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Hydatina physis Linnaeus, 1758

Description: Shell light in weight, inflated, spire slightly sunken so that apex is truncated. Shell surface smooth, glossy, or irregularly dented in large shells. Aperture wide, outer lip thin and sharp, inner lip with thin white glaze on parietal wall. Colour white with numerous black or dark brown spiral bands of varying width.

Size: NSW specimens up to 55 mm in shell length.

Distribution: Circum-global in tropical and temperate waters. In eastern Australia , as far south as eastern Vic.

Habitat: Intertidal and shallow subtidal. The animal is a specialised worm feeder, eating only cirratulid polychaete worms. In NSW the species is most frequent on seagrass flats but it also occasionally encountered on exposed rocky shores at low tide. Uncommon in NSW.

Remarks: The animal is spectacular (Fig. 2), with purple folds (enlarged head shield, metapodium and foot) much wider than the shell, edged with almost luminous blue. The Sea Slug Forum provides more illustrations of the animal (http://www.seaslugforum.net/find/hydaphys). 

The anatomy of this species was described by Rudman (1972).

Fig. 1: Narooma, NSW (C.107520).

Fig. 2: Live animal. Inscription Point, Kurnell, NSW, 2009. (Photo Jack Hannon).