Hydromedusae observed during night dives in the Gulf Stream Author Schuchert, Peter Muséum d’histoire naturelle, C. P. 6434, CH- 1211 Genève 6, Switzerland peter.schuchert@ville-ge.ch Author Collins, Richard 880 NE 33 rd Street, Boca Raton, Florida, USA rc6684@icloud.com text Revue suisse de Zoologie 2021 2021-10-21 128 2 237 356 journal article 10.35929/RSZ.0049 172fa5c5-c0c4-4bd7-b710-d608237b8458 0035-418 5639938 Melicertissa mayeri Kramp, 1959 Fig. 29 A-C Melicertissa clavigera . – Mayer, 1910: 210 , pl. 24 figs 2-3. [not Melicertissa clavigera Haeckel, 1879 ] Melicertissa mayeri Kramp, 1959a: 139 , fig. 162, new name. – Kramp, 1961: 144 . – Uribe-Palomino et al. , 2018: 26 , table 1. ? Lafoea venusta Allman, 1877: 11 , pl. 6 figs 3-4. [possible synonym] ? Hebella venusta . – Calder, 1991: 41 , fig. 25. – Boero et al., 1997: 39 , species inquirenda. – Galea, 2010: 13 , fig. 4E-G. – Calder, 2013: 17 , fig. 5a. – Calder, 2019: 33 , fig. 8a. Examined material: BFLA4451 ; 1 specimen ; 08-JUN- 2020 ; size 10 mm ; part preserved in formalin and deposited as UF-014062 , part preserved in alcohol for DNA extraction; 16S MW528729 . Observations: Melicertissa medusa with nearly hemispherical bell, diameter 10 mm , apical jelly thickened. Manubrium relatively small, cross-section an eight-rayed star, mouth with eight simple lips, base starshaped and widened, continued as eight basal perradial stomach extensions, leaf-like, replacing about 2/3 of radial canal, continued after this as thin, unbranched radial canal. Gonads on distal part of basal stomach extensions, flat and leaf-like on both sides of extension, occasional folds present ( Fig. 29B ). Eight perradial and eight interradial tentacles fully developed, three adradial small tentacles, total number thus 19. Tentacle bulbs conical to ovoid. Between each pair of tentacles 1-3 cordyli, totally about 35. Adaxial ocelli on circular canal opposite each tentacle bulband cordylus. Cirri absent. Mostly colourless, gonads white, sides of tentacle bulbs with green hue. 16S Data: A blastn search in GenBank with the obtained sequences gave a good match of 98.7% similarity with the sequence FJ550496 obtained from the hydroid Hebella venusta ( Allman, 1877 ) . The maximum likelihood tree also shows the close relationship ( Fig. 28 ). Distribution: Florida ( Kramp, 1959a );?Indian Ocean ( Navas-Pereira & Vannucci, 1991 ; a single individual only). Type locality: USA , Florida , Dry Tortugas archipelago. Remarks: Mayer (1910) identified Melicertissa medusae from Florida as M. clavigera Haeckel, 1879 despite them having 16 tentacles instead of 8 as for typical M. clavigera . He considered this as intraspecifc variation with possibly M. clavigera being a younger form. Nair (1951) questioned this, mainly based on the assumption that Haeckel described M. clavigera has having 24 cordyli at a size of 10 mm , while Mayer (1910) depicted his medusa with only 16 cordyli at a diameter of 7 mm . If Mayer’s specimens were older and had more tentacles, then they should not have fewer cordyli (although they might have been lost, they break away easily). Kramp (1959a) adopted Nair’s suggestion and introduced the new name Melicertissa mayeri for Mayer’s specimens. However, both Nair and Kramp must have overlooked comments in Mayer (1910: 211) in which he described the variation of tentacle numbers and cordyli. Some interradial tentacles may be replaced by a cordylus (resulting thus in less than 16 tentacles). Some octants of the bell may have two tentacles and three cordyli, others had three tentacles and 2 cordyli. The number of tentacles and cordyli is thus variable and Mayer’s (1910) conclusion that M. clavigera is conspecific with the Florida population is understandable and needs re-examination using genetic methods. Our specimen had 19 tentacles and about 35 cordyli, deviating thus somewhat from the scope given in Mayer (1910) and Kramp (1959a) . Only very few specimens of both M. clavigera and M. mayeri have been described so far and it is very likely that the intraspecific variation of the tentacle- and cordylus numbers has been underestimated. Therefore, we nevertheless identified the present medusa as M. mayeri . The 16S sequence matched rather closely (1.3% base pair differences, Fig. 28 ) to a sequence obtained from the hydroid Hebella venusta ( Allman, 1877 ) collected in Honduras and growing on the hydroid Thyroscyphus marginatus ( Fig. 30 ). The type material of Hebella venusta came from Loggerhead Key, Florida. Recent descriptions of Hebella venusta are given by Galea (2010) and Calder (2013 , 2019 ). It is a nominal species with very few diagnostic traits and Boero et al. (1997) are right considering it a species inquirenda as our current concept of it could include several species (viz. the same hydroid morphotype but producing different medusae). It is therefore not clear if the material from which the sequence FJ550496 was obtained corresponds really to the H. venusta at the type locality. Fig. 29. Melicertissa mayeri, BFLA 4451, size 10 mm. (A) Oblique view of whole animal. (B) Oral view. (C) Bell margin, note cordyli and ocelli. Fig. 30. Hydroid Hebella venusta , MHNG-INVE-0035476, from Honduras, stained slide preparation, voucher specimen of 16S sequence FJ550496. (A) Gonotheca with medusa buds, ocelli are faintly visible. (B) Hydrotheca with hydranth, rim with four shallow cusps. (C-D) Hydrothecae with smooth rim. (E) Unidentified structure resembling a nematotheca, but it could also be a rest of a broken caulus. However, Melicertissa mayeri is clearly not a member of the Laodiceidae ( Fig. 28 ) but must be transferred to the Hebellidae (comp. also Migotto & Andrade, 2000 ). According to the tree ( Fig. 28 ) it appears even congeneric with Staurodiscus .