Hydroids of Australia

Stems to 15 mm high, arising from a hydrorhiza reptant on worm tubes, stolons tubular.

Colony lax, monosiphonic, no discernible main stem, branching upwards from basalmost internode; branching regularly bifid or trifid. Branch segments (pedicels) with 8 -10 close, deeply incised basal annulations, pedicels thereafter long, smooth, cylindrical, with five to six distal annulations; some shorter trifid pedicels entirely annulated.

Basal chamber of hydrotheca seated directly on distalmost annulation, deep, walls slightly concave, terminating in a thin saucer-shaped diaphragm; body of hydrotheca above diaphragm cup-shaped to almost hemispherical; margin with 14 low, shallowly scalloped rounded cusps, embayments between inverse of cusps.

Hydranth large, with 20-24 tentacles.

Perisarc of colony thin, thickest on branch segments, thinner on hydrotheca, marginal cusps thin.

Colour: transparent white. less ↑

Holotype: NMV F171352, infertile colony on dead bryozoan on floating pontoon, depth 0.3 m. Clifton Springs boat harbour, Port Phillip, Victoria, Australia.

Paratype: NMV F171353, infertile colony on serpulid tube, on floating pontoon, depth 0.2 m. Clifton Springs boat harbour, Port Phillip, Victoria, Australia. NMV F171354, infertile colony on mussel shell, depth 3 m. St Leonards pier, Port Phillip, Victoria, Australia. All material ethanol preserved. less ↑

Holotype; NMV F171361, microslide, male colony, on reef, depth 8 m, North Arm Channel, Western Port, Victoria, Australia.

Paratypes: NMV F171362, microslide and remaining preserved material, infertile colony on sponge, depth 3 m. St Leonards pier, Victoria, Australia; NMV F171363, microslide and remaining preserved material. On sponge, depth 2 m, St Leonards pier, Victoria, Australia; NMV F171374, microslide, female colony, on Caulerpa, depth 2 m. Port Welshpool, Victoria, Australia. less ↑

The taxonomic history of this species has been long confused. For further details, see:

Watson, J.E. 2005. Hydroids of the Archipelago of the Recherche and Esperance, Western Australia: Annotated list, redescription of species and description of new species. pp. 495-612 in Wells, F.E., Walker, D.I., Kendrick, G.A. (ed). The Marine Flora and Fauna of Esperance, Western Australia. Perth : Western Australian Museum less ↑

P. meretricia, shows considerable variability of features usually regarded as reliable specific criteria among the Plumulariidae. The younger stems have a smooth glassy appearance, with long stem and hydrocladial internodes, and also have cauline nematothecae. The stems seldom retain their cauline nematothecae after maturity, showing only scars where the nematothecae have dropped off. Since many of the younger stems show neither scars nor nematothecae, and in some cases, the axillar nematothecae have failed to develop as well, the presence or absence of these structures cannot be regarded as diagnostic of the species. The mamilliform pore is however, a constant feature of all the stems. The regenerated athecate hydrocladial internodes common in some stems, mark the site of repeated breakage and regrowth of the hydrocladium. In these cases, a short athecate internode is first added, followed by an embryonic hydrotheca and infrathecal chamber. Nematothecae bud off later as the hydrotheca nears maturity. The athecate internode at this stage is without internal perisarcal ridges. These develop as the hydrocladium ages. less ↑

Stems to 2 cm long, fascicled, irregularly branched, with 2-3 supplementary tubes extending two thirds the distance up stem.

Hydrophore with a few reduplications.

Nematothecae rare.

Gonothecae are smooth or very slightly annulated, with curved or straight pedicels, and several have a slight constriction just below the truncated distal end.

Colour: light greenish, with black patches scattered throughout hydrocaulus. (Under the microscope these patches are black granules concentrated on the hydranth and in the coenosarc. less ↑

Stems plumose, arising from a ramified hydrorhiza; stolons tubular, perisarc very thick, internally longitudinally striated.

Hydrocaulus tubular, becoming narrower distally, divided into internodes of variable length by 4-5 deeply indented transverse nodes; hydrocladia distal on internodes, gracefully recurved from stem, up to eight annulations at junction with stem; internodes same as those on stem, one hydranth given off on a short expanding pedicel on upper, distal side of each internode; pedicel indistinctly ringed at base, perisarc thinner than on hydrocladium.

Hydranths clavate to pyriform, terminal hydranth on each hydrocladium usually larger than others, all hydranths with a whorl of 10-14 semi-filiform aboral tentacles and 14-16 scattered pedunculate capitate tentacles on upper body, aboral tentacles with a longitudinal band of nematocysts on outward facing side, peduncle of oral tentacles long, transversely wrinkled, capitulum richly armed with nematocysts.

Cnidome:

i) Stenoteles, capsule ovoid, 45 -46 x 27-30 µm, shaft 58-60 µm, 10 µm wide armed with four or five curved spines 30 µm long spines and spirals of fine bristles, tubule at least 180 µm long; abundant in capitate tentacles.

ii) Stenoteles, capsule ovoid, 37 x 25 µm, shaft 60 µm, tubule at least 60 µm, in capitate tentacles; none discharged, not common.

iii) Stenoteles, capsule ovoid, 15- 18 x 13- 17 µm, shaft 15-20 µm; in capitate tentacles.

iv) Stenoteles, capsule ovoid, 6.5-10 x 4-7 µm shaft 8 µm, very abundant; in aboral and capitate tentacles, few discharged.

v) Desmonemes ovoid to pyriform capsules, 6-8 x 4-4.5 µm, in aboral tentacles, only one seen discharged.

vi) ?Haploneme, capsules small bean shaped, 9- 12 x 3-6 µm; abundant in aboral tentacles, none discharged.

vii) Capsule ovoid narrower at anterior end, 12.5-15 x 6.5-7 µm with inclusion body; in aboral tentacles, none discharged.

viii) ?Isorhiza capsule almost spherical 10 x 7 µm, site unknown, several partially discharged, tubule with very fine spines. less ↑