Hydrodendron daidalum (Watson, 1969)

Hydrorhiza: a reticular network, very wide, flat and ribbon-like, strongly attached to the seaweed on which it grows; divided into a central inner canal and wide outer flanges. Distinct, close, dark coloured spine-shaped markings transverse between outer and inner margins of flange. Hydrorhiza widening towards base of each hydrothecal pedicel.

Stem: simple, short, erect, arising mainly at the junctions where the stolons cross: unbranched, widest at base, tapering distally, passing into hydrophore with a small constriction at narrowest part. Perisarc of stem thick, heavily ridged internally and three or four very deep oblique external constrictions dividing stem into segments, the largest at the base , becoming progressively smaller distally, the last supporting the hydrophore . Stem bending in a gentle curve toward distal end, bringing plane of margin of hydrophore almost perpendicular to hydrorhiza.

Hydrophore: small, bowl shaped, asymmetrical, the abcauline side usually a little longer than the adcauline side; expanding in a sinuous curve to an everted circular margin. Diaphragm present, extending across base of hydrophore in a shallow curve frequently not well seen, but position marked by a shoulder-like ledge and distinct notch in perisarc on the abcauline side and sometimes on adcauline side. No reduplication of margin; punctae not present; no secondary or tertiary hydrophores developed.

Hydranth: body thick, too large to fit retracted into hydrophore. Only slightly constricted below tentacles; tentacles stubby, 20 to 24 set in a circlet about a wide hypostome. A fringe of "lenticular nematocysts" packed about four deep surrounds hypostome at base of tentacles.

Nematothecae: small, elongate urn shaped, with slightly everted margin, arising from flange of hydrorhiza usually under curve of stem directly below hydrophore, only present in younger parts of colony. Nematophore very extensile, capitate head set with nematocysts.

Gonotheca: large, obovate, smooth, somewhat flattened, tapering proximally into a short, slightly bent pedicel arising from the hydrorhiza at the base of the hydrothecal pedicel below curve of stem. Width increasing to distal end; truncated by a slightly convex cap; no orifice, the contents emptied by rupture of the distal end. Male and female gonotheca same shape and size, borne on same colony.

Gonophores: Mature colonies bear gonangia thickly. Gonophores tend to grow in groups of the one sex, males outnumbering females in colony by at least five to one. Mature male gonophore comprises a mass of spermatogenic tissue almost filling gonotheca, supported by a blastostyle which expands in a cap across the distal end. Mature female gonophore composed of a cluster of 11 to 15 creamy coloured eggs with large yolk spots, almost filling gonotheca, surrounded by a thin blastostyle which spreads in a pinkish mass across distal end. Mature gonophores, December to January, Vincent Gulf (water temperature 22°C).

Colour: perisarc pale straw colour, hydranths white.