Hydractinia gelinea, Watson, 2015

Watson, Jeanette E., 2015, Five athecate hydroids (hydrozoa: anthoathecata) from south-eastern australia, Memoirs of Museum Victoria 73, pp. 19-26 : 19-20

Hydractinia gelinea sp. nov.

Figure 1A−D

Material examined. NMV F202870 , holotype, female colony, Crawfish Rock, Western Port , Victoria, coll: J. Watson, 24/04/2006, depth 10 m; material fixed in 5% formalin, later transferred to 70% ethanol .

Description. Colony comprising individuals and clusters of female polyps on a dead crustose bryozoan; no gastrozooids or dactylozooids present. Hydrorhiza ramified, firmly adherent to substrate, stolons narrow, tubular, perisarc thin and smooth.

Gonozooids sessile, robust, with a whorl of 8−12 thick tentacles surrounding a prominent dome-shaped hypostome; tentacles with prominent whorls of nematocysts. Hypostome high dome-shaped. Gonophores fixed sporosacs borne in tight clusters of up to 15 on gonozooid well below tentacles. Immature female gonophore pyriform, containing many small ova, mature gonophore balloon-shaped to spherical, seated on a cushion−shaped pad on a short peduncle and enclosed in a thick gelatinous pellicle, surface of gonophore with abundant large scattered nematocysts.

Nematocysts, probably euryteles of two sizes; none discharged:

(i) capsule bun-shaped, 18−21 x 9−10 µm, on gonophores and body of gonozooid,

(ii) capsule ovoid, 9−12 x 6−7 µm, on gonozooid tentacles.

Colour (recently preserved material): stolons yellow, gonozooids and gonophores flesh pink.

Remarks. The colony is clustered in the pores of a dead bryozoan Celleporaria sp. , some polyps encroaching onto a small sponge growing on the bryozoan. No encrusting hydrorhizal mat or dactylozoids were found. The thick ectoderm enclosing the gonophore obscures the position of the spadix, masking its structural details. The immature female gonophore contains numerous small ova c. 40 µm in diameter.

There about 130 nominal species of Hydractiniidae , mostly recorded from the northern hemisphere, although many are inadequately described ( Miglietta et al. 2009). The only presently known Hydractinia from south-eastern Australia is H. betkensis ( Watson 1978) .

Choice of substrate is important in the Hydractiniidae , encrusting colonies often being associated with mobile substrata while reticulate colonies (such as H. gelinea ) are usually associated with immobile substrata ( Miglietta and Cunningham 2012). Other known associations are with pebbles, barnacles, sponges and other hydroids ( Schuchert 2008) but there are no records of association with bryozoans.

As the nematocysts were not discharged their exact identity could not be determined, but they are probably euryteles; no desmonemes were seen. Euryteles and desmonemes are known to comprise the cnidome of Hydractinia and these were recorded in Hydractinia novaezelandiae ( Schuchert, 1996) and Hydractinia rubricata ( Schuchert, 1996) from New Zealand.

Etymology. named for the cushion-like gelatinous pellicle supporting the sporosac.