Mitrocomium

Galea, Horia R., 2008, On a collection of shallow-water hydroids (Cnidaria: Hydrozoa) from Guadeloupe and Les Saintes, French Lesser Antilles, Zootaxa 1878, pp. 1-54 : 28-30

Mitrocomium sp.

(fig. 5G–H, table 11)

? Mitrocomium cirratum — Calder, 1991d: 25, figs 15–16.

not Mitrocomium cirratum Haeckel, 1879: 182 , pl. 11 figs 9–11.

Material examined. Stn. 7: 27.03.2008 —several hydrothecae, but no gonothecae, on alga.

Description. Colony stolonal, with short (100–130 µm), upright pedicels arising individually from creeping hydrorhyza; pedicels widening gradually from base to distal end; a basal constriction separates them from stolon; one hydrotheca atop each pedicel. Hydrothecae 21–23 µm deep, widening considerably from base (75–90 µm) towards aperture (105–140 µm); margin circular, rim even, somewhat everted; a ring of desmocytes above diaphragm; no secondary hydrothecae have been observed. Hydranths with 20–22 filiform, amphicoronate tentacles, in one whorl. Intertentacular web present, although difficult to observe. Gonothecae absent. Nematocysts (undischarged capsules): large, banana-shaped (lateral view) capsules (31.0–35.5) × (6.6–6.8) µm, in intertentacular web; smaller capsules, most probably microbasic mastigophores, (6.9–7.4) × (1.9–2.1) µm, in tentacles.

Remarks. No discharged nematocysts could be seen. However, the larger capsules are generally brittle and easily broke up in squash preparations. Therefore, a well-defined, isodiametric shaft, of roughly the same length as the capsule itself, could be observed. This leads me to the conclusion that the nematocyst type corresponds to the microbasic mastigophore.

The hydroid material from Bermuda, assigned to M. cirratum Haeckel, 1879 by Calder (1991d), appears to have the same cnidome as the present specimens from Les Saintes. The large capsules in Calder’s (1991d) material seem to possess a shaft, as for instance that noticeable in his fig. 16A (more obvious in the upper side of the capsule). A comparison of measurements of the present material and that from Bermuda is available in table 11.

For the reasons explained by Schuchert (2003), the generic name adopted here is Mitrocomium Haeckel, 1879 , pending a comprehensive phylogenetic analysis of the taxa involved.

Our actual data concerning both the entire life cycle and the cnidome composition of the nominal species of Mitrocomium are limited, and do not allow a clear distinction to be made between the various world records. However, several speculations can be offered, especially based on data on the nematocyst complement available in a number of recent studies (see below). Therefore, several populations with a defined geographical distribution could be distinguished, as follows.

The Mediterranean population is more likely attributable to M. cirratum Haeckel, 1879 (see comments in Schuchert 2003), and includes the following materials: Haleciella microtheca of Hadżi (1914), Halecium torreyi and H. torreyi var. intermedia of Motz-Kossowska (1911), Campalecium medusiferum of Huvé (1954), Eucheilota cirrata of Brinckmann (1959), and C. medusiferum of Boero (1981) and Boero & Sarà (1987). The cnidome of this population is composed of large merotrichous isorhizas and smaller microbasic mastigophores ( Boero 1981, Boero & Sarà 1987), but unfortunately no measurements are available from these sources.

The recently-described Atlantic material assigned to E. medusifera ? by Altuna (2008) comes very close to the Mediterranean specimens. The cnidome of the hydroid stage is composed of merotrichous isorhizas [(28–32) × (8–9) µm] and microbasic mastigophores [(6.4–8.0) × (1.8–2.2) µm], and no apparent differences could be observed compared to those illustrated by both Brinckmann (1959) and Boero (1981) ( Altuna 2008). Additionally, the newly released medusae reported by these authors are likely identical ( Altuna 2008). On the other hand, the adult medusae raised by Altuna developed 4 perradial bulbs, each bearing one tentacle flanked by 2–3 pairs of lateral cirri, and 4 interradiar bulbs devoid of tentacles, but flanked by 2 pairs of cirri. There were 8 statocysts. Similarly, only 4 marginal tentacles were present in the medusae of E. cirrata raised by Brinckmann (1959).

The adult medusae obtained by Altuna (2008) also resemble those of Lovenella cirrata described by Pagès et al. (1992) from the Benguela current, with the difference that 8 tentacles, flanked by 3–4 pairs of lateral cirri, and 16 statocysts were reported by the latter author. However, adult medusae of M. cirratum from nature may develop up to 16 tentacles ( Kramp 1959a). The medusae raised in the laboratory often experience slow and/or incomplete growth rates as compared to those from nature, as illustrated by the aberrant medusae (with only 2 tentacles or without cirri) obtained by Altuna (2008). It is therefore reasonable to assume that Altuna’s material also belongs to M. cirratum . Therefore, the eastern Atlantic population is likely identical to the Mediterranean one, and both probably belong to M. cirratum .

Following Schuchert (2003), the Indian Ocean population is attributable to M. simplex ( Pictet, 1893) , and I concur. It includes Halecium simplex of both Pictet (1893) and Ritchie (1910), and C. cirratum of Millard & Bouillon (1975). The latter authors provided data on the cnidome, which contains large microbasic mastigophores [(33.6–36.0) × (7.2–9.0) µm] and “several other types of nematocysts in the tentacles” (sic).

The Australian M. alcoicum ( Watson, 1993) contains both large (20 × 4 µm, shaft 35 µm long) and small [(6–7) × 2 µm] microbasic mastigophores ( Watson 1993).

No data on the cnidome composition of the Japanese hydroid assigned to C. cirratum by Hirohito (1995) is presently available, and that material most probably does not belong to Haeckel’s (1879) species.

Moreover, neither the nematocysts nor the adult medusa were described in M. medusiferum ( Torrey, 1902) , and no comparisons could be established with its relatives from other geographical areas.

Therefore the present Caribbean material and that studied by Calder (1991d) could not be allocated to any of the existing species on the basis of cnidome composition. Additionally, life cycle studies are necessary in order to describe the adult medusa.

Distribution. Western Atlantic ( Calder 1991d, present study).

Dynamena crisioides Lamouroux, 1824: 613 , pl. 90 figs 11–12.— Vervoort, 1959: 260, fig. 27A–B.—Van Gemerden- Hoogeveen, 1965: 21, fig. 6.— Vervoort, 1968: 38, fig. 18.—Millard, 1975: 263, fig. 87A–F.— Calder, 1991d: 89, figs 47–48.— Migotto, 1996: 60, fig. 11E–G.— Medel & Vervoort, 1998: 21.— Schuchert, 2003: 170, fig. 28.

Material examined. Stn. 2: 26.01.2008 —two sterile stems, 1.0 and 1.5 cm high respectively, on rock.

Type locality. Moluccas, Indonesia.

Remarks. Recent descriptions of this species are available in Calder (1991d) and Migotto (1996). For an extensive synonymy, see Medel & Vervoort (1998).

Distribution. Circumglobal in tropical and subtropical waters ( Calder 1991d). The Caribbean records are summarized by Calder & Kirkendale (2005).