Ectopleura exxonia ( Watson, 1978 )

Watson, Jeanette E., 2015, Five athecate hydroids (hydrozoa: anthoathecata) from south-eastern australia, Memoirs of Museum Victoria 73, pp. 19-26 : 23-24

Ectopleura exxonia ( Watson, 1978)

Figure 4A−H

Ectopleura exxonia Watson, 1978: 303 , figs 1A, B.

Material examined. NMV F202873 , Crib Point petroleum wharf, Western Port , Victoria, coll: J. Watson, 23/04/2006, depth 2 m; abundant fertile colonies on mussel ( Mytilus galloprovincialis ) shells. Hardened in 4% formalin later transferred to 70% ethanol . NMV F203425 , from ropes at salmon farm, depth 5 m, Tory Channel , Marlborough Sounds, South Island, New Zealand , coll: J. Atalah, Cawthron Institute, Nelson, New Zealand .

Description (from preserved material, Western Port). Colonies growing thickly on mussel shells, hydrorhiza of matted closely ramified stolons. Stems crowded, to 80 mm long, unbranched but often entangled, several basal annulations on hydrocaulus and groups at intervals along stem, some stems completely annulated; perisarc smooth, firm. Hydrocaulus circular in section, with two internal longitudinal canals, one central inside the other. Distal end of hydrocaulus a shoulder surmounted by a short cylindrical section with thinner perisarc, a narrow circular indentation below hydranth, base of hydranth saucer−shaped. Hydranth with a single whorl of 12–15 short, thick oral tentacles surrounding a large hypostome and one whorl of 16–18 long, slender aboral tentacles.

Gonophores fixed sporosacs in various stages of development borne in tight clusters on short unbranched blastostyles at base of aboral tentacles. Gonophore spherical at maturity with a bun−shaped distal cap with emerging larval tentacles.

Cnidome comprising nematocysts in two categories:

(i) stenoteles, capsule spherical, diameter 11−13 µm, shaft wide; on hypostome and oral tentacles, a few on aboral tentacles; some discharged.

(ii) stenoteles, capsule spherical, diameter 5−7 µm, shaft wide; abundant on aboral tentacles, some on oral tentacles; a few discharged.

(iii) heteronemes, capsule 10−11 x 5−7 µm, abundant in tentacles and cauline coenosarc; undischarged.

Hydranths and gonophores rose pink, stems shining white, tentacles white.

Remarks. The holotype ( NMV G2801) of Ectopleura exxonia ( Watson, 1978) was a small sample collected from an oil production platform at a depth of 75 m in Bass Strait. The present abundant material permits additional description.

The hydrocaulus of E. exxonia was originally described as having four longitudinal internal canals. Examination of the present material shows however, there are only two canals − an outer perisarc-covered caulus and a single internal cylindrical canal passing up the centre of the stem.

The somewhat smaller size of the hydranths and gonophores of the Crib Point material than in the holotype may be due to the Crib Point colonies being less than eight weeks old, dating from the time of deployment of clean mussel substrate to the time of retrieval. Water temperature at deployment was 20°C declining over the immersion period to 15°C.

Schuchert (1996) predicted that E. exxonia would occur in New Zealand. Recent sampling at a salmon farm at a depth of 5 m in Tory Channel, Marlborough Sounds, South Island, New Zealand has confirmed his prediction, revealing well established colonies of E. exxonia growing on farm nets.