Phylum Mollusca
Class Gastropoda
Informal group Opisthobranchia
Order Acochlidea
Common names: freshwater slugs, acochlideans
Overview
Acochlidia are a small group of poorly known aquatic slugs. Most acochlidian species are marine, however, some occur in brackish or freshwater. Acochlideans are characterised by their uniform worm-like body shape, the lack of external gills and shell, the reduction of foot, body pigments and eyes and by the formation of a visceral sack. The head bears one or two pairs of head tentacles located near the base of the dorsal most pair of tentacles. Marine species are tiny (approx 1-5 mm in length), however most of the freshwater species are considerably larger, up to 35 mm in length.
Distribution and diversity
Around 27 species of Acochlidea have been described worldwide, most of which are marine. A small number are freshwater species are known from islands in the Caribbean, Indonesia, Palau and the South Pacific (Solomon Islands and Fiji). They have not been recorded in Australia or New Zealand.
Life cycle
Acochlideans can be simultaneous hermaphrodites, sequential hermaphrodites or have separate sexes. Little is known about the specific details of the life-cycle of freshwater acochlideans but marine opisthobranchs typically deposit eggs in transparent or semi-transparent, elaborate string-like capsules on appropriate surfaces. Development typically involves a pelagic larval stage.
Feeding
Few details are available about the feeding habits of freshwater Acochlidea though it is likely they feed on algae and detritus among the sediments.
Ecology
Most acochlidian species are marine, living interstitially living amongst sand grains. There is little known of the ecology of freshwater Acochlidea other than they are only found from clear flowing streams, swamps and pools.
Acochlidium sp., common in the Indo-West Pacific
Image credit: � Bastian Brenzinger,
used with permission
Strubellia wawrai from the Solomon Islands
Image credit: � Bastian Brenzinger,
used with permission
Acochlidea from Vanuatu
Image credit: � Timea Neusser,
used with permission
