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.
'' shows some characteristics of acochlidians, but characteristics of other acochlidians differ greatly:

f – foot

hb – heart bulb

lt – labial tentacle

rh – rhinophore

vh – visceral hump.

Description

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.
Being a small group with only 30 species worldwide known in 2010, and 32 species described in 2011, and 33 in 2012, these slugs are morphologically and biologically highly aberrant and diverse, comprising a series of unusual characters. Most acochlidians live interstitially in marine sands, while some have conquered limnic systems.

Taxonomy

established this taxon as a family in 1937, when he created the families Microhedylidae and Acochlidiidae. In 1939, he treated this taxon as an order.
Rankin treated this taxon as an order, the order Acochlidioidea.
Salvini-Plawen wrote this taxon as Acochlidiomorpha.
Anderson treated this taxon as the order Acochlidiida.
Burn in Beesley et al., wrote this taxon as the order Acochlidea.
Wawra and various authors spelled this taxon as Acochlidia.
Three families are classically recognized. Two controversial classifications have been proposed recently, but they have not been evaluated since.
An alternative classification by Burn for the Australian species recognizes 2 superfamilies and 5 families.
s like this one highly improved knowledge about acochlidians.
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 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 . 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.

2005 taxonomy

The taxonomy of Bouchet & Rocroi tentatively follows Starobogatov, but they have downgraded his taxonomic ranks. The group Acochlidiacea is arranged as follows:
A first comprehensive cladistic analysis of their phylogeny has been established by Schrödl & Neusser, but the identity of their sister group remained uncertain. Morphology-based analyses by Schrödl & Neusser, demonstrated that Acochlidia usually group with other mesopsammic taxa, if any were included. Thus, it is likely that convergent adaptations to the interstitial habitat mask the truly phylogenetic signals.
Schrödl & Neusser split Acochlidiacea into two taxa and into six families like this:

Hedylopsacea

Hedylopsacean Acochlidiacea, whose evolution involves several habitat shifts from marine interstitial to amphibious or freshwater benthic habitats, possess complex excretory and reproductive systems.
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.
Microhedylacea has no superfamilies defined:
A multi-locus molecular study by Jörger et al., 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.

2016 taxonomy

Two more families were established. Both include slugs that are unusual among acochlidians for their anatomy and habitat :
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.

Cladogram

A cladogram showing phylogenetic relations of some genera and species within Acochlidiacea:

Ecology

The life cycle of Acochlidiacea is poorly known. With a typically low reproductive output in Acochlidiacea, 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.

Overview of species

  1. Hedylopsis spiculifera – marine
  2. Hedylopsis ballantinei Sommerfeldt & Schrödl, 2005 – marine
  3. Pseudunela cornuta – marine and temporary brackish
  4. Pseudunela eirene Wawra, 1988 – marine
  5. Pseudunela espiritusanta Neusser & Schrödl, 2009 – in brackish water
  6. Pseudunela marteli Neusser, Jörger & Schrödl, 2011 – marine
  7. Pseudunela viatoris Neusser, Jörger & Schrödl, 2011 – marine
  8. Aiteng ater Swennen & Buatip, 2009 – marine
  9. Aiteng marefugitus Kano, Neusser, Fukumori, Jörger & Schrödl, 2015 – marine
  10. Aiteng mysticus Neusser, Fukuda, Jörger, Kano & Schrödl, 2011 – from Japan
  11. Bathyhedyle boucheti Neusser, Jörger, Lodde-Bensch, Strong & Schrödl, 2016 - marine
  12. Strubellia paradoxa – freshwater
  13. Strubellia wawrai Brenzinger, Neusser, Jörger & Schrödl, 2011 – freshwater
  14. Acochlidium amboinense – freshwater
  15. Acochlidium bayerfehlmanni Wawra, 1980 – freshwater
  16. Acochlidium fijiiensis Haynes & Kenchington, 1991 – freshwater
  17. Palliohedyle sutteri – freshwater
  18. Palliohedyle weberi – in brackish waters
  19. Tantulum elegans Rankin, 1979 – freshwater
  20. Asperspina brambelli
  21. Asperspina loricata
  22. Asperspina murmanica
  23. Asperspina rhopalotecta Salvini-Plawen, 1973
  24. Asperspina riseri
  25. Microhedyle gerlachi Marcus & Marcus, 1959 - synonym: Parhedyle gerlachi
  26. Microhedyle glandulifera
  27. Microhedyle nahantensis
  28. Microhedyle remanei
  29. Ganitus evelinae Marcus, 1953
  30. Paraganitus ellynnae Challis, 1968
  31. Parhedyle cryptophthalma
  32. Parhedyle odhneri - synonym: Microhedyle odhneri
  33. Parhedyle tyrtowii
  34. Pontohedyle brasilensis
  35. Pontohedyle joni Jörger & Schrödl, 2013
  36. Pontohedyle kepii Jörger & Schrödl, 2013
  37. Pontohedyle liliae Jörger & Schrödl, 2013
  38. Pontohedyle martynovi Jörger & Schrödl, 2013
  39. Pontohedyle milaschewitchii
  40. Pontohedyle neridae Jörger & Schrödl, 2013
  41. Pontohedyle peteryalli Jörger & Schrödl, 2013
  42. Pontohedyle verrucosa
  43. Pontohedyle wenzli Jörger & Schrödl, 2013
  44. Pontohedyle wiggi Jörger & Schrödl, 2013
  45. Pontohedyle yurihookeri Jörger & Schrödl, 2013
  46. Helicohedyle dikiki Drainas, Carlson, Jörger, Schrödl & Neusser, 2017 – marine