Hydractinia sp.

Watson, Jeanette W., 2003, Deep-water hydroids (Hydrozoa: Leptolida) from Macquarie Island, Memoirs of Museum Victoria 60 (2), pp. 151-180 : 152

Hydractinia sp.

Figures 1A–C

Material examined. Stn 44, TM K2768, a small infertile colony on axis of dead primnoid gorgonian, spreading onto Eudendrium stem, specimen alcohol preserved.

Description. Hydrorhiza consisting of anastomosing stolons embedded in a fine trabeculate meshwork of perisarc to 1.4 mm thick; meshwork penetrated and in some places externally covered by coenosarc. Mature gastrozooids columnar, stout, emerging from perisarcal meshwork; hypostome dome-shaped, surrounded by c. 12 stubby, slightly tapering tentacles (preserved material); tentacles often alternately long and short. Tentaculozooids styloid, tapering, very extensible, arising from a broad base in perisarc meshwork. Bases of gastrozooids and tentaculozooids may be surrounded by a collar of coenosarc or erupt from a shallow pit in perisarc. Many short zooids bearing 4 or 5 tentacles just emergent above basal meshwork. No solid spines present but instead, hollow mounds of perisarc common, circular in cross-section and invested by the perisarcal meshwork.

Nematocysts of two sizes present, probably euryteles, none discharged: (i) bean-shaped, 11–12 X 4–5 µm, in body of gastrozooid and in tentaculozooid; (ii) elongate ovoid, 8–10 X 4–5 µm, in tentacles of gastrozooid.

Gonozooids absent.

Colour. Gastrozooids and tentaculozooids white, basal perisarcal meshwork shining golden-yellow.

Remarks. The hydrorhizal meshwork is composed of chitinous strands surrounding subcircular holes which vertically penetrate the meshwork. No structures which could be identified as spines occur, the scattered mounds and peaks being hollow protrusions of the perisarcal meshwork

Apart from Millard (1975) and Schuchert (1996) few authors have adequately described the cnidome of Hydractinia . Such knowledge would undoubtedly lead to easier identification of species. In the present formalin-preserved sample the nematocysts are undischarged and cannot be further identified.

Small zooids with four or five tentacles among the larger gastrozooids are probably developing gastrozooids; other unstructured white rings flush with the surface of the hydrorhizal meshwork may be introverted gastrozooids. The finely structured hydrorhizal meshwork investing the hydroid host bears a remarkable resemblance to the ectoderm of Solanderia . Despite careful search and examination of sections of perisarc, no gonozooids were found.

Stepanjants (1979) gave a key to the following Antarctic species of Hydractinia : H. antarctica Studer, 1879 , H. angusta Hartlaub, 1904 , H. clavata, Jäderholm, 1905 , H. parvispina Hartlaub, 1905 , H. pacifica Hartlaub, 1905 and H. vallini Jäderholm, 1926 . The absence of basal spines in the perisarc considerably narrows the possible matches to known species of Hydractinia , including those of the wider concept of Hydractinia of Bouillon et al. (1997) and Boero et al. (1998). The present specimen most closely resembles H. angusta but in the absence of reproductive structures it cannot be confidently assigned any of these species nor described as new.

Figures

Figures 1A–C. Hydractinia sp., stn 44. A, part of colony. B, trabeculate meshwork of basal perisarc. C, nematocysts, possibly euryteles, undischarged. Scale bar: A, 1 mm; B, 0.1 mm; C, 10 µm.