Hydractinia dendritica Hickson & Gravely 1907

Hydractinia dendritica Hickson & Gravely, 1907

(Figs 7 I–N, 8)

Hydractinia dendritica Hickson & Gravely, 1907: 9, pl. 2 figs 7–10; Kramp, 1932: 11 M; Stechow, 1962: 417; Bouillon et al., 2006: 150; Stampar et al., 2006: 58

Hydronema dendritica — Stechow, 1921: 252; 1923: 67, fig. F.

Halorhiza dendritica — Stechow, 1962: 417.

Material examined. Discovery Antarctic Expedition 1901-04: Holotype [NHM 1907.8.20.5 (part)], W.Q. (Ross Sea), 21.III.1902, 10 fms, one long colony fragment, c. 85 mm long and c. 6 mm in maximum diameter, plus another erect colony fragment, c. 30 mm high, with gonophores; Paratype [NHM 1907.8.20.6], Locality unrecorded (label lost), several stem fragments, up to 28 mm long, with gonophores.

Description (Holotype). A long colony fragment, c. 85 mm long and c. 6 mm in diameter (Fig. 8 A), and an erect colony fragment, c. 30 mm high (Fig. 8 B). Both irregularly branched, particularly the second one. Colonies with high density of polyps and gonozooids. With perisarc ridges throughout (Fig. 8 D), sometimes forming a sort of spine.

Gastrozooids relatively large (c. 1700 µm high and c. 450 µm in maximum diameter), with cone-shaped hypostome and a distal crown of nine to ten filiform tentacles (Figs 7 I, 8C, F). Some gastrozooids with extremely wide, flattened hypostome (Fig. 8 F). There are also evaginated gastrozooids.

Gonozooids (Figs 7 K, L, 8E, F) much smaller than gastrozooids (c. 570 µm high and c. 220 µm in maximum diameter with gonophores; c. 340 µm and 125 µm, respectively, without gonophores). Much varied in development, from barely developed (with none, one, two or three tentacles) (Fig. 7 K) to well-developed polyps with up to eight tentacles (Fig. 7 L). Incipient gonozooids apparently without mouth. Gonozooids with four to six gonophores (150–180 µm in diameter) situated at their base, resting on substratum, but attached to gonozooid (Figs 7 L, 8E, F).

Dactylozooids (Figs 7 N, 8E) present in both fragments (at least three observed).

Measurements (in µm). Cnidome: Type I [range 8.5–9.0 x 3.0–3.5, mean 8.9±0.2 x 3.2±0.2 (n=10); ratio, range 2.6–3.0, mean 2.8±0.2 (n=10)], Type II [range 9.0–11.0 x 3.0–4.5, mean 9.8±0.7 x 3.9±0.5 (n=10); ratio, range 2.1– 3.0, mean 2.5±0.2 (n=10)], desmonemes [range 5.0–6.0 x 3.5, mean 5.6±0.3 x 3.5±0.0 (n=10); ratio, range 1.4–1.7, mean 1.6±0.1 (n=10)]. Dactylozooids: Type II, extremely abundant, apparently without shaft [range 10.0–11.0 x 3.5–4.5, mean 10.6±0.4 x 4.1±0.4 (n=10); ratio, range 2.3–3.1, mean 2.6±0.2 (n=10)].

Description (Paratype). Several irregularly branched stem fragments (Fig. 8 G, H), up to 28 mm long, with tree-shaped appearance. Stems formed by longitudinal tubes of perisarc.

Gastrozooids large (c. 1150 µm high and c. 350 µm in maximum diameter), with nine to ten tentacles and cone-shaped hypostome (Figs 7 J, 8H). Polyps with a very wide flattened hypostome also present (Fig. 8 H). No dactylozooids observed.

Development of gonozooids as in holotype (c. 460 µm high and c. 160 µm in maximum diameter with gonophores), with up to six tentacles (Fig. 7 M). Usually, four to five gonophores at base of polyp (Fig. 8 H), though the number might be lower when mature (c. 300 µm in diameter) (Figs 7 M, 8G).

Measurements (in µm). Cnidome: Type I (8.5– 9 x 3); Type II, microbasic mastigophores? [range 10.0–12.0 x 3.5–4.5, mean 10.8±0.6 x 4.2±0.3 (n=10); ratio, range 2.2–3.0, mean 2.6±0.2 (n=10)]; desmonemes (5.5– 6 x 3.5).

Remarks. The two colony fragments examined from the holotype clearly correspond with specimen A described by Hickson & Gravely (1907); specifically the 85-mm-long fragment to the “encrusting or basal region” and the 30-mm-high fragment to the “upright branching stem”. According to these authors specimen A “consists of a single continuous colony encrusting the stems of a specimen of Halecium arboreum. From this encrusting mass, which possesses all the general features of an ordinary Hydractinia, a single upright branching stem arises… There are two regions in this specimen, the encrusting or basal region, and the upright branching stem, the rhizocaulus. The basal part entirely surrounds the polysiphonic stems of Halecium arboreum, only a few pinnules of the supporting hydroid penetrating it and being exposed” (Hickson & Gravely 1907: 10). Concerning the upright branching stem, the authors indicated that “The ramification is irregular, the eight or nine terminal branches ending in some cases in a long filamentous process. The surface of these branches is remarkably smooth.”

In relation with specimen B, “there is no encrusting basal support as in specimen A, and the axes of the branches do not show any foreign hydroid or other kind of core. … The skeleton is composed of a series of parallel intercommunicating chitinous tubes, the superficial tubes being externally incomplete, …” (Hickson & Gravely, 1907: 11).

Stechow (1923) examined type material of this species, apparently the upright rhizocaulome of Hickson & Gravely’s specimen B. He studied sections of the stem and branches and put into evidence the presence of an inner bundle of 10–30 or more totally independent, chitinous tubes. He concluded that the inner bundle corresponds to a foreign hydroid on which Hydractinia was growing. Even though Kramp (1932) reiterated that Stechow (1923) had demonstrated that H. dendritica has no erect hydrocaulus, but simply covers the polysiphonic stems of other hydroids, I do not agree. Apart from the fact that the bundle of tubes actually belong to H. dendritica, there are several other clues that point to the formation of erect stems in this species. Hickson & Gravely (1907: 12), when comparing specimen A with specimen B, indicated “The fortunate preservation of a small and probably young branching stem of specimen A, with gastrozooids at its proximal end similar to those of the basal parts and at its distal end similar to those of specimen B; with a smooth surface similar to that of specimen B, and ramifying and growing without any axial support as specimen B does, may be regarded as conclusive evidence that the specimens belong to the same species.” In addition, concerning the upright branching stem of specimen A, Hickson & Gravely (1907: 10) also indicated that “The ramification is irregular, the eight or nine terminal branches ending in some cases in a long filamentous process ”. For me, those statements unequivocally indicate that H. dendritica has rhizocaulomic growth.

In my opinion, H. dendritica, as also H. angusta (see above), is able to grow on a substrate, as typical species of Hydractinia do, but is also able to produce an erect stem. In the material I examined, there are clear proofs of the encrusting growth. For example, it is perfectly visible that in the 85-mm-long colony fragment of the holotype, in addition to growing on Hydrodendron arboreum (as Hickson & Gravely indicated), the colony also forms a sort of hollow, thick cylinder of perisarc, filled with sponge spicules (Fig. 8 C), which indicates that the colony might also have developed growing on a stalked sponge. However, the structure of the 30-mm-high upright branching colony fragment clearly shows a rhizocaulomic growth. As indicated by Hickson & Gravely (1907) this part of the colony is “ ramifying and growing without any axial support” and “ the eight or nine terminal branches ending in some cases in a long filamentous process”.

Hydractinia dendritica had been considered conspecific with H. angusta by several authors (e.g. Totton 1930; Briggs 1938; Galea & Schories 2012). Nevertheless, after examining the type material of both species, it seems that they actually correspond to different species. Both gastrozooids and gonozooids are much larger in H. dendritica. They also differ in the position of the tentacle crown, distal in H. dendritica, but medial in H. angusta (cf. Fig. 7). Finally, no dactylozooids have been found in the type material of H. angusta, whereas they are present in H. dendritica.

Ecology and distribution. Hydractinia dendritica is only known from McMurdo Bay, in the Ross Sea, where it was collected at a depth of 9–18 m (Hickson & Gravely 1907).