Bimeria australis

Maximum length of stems 4 mm, branching irregularly alternate, a single hydranth usually terminal on branch. Stem and branches may be annulated or wrinkled, the annulations being more distinct proximally, but parts of stems, particularly those from deeper water, may be smooth.

Hydranths not distinctly set off from stem, but this is frequently difficult to observe due to infolded habit of tentacles when preserved, and the heavy coating of foreign matter on and below the hydranth. The proximal half of the tentacles of preserved specimens is always obscured by adherent agglutinated material, but close-up underwater photographs show that in life the tentacles are capable of considerable extension beyond this protective sheath.

Gonophores arising singly on a short pedicel from stems, branches and hydrorhiza. Mature gonophores 0.2—0.28 mm diameter, globular or sometimes slightly flattened in section, covered in a thick gelatinous envelope heavily coated in foreign material. Male and female gonophores fixed sporosacs, borne on same stems, female with a bright red spadix in the proximal region supporting a single egg which develops into a planula in situ. Male gonophore spherical; radical canals and tentacles rudiments absent.

Nematocysts rare in hydroid, of three types:

— Large stenoteles, capsule oval, 7 x 5 5—1 0 x 9 µm, butt about half the length of capsule.

— Smaller stenoteles, capsule spherical, very rare, 3 µm in diameter.

— Microbasic mastigophores, capsule bean-shaped, 7 x 4.5 µm in length, tube long.

The stenoteles are concentrated in the tentacles and the mastigophores are scattered throughout the coenosarc of the hydrocaulus.

Colour: The colonies are uniformly buff coloured from the adherent matter, but parts of the hydranths visible are pinkish and tentacles white (from colour photographs in situ), gonophores white, spadix of female bright red. Specimens from deeper water are more uniform pink in colour than those from shallow water.

Nematocysts are extremely rare in B. australis. Possibly the protection afforded to the hydranths and gonophores by the thick encrustation of foreign material obviates the necessity for production of large concentrations of nematocysts.

Western and southern Australia (South Australia, Tasmania, Victoria, Western Australia).

Subtidal, epizoic on bryozoans, ascidians, sponges and other benthic fauna.