Acochlidiacea
Acochlidiacea, common name acochlidians, are a taxonomic clade of very unusual sea snails and sea and freshwater slugs, aquatic gastropod mollusks within the large clade Heterobranchia. Acochlidia is a variant spelling.
These are mostly very small animals, without a shell or gills, distinguished by the visceral mass being sharply set off from the rest of the body.[4]
Being a small group with only 47 species worldwide known in 2023,[5] this group has been the subject of active research since 2010, by which date only 32 species were named.[6] These slugs are morphologically and biologically highly aberrant and diverse, comprising a series of unusual characters (e.g. secondary gonochorism, lack of copulatory organs, asymmetric radulae).[7] Most acochlidians live interstitially in marine sands, while some have conquered limnic systems (uniquely within opisthobranch gastropods).[7]
Nils Hjalmar Odhner established this taxon as a family in 1937, when he created the families Microhedylidae and Acochlidiidae.[1][2] In 1939, he treated this taxon as an order.[2][8]
Three families (Hedylopsidae, Microhedylidae and Acochlidiidae) are classically recognized.[2][3] Two controversial classifications (Rankin 1979,[9] Starobogatov 1983)[16] have been proposed recently, but they have not been evaluated since.[2][3]
An alternative classification by Burn (in Beesley et al., 1998) for the Australian species recognizes 2 superfamilies and 5 families.[2][3]
f – foot
hb – heart bulb
lt – labial tentacle
rh – rhinophore
vh – visceral hump.
