Zancleopsis dichotoma ( Mayer, 1900 )

Schuchert, Peter & Collins, Richard, 2021, Hydromedusae observed during night dives in the Gulf Stream, Revue suisse de Zoologie 128 (2), pp. 237-356 : 272-273

Zancleopsis dichotoma ( Mayer, 1900)

Figs 22 A-F, 23A-F

Gemmaria dichotoma Mayer, 1900: 35 , pl. 17 fig. 40.

Zancleopsis dichotoma . – Hartlaub, 1907: 115, fig. 105. – Mayer, 1910: 91, pl. 8 fig. 1. – Bigelow, 1938: 102, figs 1-2. – Kramp, 1959a: 95, fig. 53. – Kramp, 1961: 56. – Kramp, 1965: 25, correction of Kramp, 1959a. – Kramp, 1968: 39. – Bouillon, 1978b: 290. – Bouillon, 1985: Table 1 .

Cnidotiara gotoi . – Kramp, 1959a: 12, pl. 1 figs 7-8. [misidentification, not Zancleopsis gotoi ( Uchida, 1927) ]

? Zancleopsis dichotoma . – Xu et al., 2014: 363, fig. 228, juvenile specimen.

Examined material

Small form: BFLA4170 ; 1 specimen; 09-AUG-2019; size 3 mm; preserved in alcohol for DNA extraction; 16S sequence MW528683 . – BFLA4171 ; 1 specimen; 09- AUG-2019; size 3 mm; preserved in alcohol for DNA extraction; 16S sequence MW528684 . – BFLA4248 ; 1 specimen; 23-NOV-2019; size 2 mm; preserved in alcohol for DNA extraction; 16S sequence MW528692 . – 13-AUG- 2019, 3 mm, 1 specimen photographed, not collected. – 16-OCT-2019, no size estimate, 1 specimen photographed, not collected.

Large form: BFLA4408 ; 1 specimen; 26-MAY-2020; height 15 mm, 5 mm wide, male; preserved in alcohol for DNA extraction; 16S sequence MW528723 . – BFLA4436 ; 1 specimen; 07-JUN-2020; height 12 mm, female; preserved in formalin and deposited as UF- 014054 . – BFLA4467 ; 1 specimen; 17-JUN-2020; height 15 mm, 5 mm wide, female; preserved in formalin and deposited as UF-014072 , part of broken tentacle preserved in alcohol for DNA extraction; 16S sequence MW528732 . – 11-APR-2019; 1 specimen photographed; size 10-15 mm, with male gonads; not collected.

Observations

Small form ( Fig. 22 ): Total bell height up to 3 mm, 1/4 to 1/3 of the height taken by pointed apical process; umbrella bell-shaped to conical, relatively thick walls, with shallow interradial subumbrellar pockets, tip of apical process green ( Fig. 22A, E ). Manubrium height about half the subumbrellar height when gonads start to develop, pear-shaped, short tubular oral part, mouth rim with four perradial white regions, upper part of manubrium (stomach) ochre coloured, with about 10 longitudinal, shallow gonad folds, folds mostly adradial, irregular. Radial canals not forming mesenteries, smooth. Tentacle bulbs all equally developed, almost spherical, placed adaxial of origin of tentacles, white or faintly yellow. Two long, opposite tentacles, much extendable/ contractible, with up to 25 short, abaxial, side branches ending in nematocyst knobs, size of knobs gradually increasing towards distal ( Fig. 22F ). The other tentacle pair very short, ending in spherical nematocyst knob. In youngest animals observed these short tentacles either missing or just beginning to develop ( Fig. 22E ). All tentacle bases with a red ocellus on abaxial side.

Nematocysts (alcohol preserved tissue, BFLA4171): larger stenoteles [(24-26)x(22-23) µm], smaller stenoteles [(18+21)x(14-17) µm], desmonemes [8.5x5 µm], and macrobasic euryteles [(15-16)x(6-7) µm].

Large form ( Fig. 23 ): Much larger than small form, height 8 to 15 mm and 5 mm diameter, apical process larger reaching 1/2 of total height, tip of apical process whitish, gonads always present, more and larger vertical gonad folds, approximately up to 15, 2-3 folds adradial, brownish, oocytes yellow ( Fig. 23A ), tentacle bulbs with intense yellow colour, shorter tentacle pair longer than in small form, sometimes terminal knob missing ( Fig. 23B ).

16S Data: The haplotypes of the small form differed in 0.5-0.8% of the base pairs, in the large form 0.3%. The maximal divergence observed between the two forms was 6.2% base pair difference.

The maximum likelihood analysis ( Fig. 21 ) identified Astrocoryne cabela Maggioni et al., 2017 polyps from the Red Sea and the Maldives Islands as relatively closely related, if not conspecific.

Distribution: Florida, Bermuda Islands, mid North Atlantic, Brazil ( Kramp, 1959a; Navas-Pereira, 1980). Type locality: USA, Florida, Dry Tortugas archipelago. The record for the China Sea ( Xu et al., 2014) is based on a juvenile, indeterminable animal.

Remarks: This is a rare species of which only a few specimens have been reported. Our samples separated into two size categories which also had slight morphological differences, but explainable by different developmental stages being present. The smaller (bell size up to 3 mm) corresponded more to Mayer’s (1910) description, while the larger ones corresponds to Bigelow’s (1938) specimen. The only difference to Bigelow’s material was the length of the tentacles and the presence of terminal knobs in the shorter pair of tentacles. However, this is easily explained by damage during the collecting process of Bigelow’s material. Also Kramp (1965: 25) observed that the distal parts of the tentacles are often lost in net material. In one of our specimens, one knob was also missing ( Fig. 23B ). Mayer (1900) described Gemmaria dichotoma based on several, but apparently all immature medusa of 3 mm bell height. He characterised them as having only a single pair of branched tentacles. This is certainly attributable to the rather young stages he had. Also in some of our younger stages the short tentacles were either just developing or absent ( Fig. 22E ).

Were it not for the 16S results, we would have attributed without hesitation the two forms to two different developmental stages of the same species. The 16S barcode sequences separated the large and small forms into two distinct lineages ( Fig. 22 ) which differed maximally in 6.2% of their aligned bases. We have no reference values of what constitutes intraspecific variation for this family, but the value is quite high compared to other species (comp. Table 1 ).

Due to the absence of any other diagnostic feature, we preferred for the moment to regard these two forms as belonging to the same species and representing younger and older stages. Many hydromedusae are known to continue their growth even when their gonads get mature. Note also that both forms were found at different times of the year (August to October versus April to June). If the two forms belong to the same species, then the differences in the 16S sequences deserves nevertheless some thoughts, beyond the possibility that they are simply stochastic variations observed in the small number of specimens. It can be argued that the larger forms are certainly also older and have thus spent more time in the current of the Gulfstream. This would then imply that the two forms come from quite different localities, the different 16S haplotypes might therefore come from separate populations.

Kramp (1968: 39) suspected that Z. dichotoma and Z. tentaculata are conspecific [for descriptions of Z. tentaculata see Kramp (1965, 1968) and Bouillon (1978b)]. Bouillon (1978b; 1985) kept them distinct, but he also had some doubts about the suitability of the distinguishing traits. Some of them are linked to the developmental stage (size and shape of bell, position and shape of the gonads, length and form of tentacles), others are clearly due to damages and losses during the collecting process (tentacle lengths and terminal knobs).

Figures

Fig. 22. Zancleopsis dichotoma, small form. (A-C) BFLA4171, bell height 3 mm, stomach filled with prey items, note well developed short tentacles. (D) BFLA4170, bell height 3 mm, gonads just begin to develop.(E-F) BFLA4248, bell height 2 mm, immature, note that one of the short tentacles (upper one) is only just developing, while the opposite bulb appears to be devoid of a tentacle.

Fig. 23. Zancleopsis dichotoma, large form, sizes refer to the total bell height, including the apical process. (A-C) BFLA4467, size 12 mm, female, note in B that one of the short tentacles lacks a terminal knob. (D) BFLA4436, size 10 mm, part of long tentacles with capitate side branches. (E) BFLA4408, size 15 mm, close up of manubrium with male gonads. (F) Animal observed 11-APR-2019, size approximately 10-15 mm.

Fig. 21. 16S maximum likelihood phylogenetic tree of Zancleidae and related taxa obtained with PhyML (GTR+G model) and based on about 600 bp positions of the mitochondrial 16S gene. Node-support values are bootstrap values of 100 pseudoreplicates (shown only if> 70%). All sequence labels start with the GenBank numbers permitting the retrieval of more information. Red labels are new sequences from this study.