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.
Being a small group with only 30 species worldwide known in 2010, and 32 species described in 2011, and 33 in 2012 (+9 undescribed Pontohedyle species), 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). Most acochlidians live interstitially in marine sands, while some have conquered limnic systems (uniquely within opisthobranch gastropods).
Burn in Beesley et al. (1998), wrote this taxon as the order Acochlidea.
Three families (Hedylopsidae, Microhedylidae and Acochlidiidae) are classically recognized. Two controversial classifications (Rankin 1979, Starobogatov 1983) have been proposed recently, but they have not been evaluated since.
The Acochlidia, a traditional "order" of the Opisthobranchia since their establishment by Odhner have formed one of the unsolved mysteries within Euthyneura. Their monophyly is widely accepted especially since a proposed sister group relationship of the acochlidian family Ganitidae with Sacoglossa (based on the dagger-shaped radula teeth) could be rejected based on a comprehensive parsimony analysis of morphological characters. During the last years a series of studies have redescribed key acochlidian taxa in great detail, including 3D reconstructions, and added considerably to the morphological and biological knowledge of this previously little understood group.
Most recent morphological analyses suggested a common origin with either the equally enigmatic Rhodopemorpha, the diaphanid cephalaspidean Toledonia, or with runcinid or philinoid cephalaspideans. Molecular markers independent from direct ecological pressures suggested an unresolved basal opisthobranch origin for Acochlidia (based on nuclear 18S rRNA and 28S rRNA) (Vonnemann et al. 2005). A first combined multi-gene dataset led to the surprising result of Acochlidia clustering in a pulmonate relationship, united in a clade with Pyramidelloidea, Amphiboloidea and Eupulmonata. However, only three derived acochlids were included into analysis prior to 2010, with partially missing data.
The taxonomy of Bouchet & Rocroi (2005) tentatively follows Starobogatov (1983), but they have downgraded his taxonomic ranks (suborders to superfamilies, superfamilies to families). The group Acochlidiacea is arranged as follows:
A first comprehensive cladistic analysis of their phylogeny has been established by Schrödl & Neusser (2010), but the identity of their sister group remained uncertain. Morphology-based analyses by Schrödl & Neusser, demonstrated that Acochlidia usually group with other mesopsammic (they live in interstitial spaces of marine sands) taxa, if any were included (i.e. with the sacoglossan Platyhedyle, the rhodopemorph Rhodope or the cephalaspideans Philinoglossa or Philine exigua). Thus, it is likely that convergent adaptations to the interstitial habitat mask the truly phylogenetic signals.
Schrödl & Neusser (2010) split Acochlidiacea into two (unranked) taxa and into six families like this:
Hedylopsacean Acochlidiacea, whose evolution involves several habitat shifts from marine interstitial to amphibious or freshwater benthic habitats, possess complex excretory and reproductive systems.
(unranked) Hedylopsacea has no superfamilies defined:
Microhedylacean Acochlidiacea are exclusively found in interstitial spaces in sediment, and show a tendency toward reduction of complexity in major organ systems.
(unranked) Microhedylacea has no superfamilies defined:
A multi-locus molecular study by Jörger et al. (2010), included six out of seven acochlidian families. It confirmed Acochlidiacea in a pulmonate relationship, as sister to Eupulmonata. Euthyneura, Opisthobranchia and Pulmonata as traditionally defined were found non-monophyletic. The enigmatic amphibious and insectivorous Aitengidae clusters within Acochlidiacea, as sister to meiofaunal and brackish Pseudunelidae and limnic Acochlidiidae. Inclusion of this small acochlidian group resulted in redefinition of major groups within Heterobranchia, that has led to creation of the new clades Euopisthobranchia and Panpulmonata.
Two more families were established. Both include slugs that are unusual among acochlidians for their anatomy and habitat (amphibious, terrestrial, or benthic in deeper waters):
There is no fossil record of Acochlidiacea. Application of a molecular clock allowed estimation of divergence times for these groups. The split between Eupulmonata and Acochlidiacea took place in the Mesozoic, between the Triassic and Jurassic periods. The diversification of Acochlidia is estimated to have happened in the Jurassic with the split between Hedylopsacea and Microhedylacea.
The life cycle of Acochlidiacea is poorly known. With a typically low reproductive output in Acochlidiacea (max. of 40 eggs in Pontohedyle milaschewitchii), free veliger larvae are assumed to stay in the interstices of the sand grains rather than entering the water column thereby avoiding long distance dispersal. Fertilized eggs are attached to sand grains and might promote dispersal via current driven sediment transport along shorelines.