Hydranthea margarica (Hincks, 1862)

Colony stolonal, hydrorhiza creeping, stolons tubular, walls thin, fairly smooth with a thin coating of adventitious material. Hydrophores small, vasiform, smooth, arising at irregular intervals along hydrorhiza, variable in length but typically short.

Hydrotheca shallow saucer-shaped, expanding from a transverse diaphragm to margin, rim sometimes everted. Secondary hydrophores rare but when present arising from diaphragm of primary hydrotheca. Column of hydranth tall, slender, expanding just below tentacles; c. 30 tentacles in a single whorl, long and slender, surrounding a dome-shaped hypostome; tentacles connected at their bases by a conspicuous intertentacular web; two or three large nematocysts on web between each set of tentacles.

Gonophores borne on hydrorhiza, on a short pedicel, ovoid in shape, with an aureole of large nematocysts; contents too immature for more detailed description.

Cnidome comprising mastigophores of two categories:

i) macrobasic mastigophores, capsule banana-shaped, 39–42 x 8–9 μm, shaft thick, with a row of large spines at c. 100 μm above capsule, tubule long; in tentacle web, hydranth column and hydrorhiza.

ii) microbasic mastigophores, capsule elliptical, 8–9 x 3 μm, shaft 6–8 μm, tubule fairly short, grouped in circlets in tentacles.

Perisarc thin throughout.

Colour: Hydranth body pale gold with a narrow white band below hypostome, tentacles colourless.