Hydractinia angusta Hartlaub 1904

Hydractinia angusta Hartlaub, 1904

(Fig. 7 A–H)

Hydractinia angusta Hartlaub, 1904: 7, pl. 4, figs 1–7; Totton, 1930: 138; Peña Cantero & Ramil, 2006: 950 –951, fig 2; Peña Cantero, 2004: 768; 2014b: 1714.

? Hydronema angustum — Briggs, 1938: 10.

Halorhiza angusta — Stechow, 1962: 418.

? Hydractinia angusta — Naumov & Stepanjants, 1972: 33, 35, fig 1; Stepanjants, 1979: 13, pl. 1, fig 7; Cerrano et al., 2000: 488 –493, figs 2–5; 2001: 577–580; Galea & Schories, 2012: 2, pl. 1A–E, fig 2A–C; Peña Cantero et al. 2013: 737 –739, figs 3g –h, 4a–c.

Material examined. Belgica Antarctic Expedition 1897–1899: labelled “cotype” (RBINS 539798 INV 38500), (nº 1059), 28.V.1898, 71°19' S, 87°37' W (Bellingshausen Sea), 400 m, c. 22-mm-long colony fragment, with gonophores.

Description. Colony growing along three sponge spicules, forming a sort of cylinder of perisarc filled with coenosarc; structure very soft.

Polyps white. Gastrozooids and gonozooids present. No dactylozooids. Gastrozooids of varied size (longest c. 1 mm, usually smaller, between 330 and 390 µm) and with eight to ten filiform tentacles arranged in a crown near the middle of the polyp (Fig. 7 A–C). Hypostome relatively large, finger-shaped (Fig. 7 A–C).

Gonozooids much smaller, from 140 to 250 µm high, and provided with up to six filiform tentacles (Fig. 7 D– G); gonozooids with fewer tentacles or even completely devoid of tentacles also present. Gonozooids with up to five tiny (from 50 to 70 µm in diameter), rounded gonophores at base (Fig. 7 D–G).

Some blunt spines present (Fig. 7 H).

Measurements (in µm). Cnidome: Type I [range 7.0–10.0 x 2.0–3.0, mean 8.6±1.0 x 2.7±0.5 (n=10); ratio, range 2.3–4.0, mean 3.3±0.6 (n=10)], Type II [range 11.0–15.0 x 2.5–4.0, mean 12.7±1.4 x 3.5±0.6 (n=6); ratio, range 2.8–4.8, mean 3.8±0.9 (n=6)], desmonemes [range 5.0–6.5 x 2.0–2.5, mean 5.8±0.5 x 2.2±0.2 (n=5); ratio, range 2.4–3.3, mean 2.7±0.3 (n=5)].

Remarks. I have only examined part of the material studied by Hartlaub (1904), in particular the c. 20-mmlong fragment that according to him formed part of the material labelled 1059.

The preservation in ethanol has made nematocysts barely distinguishable, so that the information related to the cnidome must be taken with caution. Nevertheless, there are apparently three types of nematocysts.

Our study has confirmed Hartlaub’s description. The most remarkable features for the species are the relatively long hypostome, the exceedingly small size of the gonozooids and the rhizocaulomic growth, i.e., having erect hydrorhiza formed by a bunch of parallel stolons. Hartlaub (1904) already indicated that the polyps are provided with long hypostome and that the gonozooids are very tiny in spite of having well-developed tentacles and proboscis, the largest with c. five tentacles and up to five gonophores forming a crown at the polyp's basal part. Hartlaub also remarked the scarcity and low-density of gastrozooids and the relatively high number of gonozooids. He also pointed out the presence of contracted hydranths with an almost flat oral disc, the absence of dactylozooids and the presence of blunt spines.

As indicated above, H. angusta also seems to be characterized by the colony structure. Apparently, apart from forming creeping colonies, it can also give rise to rhizocaulomic stems. Unfortunately, I could not confirm this point with the material examined, but Hartlaub was convinced that part of his material clearly showed that type of growth.

Briggs (1938: 10) assigned to this species material saying that ‘the gastrozooids and the skeletal features agree very closely with Hartlaub's description and figures’, but without giving description or figures, so that it is not possible to define its systematic position.

Cerrano et al. (2000) studied the trophic ecology of a species they identified as H. angusta by the shape of the gonophores arranged in a single ring at the base of the gonozooids, and by the particularly small size of these gonozooids. They, however, pointed out that their material differs from Hartlaub’s in the absence of spines and the presence of dactylozooids, but they concluded that, as the presence of spines and dactylozooids is related to environmental conditions and the life-cycle of the polyps (Braverman 1960; Cerrano et al. 1998), those features are not suitable as taxonomic characters. Peña Cantero et al. (2013) also assigned to H. angusta material from the same area and certainly conspecific with that studied by Cerrano et al. (2000).

Galea & Schories (2012) assigned to this species material having several characters in common, such as the presence of spines and the dwarf gonozooids (less than 500 µm high), with one to five tentacles, and with four to six basal, rounded gonophores. It differs, however, in simply forming an encrusting mat on worm tubes (there is no rhizocaulomic form), in the size of the gastrozooids (up to 5 mm high in fixed material, notably longer in life) and in the shape of the polyps: those of H. angusta, as indicated above, have a very large, finger-shaped hypostome, so that the crown of tentacles is placed in a relatively low position. In addition, H. angusta has gastrozooids with a distinctly lower number of tentacles [eight to ten vs 12–14 in Galea & Schories (2012)], the spines in the material by Galea & Schories (2012) are often continuous with each other and are also provided with low ridges of perisarc and the gonophores carry three eggs; neither Hartlaub (1904) nor myself were able to observe eggs in the gonophores.

In all those previous records, including also those by Naumov & Stepanjants (1972) and Stepanjants (1972, 1979), there is no sign of rhizocaulomic growth; colonies always have a lamellate hydrorhiza. The material studied by Galea & Schories (2012) and Peña Cantero et al. (2013) had gonozooids with gonophores. Notwithstanding the tiny size of gonozooids and gonophores in that material (cf. Table 1), it is distinctly smaller in the type material of H. angusta. Galea & Schories (2012) and Peña Cantero et al. (2013) also studied the cnidome, finding it composed of microbasic euryteles and desmonemes (cf. Table 1). I found desmonemes and two other putative types of unidentifiable nematocysts in the type material of H. angusta, one of these distinctly larger than those found by Galea & Schories (2012) and Peña Cantero et al. (2013) (cf. Table 1). The material studied by Peña Cantero et al. (2013) also has dactylozooids and lack spines.

From the other records of species of Hydractinia from Antarctic waters, only Hickson & Gravely (1907), as Hydractinia dendritica, and Peña Cantero & Ramil (2006) and Peña Cantero (2014b), as H. angusta, reported the presence of erect, rhizocaulomic colonies. For H. dendritica see below when dealing with this species. Concerning the material studied by Peña Cantero & Ramil (2006) and Peña Cantero (2014b), there is no doubt about the formation of erect colonies not developing on any other organism. As Peña Cantero (2014b) indicated, this species usually starts development by spreading on a substrate (e.g. hydrocoral, hydroid stem), later on developing erect, irregularly branched stems. However, in that material there were putative dactylozooids and the spines are similar to fin rays. In fact, some of them are joined together with perisarc, giving the appearance of fish fins. All structure is irregular, forming mountain-like ridges.

It is not clear if all the material previously assigned to H. angusta actually belongs to it. According to the original description, H. angusta is characterized by the tiny gonozooids, the relatively large hypostome, the absence of dactylozooids, the presence of isolated spines and the possibility of rhizocaulomic growth. With these characteristics, none of the previous records will actually belong to Hartlaub’s species. Alternatively, however, if all those previous records were considered valid, we would be dealing with a species with a large morphological plasticity. Future findings might clarify this question.

Ecology and distribution. Hydractinia angusta has been found at depths from three (Stepanjants 1979) to 922 m (Peña Cantero & Ramil 2006), although the only reliable record is from 400 to 500 m (Hartlaub 1904). Epibiotic on hydroids and other invertebrates (cf. Peña Cantero & Ramil 2006; Peña Cantero et al. 2013).

The only reliable record is that by Hartlaub (1904), from the Bellingshausen Sea (see discussion above). If future studies confirm all previous records, H. angusta would have a circum-Antarctic distribution (Stepanjants 1979): it has also been reported from the South Shetland Islands area (Peña Cantero & Ramil 2006; Galea & Schories 2012), in West Antarctica, and from off King George Land (Briggs 1938; Stepanjants 1979), Adélie Land (Naumov & Stepanjants 1972), the Ross Sea (Totton 1930; Stepanjants 1979; Peña Cantero et al. 2013), the Davis Sea (Stepanjants 1972, 1979; Peña Cantero 2014b) and the Sodruzestva and the Cosmonauts seas (Stepanjants 1979), in East Antarctica.