Sertularella diaphana ( Allman, 1885 )

Galea, Horia R., 2010, Additional shallow-water thecate hydroids (Cnidaria: Hydrozoa) from Guadeloupe and Les Saintes, French Lesser Antilles, Zootaxa 2570, pp. 1-40 : 16-18

Sertularella diaphana ( Allman, 1885)

(figs 1B, C; 4L–R)

Thuiaria diaphana Allman, 1885: 145 , pl. 18 figs 1–3.

Sertularella diaphana ― Calder, 1991: 101, fig. 53.― Vervoort, 1993: 214, fig. 45D, E; 46D. Hincksella brevitheca Galea, 2009: 61 , figs 1B, 2, syn. nov.

Material examined. Stn.9: 21.11.2009, 21 m—three sterile, fan-shaped, highly polysiphonic colonies (morphotype 1), 3.7–6.2 cm high, on worm tubes; several infertile stems (morphotype 2), 3.4–6.0 cm high, growing up through sandy bottom (MHNG-INVE-68721). Stn.11: 30.11.2009, 15–18 m—two large, sterile, much branched, highly polysiphonic colonies (morphotype 1), 13–14 cm high, on bivalves attached to black gorgonian; numerous stems (morphotype 2), to 10.5 cm high, some bearing gonothecae, growing up through sandy bottom (four stems as MHNG-INVE-68722). Stn.12: 30.11.2009, 12–17 m—several sterile, fanshaped, polysiphonic stems and fragments (morphotype 1), to 9.5 cm high, from hard substrate (MNHN- IK.2009-830); four sterile colonies (morphotype 2), 5.5–9.5 cm high, growing up through sandy bottom. Stn.13: 19.11.2009, 13 m—several sterile stems and fragments (morphotype 2), to 8 cm high, growing up through sandy bottom; 0 3.12.2009, 10 m—numerous stems to 9.5 cm high, some bearing gonothecae, growing up through sandy bottom (MNHN-IK.2009-831). Stn.14: 0 1.12.2009, 20 m—three polysiphonic, much branched colonies and fragments (morphotype 1), to 12 cm high, some with gonothecae, removed from hard substrate (MHNG-INVE-68736). Stn.21: 24.11.2009, 10–15 m—three polysiphonic, branched stems (morphotype 1), to 11 cm high, and several smaller fragments, all sterile, growing on bivalve shell attached to sponge; additional sample comprising numerous hydrocladia arising epizootically from stolons creeping on Plumularia setacea ( Linnaeus, 1758) .

Remarks. For complete lists of synonyms and descriptions of S. diaphana , see Calder (1991) and Vervoort (1993).

Two distinct morphotypes occur in the present material. Morphotype 1 is typically represented by branched, fan-shaped, highly polysiphonic colonies, growing upside down from overhangs and ceilings of submarine arches and cavities (fig. 1C). Color in life pale pink. Perisarc thin, transparent; cladia brittle, easily broken off upon colony manipulation. All hydrothecae exhibit a rim provided with four pointed triangular cusps separated by shallow embayments; the opercular apparatus is composed of four triangular flaps, often remaining attached to the hydrothecal rim. There is no hydrothecal base and the adcauline wall of hydrotheca ends proximally in a perisarc plug. A single nematocyst type was found in all samples examined: microbasic mastigophores (5.6–6.0) × (1.6–1.7) µm.

From all synonym species of S. diaphana , the present morphotype is closest to Thuiaria pinnata Allman, 1877 , originally described from the Straits of Florida.

Morphotype 2 is sympatric with the former and is characterized by unbranched, pinnate colonies growing more or less individually on sandy bottom (fig. 1B). The stems are always monosiphonic, the perisarc is thick, horn colored, rather elastic and never brittle. The coenosarc is canaliculated, often anastomozing, milky in color. The very basal part of the stems (1–13 mm in length) may appear polysiphonic but, upon careful inspection, it is actually formed by several stolonal tubes arising from the main stem and running downwards to form the complex, root-like hydrorhizal network anchoring the colony to its substrate. No secondary tubes, running upwards along the main stem, were observed in any of the available material.

When colonies of similar sizes are considered (e.g. samples from Stn.9), those belonging to morphotype 1 have strongly polysiphonic stems, while those of morphotype 2 have always monosiphonic though rather thick stems.

The most conspicuous difference is to be found in the shape of hydrothecae, which, in this variety, are Hincksella -like. Each hydrotheca has a well-defined, transverse base, the rim is even or nearly so, and no closing apparatus is present. The adcauline hydrothecal wall is somewhat shorter in this morphotype than in the former (345–370 µm vs. 380–450 µm).

There are also differences in cnidome composition: in addition to the microbasic mastigophores [(6.2–6.5) × (1.9–2.0) µm] observed in morphotype 1, conspicuous, larger capsules [(8.0–8.3) × (2.7–3.0) µm] of unidentified type co-occur in morphotype 2.

Gonothecae are present in some of the examined specimens and are morphologically indistinguishable from those normally observed in S. diaphana ; the fine, internal structure of the gonophore could not be evaluated properly in fixed material.

At present, it is impossible to state whether the morphological differences observed between the two morphotypes are solely due to ecological factors. It is worth noting that specimens from the same station (e.g. Stn.9, 11 and 12) collected from the bottom and the overhangs, respectively (each of which found distantly at not more than 2–3 m), can exhibit such profound contrasts. Molecular analyses would allow for a better understanding of their true relationship.

Of all the synonym species of S. diaphana , the present morphotype is closest to Thuiaria distans Allman (1877) , originally described from the Tortugas. Both share in common the colony shape, their monosiphonic stems, the canaliculated coenosarc, and the even to wavy hydrothecal margin.

Gravier-Bonnet & Lebon (2002) stated, without providing any comparison, that differences were present in two sympatric populations from Reunion Island, one assigned to Allman’s (1885) species, the other to a new, yet unnamed taxon, whose “important characters for specific diagnosis” have not since been provided.

Taken together, these scant data suggest that we are probably dealing with several cryptic species and, in the absence of conspicuous morphological differences, molecular studies are expected to elucidate their relationships. It is also worth mentioning the amazingly long list of synonyms (see Calder 1991) for S. diaphana , which partly accounts for the apparently wide morphological variation observed in this taxon. Caribbean records. Belize ( Spracklin 1982, as Sertularella speciosa Congdon, 1907 ).

World distribution. Circumglobal in tropical and subtropical waters ( Schuchert 2003).