids, however, and in having what appear to be morphologically distinctive gonophores, we conclude that the species should be assigned at least to a different genus. It is therefore returned here to Croatella , a taxon originally based primarily on L. gigas but also provisionally including Campanularia corrugata Thornely, 1904 ( Hadži 1915: 78) . Hadži distinguished the genus largely by the morphology of its diaphragm, which was said to be two-layered with a peripheral cleft.

Given all this, specimens from the Northwestern Hawaiian Islands have been assigned, largely on zoogeographic grounds, to the western tropical Pacific hydroid H. sibogae . This is only the second collection record of the species. With no information yet available on either the gonosome or the molecular phylogeny of the species, however, it is kept here in Hebellopsis rather than being assigned to Croatella . The holotype ( Van Soest 1976), on an alga ( Billard 1942), is now at the Naturalis Biodiversity Center, Leiden, the Netherlands (ZMA.COEL.5224).

Reported Distribution. Hawaiian archipelago. First record.

Elsewhere. Indonesia ( Billard 1942).

Figures

FIGURE 4. Hebellidae. a, Hebella furax Millard, 1957, hydrotheca, Midway Atoll, ROMIZ B5407. Scale equals 0.1 mm. b. Hebella sp., hydrotheca, pedicel, and stolon, Midway Atoll, ROMIZ B5493. Scale equals 0.2 mm. c, Hebellopsis scandens (Bale, 1888), hydrotheca and pedicel, Pearl & Hermes Atoll, ROMIZ B5413. Scale equals 0.1 mm. d, Hebellopsis sibogae Billard, 1942, hydrotheca and pedicel, Pearl & Hermes Atoll, ROMIZ B5410. Scales equal 0.1 mm. e, Hebellopsis sibogae Billard, 1942, hydrotheca and pedicel, Pearl & Hermes Atoll, ROMIZ B5411. Scale equal 0.1 mm. e,