Acochlidiacea
Encyclopedia
Acochlidiacea, common name acochlidians, are a taxonomic clade of very unusual sea snail
s and sea and freshwater slug
s, aquatic
gastropod mollusks within the large clade Heterobranchia
. Acochlidia is a variant spelling.
s, 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, 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 radula
e). Most acochlidians live interstitially in marine sands, while some have conquered limnic systems (uniquely within opisthobranch gastropods).
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 (1979) treated this taxon as an order, the order Acochlidioidea.
Salvini-Plawen (1983) wrote this taxon as Acochlidiomorpha.
Anderson (1992) treated this taxon as the order Acochlidiida.
Burn in Beesley et al (1998), wrote this taxon as the order Acochlidea.
Wawra (1987) and various authors (2007–2010) spelled this taxon as Acochlidia.
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.
An alternative classification by Burn (in Beesley et al., 1998) for the Australian species recognizes 2 superfamilies and 5 families.
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 reconstruction
s, 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 cephalaspidea
ns. 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.
(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:
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) have splitted Acochlidiacea into two (unranked) taxa and into six families like this:
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 new clades Euopisthobranchia
and Panpulmonata
.
allowed to estimate 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.
Sea snail
Sea snail is a common name for those snails that normally live in saltwater, marine gastropod molluscs....
s and sea and freshwater slug
Slug
Slug is a common name that is normally applied to any gastropod mollusc that lacks a shell, has a very reduced shell, or has a small internal shell...
s, aquatic
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
gastropod mollusks within the large clade Heterobranchia
Heterobranchia
Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....
. Acochlidia is a variant spelling.
Description
These are mostly very small animals, without a shell or gillGill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s, 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, these slugs are morphologically and biologically highly aberrant and diverse, comprising a series of unusual characters (e.g. secondary gonochorism
Gonochorism
In biology, gonochorism or unisexualism describes sexually reproducing species in which individuals have just one of at least two distinct sexes. The term is most often used with animals . The sex of an individual may change during its lifetime, this can for example be found in parrotfish...
, lack of copulatory organs, asymmetric radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
e). Most acochlidians live interstitially in marine sands, while some have conquered limnic systems (uniquely within opisthobranch gastropods).
Taxonomy
Nils Hjalmar OdhnerNils Hjalmar Odhner
Nils Hjalmar Odhner was a Swedish zoologist who studied mollusks, a malacologist. During his lifetime he was professor of invertebrate zoology at the Swedish Museum of Natural History, Stockholm...
established this taxon as a family in 1937, when he created the families Microhedylidae
Microhedylidae
Microhedylidae are a taxonomic family of sea slugs, marine gastropod mollusks within the clade Acochlidiacea.- 2005 taxonomy :Microhedylidae has been listed as a synonym of Parhedylidae in the taxonomy of Bouchet & Rocroi . Ganitidae Rankin, 1979 has been listed as a sole family within Hedylopsoidea...
and Acochlidiidae
Acochlidiidae
Acochlidiidae are a taxonomic family of shell-less freshwater gastropods , aquatic gastropod mollusks within the clade Acochlidiacea.- 2005 taxonomy :...
. In 1939, he treated this taxon as an order.
Rankin (1979) treated this taxon as an order, the order Acochlidioidea.
Salvini-Plawen (1983) wrote this taxon as Acochlidiomorpha.
Anderson (1992) treated this taxon as the order Acochlidiida.
Burn in Beesley et al (1998), wrote this taxon as the order Acochlidea.
Wawra (1987) and various authors (2007–2010) spelled this taxon as Acochlidia.
Three families (Hedylopsidae
Hedylopsidae
Hedylopsis is a genus of sea slugs, marine gastropod mollusks within the clade Acochlidiacea.- Taxonomy :Hedylopsidae has been classified within the superfamily Hedylopsoidea in the taxonomy of Bouchet & Rocroi ....
, Microhedylidae
Microhedylidae
Microhedylidae are a taxonomic family of sea slugs, marine gastropod mollusks within the clade Acochlidiacea.- 2005 taxonomy :Microhedylidae has been listed as a synonym of Parhedylidae in the taxonomy of Bouchet & Rocroi . Ganitidae Rankin, 1979 has been listed as a sole family within Hedylopsoidea...
and Acochlidiidae
Acochlidiidae
Acochlidiidae are a taxonomic family of shell-less freshwater gastropods , aquatic gastropod mollusks within the clade Acochlidiacea.- 2005 taxonomy :...
) are classically recognized. Two controversial classifications (Rankin 1979, Starobogatov
Yaroslav Starobogatov
Yaroslav Igorevich Starobogatov was a Russian malacologist.He was a researcher of the Zoological Institute of the Russian Academy of Sciences.- Taxa described :New taxa described by Yaroslav Igorevich Starobogatov include:- Taxa named in honour :...
1983) have been proposed recently, but they have not been evaluated since.
An alternative classification by Burn (in Beesley et al., 1998) for the Australian species recognizes 2 superfamilies and 5 families.
The Acochlidia, a traditional "order" of the Opisthobranchia since their establishment by Odhner have formed one of the unsolved mysteries within Euthyneura
Euthyneura
Euthyneura is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks in the clade Heterobranchia....
. Their monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
is widely accepted especially since a proposed sister group relationship of the acochlidian family Ganitidae with Sacoglossa
Sacoglossa
Sacoglossa, commonly known as the sacoglossans or the "sap-sucking sea slugs", are a clade of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia...
(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 reconstruction
3D reconstruction
See Iterative reconstruction for 3D reconstruction in Medical imaging.In computer vision and computer graphics, 3D reconstruction is the process of capturing the shape and appearance of real objects....
s, 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
Toledonia
Toledonia is a genus of sea snails, marine gastropod molluscs in the family Diaphanidae.Toledonia is the type genus of the subfamily Toledoniinae.- Description :...
, or with runcinid or philinoid cephalaspidea
Cephalaspidea
The suborder Cephalaspidea, also known as the headshield slugs and bubble snails, is a clade of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia....
ns. 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
Pyramidelloidea
Pyramidelloidea is a superfamily of mostly very small sea snails, marine gastropod mollusks and micromollusks within the clade Panpulmonata.This is a voluminous taxon: above the species level close to 400 named taxa are referred to this gastropod superfamily.Pyramidelloidea has both fossil and...
, Amphiboloidea
Amphiboloidea
Amphiboloidea is a taxonomic superfamily of air-breathing land snails.- 2005 taxonomy :According to the taxonomy of the Gastropoda , it is a superfamily in the informal group Basommatophora, within the Pulmonata....
and Eupulmonata
Eupulmonata
The Eupulmonata is a taxonomic clade of air-breathing snails. The great majority of this group are land snails and slugs, but some are marine and some are saltmarsh snails that can tolerate salty conditions.-Linnean taxonomy:...
. However, only three derived acochlids were included into analysis prior to 2010, with partially missing data.
2005 taxonomy
The taxonomy of Bouchet & RocroiTaxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...
(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:
- Superfamily Acochlidioidea
- Family AcochlidiidaeAcochlidiidaeAcochlidiidae are a taxonomic family of shell-less freshwater gastropods , aquatic gastropod mollusks within the clade Acochlidiacea.- 2005 taxonomy :...
- Family Acochlidiidae
- Superfamily HedylopsoideaHedylopsoideaHedylopsoidea was defined as a taxonomic superfamily of sea slugs, mostly marine gastropod mollusks within the informal group Opisthobranchia according to the taxonomy of Bouchet & Rocroi ....
- Family HedylopsidaeHedylopsidaeHedylopsis is a genus of sea slugs, marine gastropod mollusks within the clade Acochlidiacea.- Taxonomy :Hedylopsidae has been classified within the superfamily Hedylopsoidea in the taxonomy of Bouchet & Rocroi ....
- Family Ganitidae
- Family Livorniellidae
- Family MinicheviellidaeMinicheviellidaeAsperspina is a genus of sea slugs, marine gastropod mollusks within the clade Acochlidiacea.- Taxonomy :Asperspinidae has been listed as a synonym of Parhedylidae in the taxonomy of Bouchet & Rocroi ....
- Family Parhedylidae
- Family TantulidaeTantulidaeNot to be confused with Tantalum.Tantulum elegans is a species of freshwater gastropod, aquatic gastropod mollusk within the clade Acochlidiacea.This species has no shell...
- Family Hedylopsidae
- Superfamily Palliohedyloidea
- Family Palliohedylidae
- Superfamily StrubellioideaStrubellioideaStrubellioidea was defined as a taxonomic superfamily of sea slugs, mostly marine gastropod mollusks within the informal group Opisthobranchia according to the taxonomy of Bouchet & Rocroi ....
- Family StrubelliidaeStrubelliidaeStrubellia paradoxa is a shell-less freshwater gastropod, aquatic gastropod mollusks within the clade Acochlidiacea.Strubellia paradoxa is the only species in the genus Strubellia.- Taxonomy :...
- Family Pseudunelidae
- Family Strubelliidae
2010 taxonomy
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 RhodopeRhodope
Rhodope may mean:* Queen Rhodope, a figure of Greek mythology* Rhodope Mountains, in Bulgaria and Greece* Rhodope Prefecture, of Greece* Rhodope * 166 Rhodope, an asteroid...
or the cephalaspideans Philinoglossa or Philine exigua). Thus, it is likely that convergent adaptations to the interstitial
Interstitial
An interstitial space or interstice is an empty space or gap between spaces full of structure or matter.In particular, interstitial may refer to:-Physical sciences:...
habitat mask the truly phylogenetic signals.
Schrödl & Neusser (2010) have splitted Acochlidiacea into two (unranked) taxa and into six families like this:
Hedylopsacea
(unranked) Hedylopsacea has no superfamilies defined:- AcochlidiidaeAcochlidiidaeAcochlidiidae are a taxonomic family of shell-less freshwater gastropods , aquatic gastropod mollusks within the clade Acochlidiacea.- 2005 taxonomy :...
: AcochlidiumAcochlidiumAcochlidium is a genus of freshwater slugs, an aquatic gastropod mollusks in the family Acochlidiidae.Acochlidium species have no shell.-Species:Species within the genus Acochlidium include:* Acochlidium amboinense Strubell, 1892...
, Palliohedyle, including Strubellia - Pseudunelidae: with the only genus PseudunelaPseudunelaPseudunela is a genus of minute sea slugs, acochlidians, shell-less marine or temporary brackish or brackish gastropod mollusks in the clade Acochlidiacea.Pseudunela is the only genus in the family Pseudunelidae...
- HedylopsidaeHedylopsidaeHedylopsis is a genus of sea slugs, marine gastropod mollusks within the clade Acochlidiacea.- Taxonomy :Hedylopsidae has been classified within the superfamily Hedylopsoidea in the taxonomy of Bouchet & Rocroi ....
: with the only genus Hedylopsis - TantulidaeTantulidaeNot to be confused with Tantalum.Tantulum elegans is a species of freshwater gastropod, aquatic gastropod mollusk within the clade Acochlidiacea.This species has no shell...
: with the only species Tantulum elegans
Microhedylacea
(unranked) Microhedylacea has no superfamilies defined:- Asperspinidae: with the only genus Asperspina – junior synonym: MinicheviellidaeMinicheviellidaeAsperspina is a genus of sea slugs, marine gastropod mollusks within the clade Acochlidiacea.- Taxonomy :Asperspinidae has been listed as a synonym of Parhedylidae in the taxonomy of Bouchet & Rocroi ....
- MicrohedylidaeMicrohedylidaeMicrohedylidae are a taxonomic family of sea slugs, marine gastropod mollusks within the clade Acochlidiacea.- 2005 taxonomy :Microhedylidae has been listed as a synonym of Parhedylidae in the taxonomy of Bouchet & Rocroi . Ganitidae Rankin, 1979 has been listed as a sole family within Hedylopsoidea...
s.l.: Pontohedyle, Parhedyle, Microhedyle – including Ganitidae: Ganitus and Paraganitus. Inclusion of Ganitidae within Microhedylidae requires further research and higher statistical support.
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
Heterobranchia
Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....
, that has led to creation new clades Euopisthobranchia
Euopisthobranchia
Euopisthobranchia is a taxonomic clade of snails and slugs in the clade Heterobranchia within the clade Euthyneura.Euopisthobranchia was established as a new taxon by Jörger et al...
and Panpulmonata
Panpulmonata
Panpulmonata is a taxonomic clade of snails and slugs in the clade Heterobranchia within the clade Euthyneura.Panpulmonata was established as a new taxon by Jörger et al...
.
Phylogeny
There is no fossil record of Acochlidiacea. Application of molecular clockMolecular clock
The molecular clock is a technique in molecular evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
allowed to estimate divergence times for these groups. The split between Eupulmonata
Eupulmonata
The Eupulmonata is a taxonomic clade of air-breathing snails. The great majority of this group are land snails and slugs, but some are marine and some are saltmarsh snails that can tolerate salty conditions.-Linnean taxonomy:...
and Acochlidiacea took place in the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
, 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.
Overview of species
- Hedylopsis spiculifera (Kowalevsky, 1901) (Hedylopsidae)
- Hedylopsis ballantinei Sommerfeldt & Schrödl, 2005 (Hedylopsidae)
- Pseudunela cornutaPseudunela cornutaPseudunela cornuta is a species of minute sea slug, an acochlidian, a shell-less marine and temporary brackish gastropod mollusk in the family Pseudunelidae....
(Challis, 1970) (Pseudunelidae) – marine and temporary brackish - Pseudunela eirene Wawra, 1988 (Pseudunelidae) – marine
- Pseudunela espiritusantaPseudunela espiritusantaPseudunela espiritusanta is a species of sea slug, an acochlidian, a shell-less marine gastropod mollusk in the family Pseudunelidae.Its specific name is in honor of Espiritu Santo Island in Vanuatu, where it was found....
Neusser & Schrödl, 2009 (Pseudunelidae) – in brackish water - Pseudunela marteli Neusser, Jörger & Schrödl, 2011 (Pseudunelidae) – marine
- Pseudunela viatoris Neusser, Jörger & Schrödl, 2011 (Pseudunelidae) – marine
- Aiteng aterAiteng aterAiteng ater is a species of sea slug, a marine gastropod mollusk in the family Aitengidae.The specific name ater is from Latin language and means black, in reference to the appearance of the slug on the mud....
Swennen & Buatip, 2009 (Aitengidae) – marine (and brackish) - undescribed species Aiteng mysticusAiteng mysticusAiteng mysticus is an undescribed species of sea slug, a marine gastropod mollusk in the family Aitengidae....
from Japan (Aitengidae) - Strubellia paradoxa (Strubell, 1892) (Acochlidiidae) – freshwater
- Acochlidium amboinense (Strubell, 1892) (Acochlidiidae) – freshwater
- Acochlidium bayerfehlmanni Wawra, 1980 (Acochlidiidae) – freshwater
- Acochlidium fijiiensisAcochlidium fijiiensisAcochlidium fijiiensis is a species of freshwater gastropod, an aquatic gastropod mollusk within the family Acochlidiidae.Acochlidium fijiiensis has no shell.It lives in two rivers on Fiji.- Further reading :...
Haynes & Kenchington, 1991 (Acochlidiidae) – freshwater - Palliohedyle sutteri (Wawra, 1979) (Acochlidiidae) – freshwater
- Palliohedyle weberi (Bergh, 1895) (Acochlidiidae) – in brackish waters
- Tantulum elegans Rankin, 1979 (Tantulidae) – freshwater
- Asperspina brambelli (Swedmark, 1968) (Asperspinidae)
- Asperspina murmanica (Kudinskaya & Minichev, 1978) (Asperspinidae)
- Asperspina rhopalotecta Salvini-Plawen, 1973 (Asperspinidae)
- Asperspina loricata (Swedmark, 1968) (Asperspinidae)
- Asperspina riseri (Morse, 1976) (Asperspinidae)
- Microhedyle glandulifera (Kowalevsky, 1901) (Microhedylidae)
- Microhedyle nahantensis (Doe, 1974) (Microhedylidae)
- Microhedyle odhneri (Ev. Marcus & Er. Marcus, 1955) (Microhedylidae)
- Microhedyle remanei (Er. Marcus, 1953) (Microhedylidae)
- Ganitus evelinae Marcus, 1953 (Microhedylidae s.l. / Ganitidae)
- Paraganitus ellynnaeParaganitus ellynnaeParaganitus ellynnae is a species of sea slug, an acochlidian, a shell-less marine gastropod mollusk in the family Microhedylidae.Paraganitus ellynnae is the only species in the genus Paraganitus.- Description :...
Challis, 1968 (Microhedylidae s.l. / Ganitidae) - Parhedyle cryptophthalmaParhedyle cryptophthalmaParhedyle cryptophthalma is a species of sea slug, an acochlidian, a shell-less marine gastropod mollusk in the family Microhedylidae.- Ecology :...
(Westheide & Wawra, 1974) (Microhedylidae) - Parhedyle tyrtowii (Kowalevsky, 1900) (Microhedylidae)
- Parhedyle gerlachi (Ev. Marcus & Er. Marcus, 1959) (Microhedylidae)
- Pontohedyle milaschewitchii (Kowalevsky, 1901) (Microhedylidae)
- Pontohedyle verrucosa (Challis, 1970) (Microhedylidae)