Rhopalonema velatum Gegenbaur, 1857

Schuchert, Peter & Collins, Richard, 2021, Hydromedusae observed during night dives in the Gulf Stream, Revue suisse de Zoologie 128 (2), pp. 237-356 : 334-336

Rhopalonema velatum Gegenbaur, 1857

Fig. 60 A-E

? Trachynema ciliatum Gegenbaur, 1854: 53 , pl. 2 figs 17-23. – Gegenbaur, 1857: 259, pl. 9 fig. 6.

? Calyptra umbilicata Leuckart, 1856: 14 , pl. 1 figs 9-10. – Mayer, 1910: 378, possibly R. velatum .

Rhopalonema velatum Gegenbaur, 1857: 251 , pl. 9 figs 1-5. – Bigelow, 1909: 129, pl. 2 fig. 8, pl. 45 fig. 11. – Mayer, 1910: 378, figs 213-222, 224, synonymy. – Russell, 1953: 430, figs 283-284, synonymy. – Kramp, 1959a: 185, fig. 270. – Kramp, 1961: 262. – Kramp, 1968: 114, fig. 307. – Horridge, 1969: 345, figs 3-4, statocysts. – Segura-Puertas, 1984: 38, pl. 10 fig. 4. – Bleeker & Van der Spoel, 1988: 239, fig. 28. – Pagès et al., 1992: 45, fig. 55.

? Sminthea leptogaster Gegenbaur, 1857: 246 , pl. 9 fig. 11, Mediterranean. ‒ Mayer, 1910: 383, possible synonym.

Trachynema octonarium Haeckel, 1879: 260 , Canary Islands. – Mayer, 1910: 378, synonym.

Marmanema tympanum Haeckel, 1879: 262 , Mediterranean. – Mayer, 1910: 378, synonym.

Marmanema clavigerum Haeckel, 1879: 263 , pl. 17 figs 1-2, Canary Islands. – Kramp, 1961: 262, synonym.

Rhopalonema coerulum Haeckel, 1879: 264 , pl. 17 figs 3-6, Canary Islands. – Kramp, 1961: 262, synonym.

Rhopalonema polydactylum Haeckel, 1879: 265 , pl. 17 figs 7-1, Mediterranean. – Kramp, 1961: 262, synonym.

Marmanema velatoides Maas, 1893: 13 , pl. 1 fig. 6, Atlantic. – Mayer, 1910: 378, synonym.

Rhopalonema striatum Maas, 1893: 15 , pl. 1 figs 3-4, Atlantic.

– Kramp, 1961: 262, synonym.

Examined material: BFLA4289 ; 1 specimen; 09-DEC- 2019; size 8 mm; no photos taken, preserved in alcohol for DNA extraction; 16S sequence MW528700 . – BFLA4292 ; 1 specimen; 09-DEC-2019; size 7 mm; preserved in alcohol for DNA extraction; 16S sequence MW528702 . – 04-MAR-2019; 1 specimen; size 11 mm, photographed, not collected.

Mediterranean, France, Bay of Villefranche-sur-Mer , 43.6860° N 7.3170° E, depth 0-70 m; 3 specimens collected 09-MAY-2001, 04-APR-2005, 29-APR- 2014, used for DNA extractions, 16S EU293992 and KP776749 .

Observations: Specimens from Florida with bell size of 7-11 mm, bell flatter than hemisphere, at least in some a very shallow apical process visible ( Fig. 60E ). Velum very broad and hanging down like a curtain. Manubrium small, shorter than bell cavity height. Eight thin radial canals. Gonads on radial canals, elongated ovals, about 1/3 as long as the radial canal, placed in middle of the radial canals. About 16 statocysts, positioned closer to tentacles and not in middle of spaces between tentacles. Eight long, extensible tentacles ending in club-shaped swellings ( Fig. 60B, C ); eight short, thin tentacles in interradial position, with terminal swelling ( Fig. 60B ); up to two tentacle stumps in adradial position per octant ( Fig. 60C , not well visible).

16S data: The two halplotypes found in the Florida material differ only in one base pair position and likewise also from a Mediterranean haplotype ( Table 1 ). A further haplotype from Tierra del Fuego is more divergent ( Table 1 ).

Distribution: Abundant in the warm and temperate waters of Atlantic, Indian, and Pacific oceans, including also the Mediterranean. Type locality: Mediterranean.

Remarks: The medusa from Florida was identified as R. velatum although it differed somewhat from typical specimens as described in Kramp (1959a). There are currently two accepted species in the genus Rhopalonema : R. velatum and R. funerarium Vanhöffen, 1902 . Rhopalonema velatum differs from the former in having a small apical process, marked off from the rest of the umbrella by a transverse circular furrow or depression. The process varies considerably in size and form ( Russell, 1953). Rhopalonema funerarium lacks this proces, has long gonads on the distal 2/3 of the radial canals, a very short manubrium, and 32 statocysts. In one of the sample photos ( Fig. 60E ), a shallow apical process is present. Additionally, the size and position of the gonads and the statocyst number and position matched more R. velatum , but the short manubrium was more like in R. funerarium . Although the latter species has been regarded as distinct by Kramp (1947, 1959a), others had doubts [see discussion and references in Russell (1953: 435) and Bouillon et al. (2004: 244)]. It could well be that R. funerarium is only an advanced growth stage of R. velatum .

Figures

Fig. 60. Rhopalonema velatum. (A-B) Specimen of 04-MAR-2019, diameter ca. 11 mm. (A) Oblique view from aboral side. (B) Tentacle tips. (C-E) BFLA4292, 7 mm diameter. (C) Oblique view from aboral side, yellow arrow points to short, interradial tentacle, red arrow points to long, perradial tentacle. (D) Oblique view from oral side. (E) Near lateral view, note the slight apical process.