Hebella dyssymetra Billard 1933

Hebella dyssymetra Billard, 1933

(Figs 3 K, S, T; 4E)

Material examined. Stn. 7, 24.i.2012, 6– 8 m, M044: hydrothecae on stems of Ventromma halecioides (Alder, 1859), some spreading on the substrate of the latter, a dead gorgonian.

Remarks. Boero et al. (1997) provide a description of this species and a list of synonyms. This material shows that the portion of colony creeping on the host's stems, Ventromma halecioides (Alder, 1859), has conspicuously asymmetric hydrothecae, nearly adnate basally to their substrate (Fig. 3 S), identical to those observed earlier by Calder (1991) in material from Bermuda. In addition, a portion of the same colony extends over the substrate of V. halecioides, a dead gorgonian, and has pedicellate, nearly radially symmetrical hydrothecae (Fig. 3 T).

The cnidome of the hydroid is composed of four types of capsules (none seen discharged): 1) minute, eggshaped microbasic heteronemes, ca. 3.7×1.6 µm, occurring occasionally in the coenosarc; 2) fusiform microbasic mastigophores, ca. 6.6×2.1 µm, abundant in the tentacles, also present in the coenosarc; 3) ovoid capsules provided with a lateral "beak" at the insertion of the shaft, ca. 6.1×2.6 µm, sparsely found in the coenosarc; 4) elongateovoid capsules, (16.5–18.9)×(5.6–6.4) µm, exclusively occurring in the hydrorhiza.

Boero et al. (1997) reported on the presence of only two size classes of microbasic mastigophores in the hydroid of this species, of which one is illustrated in their Fig. 8F, and corresponds to type 2 described here. Since some nematocysts (types 1 and 3) occur rarely in the coenosarc, and type 4 is only present in the hydrorhiza, I assume that these authors overlooked them. However, they illustrated (Fig. 8G, H) two additional microbasic mastigophores of the medusoid, which correspond well to types 3 and 4 described above.

5. In the absence of morphometric data, knowledge of the cnidome, and even the nature of the gonophore, this record is only provisionally assigned to A. communis (Calder, 1991). It may well belong to Hebella scandens (Bale, 1888).

Geographical distribution. Circumglobal in tropical to temperate seas (Calder 1991).