Obelia spongicola, Watson, Jeanette E., 2011

Watson, Jeanette E., 2011, New species, new records and redescriptions of Thecate hydroids (Cnidaria: Hydrozoa: Leptothecata) from Southern Australia, Zootaxa 3122, pp. 1-36 : 28-30

Obelia spongicola sp. nov.

Fig. 13 A–F

Material examined. Holotype, NMV F171370, St Leonards pier, Port Phillip, preserved colony and microslide from colony on soft yellow sponge, depth 2 m, coll: J.Watson, 26/03/2008. Paratype, NMV F171371, colony on sponge, alcohol preserved, and one microslide, St Leonards pier, on digitate orange sponge, depth 2 m, coll: J. Watson, 11/11/2009.

Description. Hydrorhiza creeping on sponge; stolons tubular, rugose to smooth, spicules of the sponge embedded in the hydrorhizal perisarc but not penetrating coenosarc.

Colony a mixture of simple pedicels interspersed with taller erect stems to 4 mm high. Pedicillate stems short, variable in length with a single distal hydrotheca; stems deeply ringed below hydrotheca, typically with 6–8 annulations or sometimes annulated throughout. Erect stems straight to weakly sympodial, base of stem with several deep annulations, following internodes narrowly cylindrical, each with up to 10 proximal annulations. Hydrothecal pedicels of erect stems arising alternately at an acute upward angle to internode from a distal thickening of internode; pedicels variable in length but always shorter than cauline internode, typically annulated throughout.

Hydrotheca long and slenderly conical, perisarc thin and smooth, very thin at margin. Basal chamber deep, almost rectangular, diaphragm thin, visible as a concave ring of tissue supporting the hydranth. Margin of hydrotheca circular, fragile, with 6–10 sharp bimucronate cusps separated by shallow embayments. Hydranth with 8–10 long tentacles.

Gonotheca borne on hydrorhiza among pedicellate stems; erect stems on an annulated pedicel in axil between stem internode and hydrothecal pedicel. Gonotheca narrowly top-shaped to conical, walls smooth, thin; apex a broadly cylindrical neck, aperture closed by a thin sheet of tissue. Gonophore with 4–5, sometimes up to 8 developing medusae.

Cnidome comprising nematocysts of one size: microbasic mastigophores (see Bouillon et al. 2006) capsule 8– 9 x 2–3 µm, shaft 5–8µm, tubule to 85 µm long.

Medusa typically obeliate at release, tiny and fragile with 24 transparent equidistant tentacles and a goldenbrown manubrium.

Colonies translucent white.

Remarks. Obelia spongicola resembles Obelia bidentata but is distinguished from that species by its smaller size and fragile, strictly monosiphonic colonies and smaller hydrothecae.

The species is very common in Port Phillip where it grows on several species of soft-textured yellow and orange sponges (? Clathria spp.). It occurs in sheltered shallow water habitat of jetty pilings and reef. The colonies are very fragile and easily brushed from the sponge and can only be successfully collected by sealing samples of the sponge and hydroid in situ in a container. The undersides of the stolons retain rows of embedded needle-shaped sponge spicules. Colonies are present in the warmer months of the year becoming fertile in late summer (March) with water temperatures of 18 to 20º C.

Obelia spongicola was reported by Watson (2003) from the Isles of St Francis, South Australia, as Obelia spinulosa Bale, 1894 .

Etymology. The species is named for its association with sponges.

Distribution. Port Phillip, Victoria and South Australia. The species is probably more widely distributed in suitable habitat in southern Australia.

Hydrorhiza, diameter 52–60

Stem, internode length 640–1160

Pedicel

length to basal hydrothecal segment 120–400 diameter 44–48 basal section, length to diaphragm 44–76

Hydrotheca

length, margin to diaphragm 300–360 width at diaphragm 60–76 diameter at margin 160–184 height of cusp 20–24 width of saddle 24–26

Gonotheca

length of pedicel 80–120 length excluding pedicel 600–680 maximum width 192–256 width of orifice 124–152

Figures

FIGURE 13. Obelia spongicola sp. nov. A, fertile colony on sponge. B, fertile stem internodes. C, hydrotheca with castellate margin and diaphragm. D, pedicellate hydrotheca and gonotheca on hydrorhiza. E, hydrorhiza with embedded sponge spicules. F, microbasic mastigophore. Scale bar: A, B, 1 mm, C – E, 0.5 mm, F, 10 µm.