Acochlidiacea, common name acochlidians, are a taxonomic clade of very unusual

sea snail Sea snails are slow-moving marine (ocean), marine gastropod Mollusca, molluscs, usually with visible external shells, such as whelk or abalone. They share the Taxonomic classification, taxonomic class Gastropoda with slugs, which are distinguishe ...

s and sea and freshwater

slug Slug, or land slug, is a common name for any apparently shell-less Terrestrial mollusc, terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced ...

s, aquatic

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

s within the large clade

Heterobranchia Heterobranchia, the heterobranchs (meaning "different gill"), is a taxonomic clade of snails and slugs, which includes marine, aquatic, and terrestrial gastropod molluscs. Heterobranchia is one of the main clades of gastropods. Currently Het ...

. Acochlidia is a variant spelling. Pseudunela cornuta svg hariadhi

Description

These are mostly very small animals, without a

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s, distinguished by the

viscera In a multicellular organism, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to a ...

l mass being sharply set off from the rest of the body. Being a small group with only 47 species worldwide known in 2023,MolluscaBase eds. (2021)
MolluscaBase. Acochlidiimorpha.
Accessed on 2023-02-13. this group has been the subject of active research since 2010, by which date only 32 species were named. These slugs are morphologically and biologically highly aberrant and diverse, comprising a series of unusual characters (e.g. secondary

gonochorism In biology, gonochorism is a sexual system where there are two Sex, sexes and each individual organism is either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric. Gonochorism con ...

, lack of copulatory organs, asymmetric

radula The radula (; : radulae or radulas) is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters ...

e). Most acochlidians live interstitially in marine sands, while some have conquered limnic systems (uniquely within opisthobranch gastropods).

Taxonomy

Nils Hjalmar Odhner 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)Rankin J. J. (1979). "A freshwater shell-less mollusc from the Caribbean: structure, biotics, and contribution to a new understanding of the Acochlidioidea". ''

Royal Ontario Museum The Royal Ontario Museum (ROM) is a museum of art, world culture and natural history in Toronto, Ontario, Canada. It is one of the largest museums in North America and the largest in Canada. It attracts more than one million visitors every year ...

, Life Sciences Contributions'' 116: 123 pp., page 83. 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)Schrödl M. & Neusser T. P. (2010). "Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia)". ''

Zoological Journal of the Linnean Society The ''Zoological Journal of the Linnean Society'' is a monthly peer-reviewed scientific journal covering zoology published by Oxford University Press on behalf of the Linnean Society. The editor-in-chief is Maarten Christenhusz (Linnean Society) ...

'' 158: 124–154. . spelled this taxon as Acochlidia. Three families ( Hedylopsidae, Microhedylidae and Acochlidiidae) are classically recognized. Two controversial classifications (Rankin 1979, Starobogatov 1983) Starobogatov Ya. I. (1983). "Sistema otriada Acochlidiiformes ystem of the order Acochlidiiformes" ''Vsesoiuznoe soveshchanie po izucheniiu molliuskov'', Leningrad, 7: 30–32. 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. Pseudunela cornuta 2

The Acochlidia, a traditional "order" of the Opisthobranchia since their establishment by Odhner have formed one of the unsolved mysteries within

Euthyneura Euthyneura is a taxonomic infraclass of snails and slugs, which includes species exclusively from marine, aquatic and terrestrial gastropod mollusks in the clade Heterobranchia. Euthyneura are characterised by several autapomorphies, bu ...

. Their

monophyly In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent comm ...

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

cephalaspidea The order Cephalaspidea, also known as the headshield slugs and bubble snails, is a major taxon of sea slugs and bubble snails, marine (ocean), marine gastropod mollusks within the larger clade Euopisthobranchia. Bubble shells is another commo ...

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 taxonomy

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: *Superfamily Acochlidioidea **Family Acochlidiidae *Superfamily Hedylopsoidea **Family Hedylopsidae **Family Ganitidae **Family Livorniellidae **Family Minicheviellidae **Family Parhedylidae **Family Tantulidae *Superfamily Palliohedyloidea **Family Palliohedylidae *Superfamily Strubellioidea **Family Strubelliidae **Family Pseudunelidae

2010 taxonomy

A first comprehensive cladistic analysis of their

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

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:

Hedylopsacea

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: * Acochlidiidae: '' Acochlidium'', '' Palliohedyle'', including '' Strubellia'' * Pseudunelidae: with the only genus '' Pseudunela'' * Hedylopsidae: with the only genus '' Hedylopsis'' * Tantulidae: with the only species '' Tantulum elegans''

Microhedylacea

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: * Asperspinidae: with the only genus '' Asperspina'' – junior synonym: Minicheviellidae * Microhedylidae 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 ''Aiteng'' is a genus comprising three species of sea slug, ''A. ater'' and ''A. mysticus'' being found in intertidal zones while ''A. marefugitus'' is fully terrestrial. ''Aiteng'' is the only genus in the family Aitengidae. The name of a biolog ...

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

, that has led to creation of the new clades

Euopisthobranchia Euopisthobranchia is a taxonomic clade of snails and slugs in the clade Heterobranchia within the clade Euthyneura. This clade was established as a new taxon by Jörger et al. in October 2010. Euopisthobranchia is a monophyletic portion of the ...

and

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. in October 2010. The older name "Pulmonata" referred to a group of euth ...

2016 taxonomy

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): *

Swennen & Buatip, 2009 * Bathyhedylidae Neusser, Jörger, Lodde-Bensch, Strong & Schrödl, 2016Neusser T. P., Jörger K. M., Lodde-Bensch E., Strong E. E. & Schrödl M. (2016). "The unique deep sea - land connection: Interactive 3D visualization and molecular phylogeny of ''Bathyhedyle boucheti'' n. sp. (Bathyhedylidae n. fam.) - the first panpulmonate slug from bathyal zones". ''PeerJ'' 4: e2738. .

Phylogeny

There is no fossil record of Acochlidiacea. Application of a

molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleot ...

allowed estimation of divergence times for these groups. The split between Eupulmonata and Acochlidiacea took place in the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

, 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 Acochlidiacea, common name acochlidians, are a Taxonomy (biology), taxonomic clade of very unusual sea snails and sea and freshwater slugs, aquatic animal, aquatic gastropod mollusks within the large clade Heterobranchia. Acochlidia is a variant ...

Cladogram

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

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 (max. of 40 eggs in '' Pontohedyle milaschewitchii''), free

veliger A veliger is the planktonic larva of many kinds of sea snails and freshwater snails, as well as most bivalve molluscs (clams) and tusk shells. Description The veliger is the characteristic larva of the gastropod, bivalve and scaphopod taxono ...

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

# '' Hedylopsis spiculifera'' (Kowalevsky, 1901) (Hedylopsidae) – marine # '' Hedylopsis ballantinei'' Sommerfeldt & Schrödl, 2005 (Hedylopsidae) – marine # ''

Pseudunela cornuta ''Pseudunela cornuta'' is a species of small sea slug, an acochlidian, a shell-less marine (ocean), marine and temporarily brackishNeusser T. P., Jörger K. M. & Schrödl M. (2011). "Cryptic Species in Tropic Sands – Interactive 3D Anatomy ...

'' (Challis, 1970) (Pseudunelidae) – marine and temporary brackish # '' Pseudunela eirene'' Wawra, 1988 (Pseudunelidae) – marine # '' Pseudunela espiritusanta'' Neusser & Schrödl, 2009 (Pseudunelidae) – in brackish water # '' Pseudunela marteli'' Neusser, Jörger & Schrödl, 2011 (Pseudunelidae) – marine # '' Pseudunela viatoris'' Neusser, Jörger & Schrödl, 2011 (Pseudunelidae) – marineNeusser T. P., Jörger K. M. & Schrödl M. (2011). "Cryptic Species in Tropic Sands - Interactive 3D Anatomy, Molecular Phylogeny and Evolution of Meiofaunal Pseudunelidae (Gastropoda, Acochlidia)". '' PLoS ONE'' 6(8): e23313. . # '' Aiteng ater'' Swennen & Buatip, 2009 (Aitengidae) – marine (and brackish) # '' Aiteng marefugitus'' Kano, Neusser, Fukumori, Jörger & Schrödl, 2015 (Aitengidae) – marine # '' Aiteng mysticus'' Neusser, Fukuda, Jörger, Kano & Schrödl, 2011 (Aitengidae) – from Japan # ''

Bathyhedyle boucheti ''Bathyhedyle boucheti'' is a species of Panpulmonata, panpulmonate slug, a deep-sea dwelling gastropod native to the continental slope off the coast of Mozambique. It is the first ever such panpulmonate slug to be discovered at such depths. It ...

'' Neusser, Jörger, Lodde-Bensch, Strong & Schrödl, 2016 (Bathyhedylidae) - marine # '' Strubellia paradoxa'' (Strubell, 1892) (Acochlidiidae) – freshwater # '' Strubellia wawrai'' Brenzinger, Neusser, Jörger & Schrödl, 2011 (Acochlidiidae) – freshwater # '' Acochlidium amboinense'' (Strubell, 1892) (Acochlidiidae) – freshwater # '' Acochlidium bayerfehlmanni'' Wawra, 1980 (Acochlidiidae) – freshwater # '' Acochlidium fijiiensis'' Haynes & Kenchington, 1991Haynes A. & Kenchington W. (1991). "''Acochlidium fijiiensis'' sp. nov. (Gastropoda: Opisthobranchia: Acochlidiacea) from Fiji". ''The Veliger'' 34(2): 166–171. (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 loricata'' (Swedmark, 1968) (Asperspinidae) # '' Asperspina murmanica'' (Kudinskaya & Minichev, 1978) (Asperspinidae) # '' Asperspina rhopalotecta'' Salvini-Plawen, 1973 (Asperspinidae) # '' Asperspina riseri'' (Morse, 1976) (Asperspinidae) # '' Microhedyle gerlachi'' Marcus & Marcus, 1959 - synonym: '' Parhedyle gerlachi'' (Ev. Marcus & Er. Marcus, 1959) (Microhedylidae) # '' Microhedyle glandulifera'' (Kowalevsky, 1901) (Microhedylidae) # '' Microhedyle nahantensis'' (Doe, 1974) (Microhedylidae) # '' Microhedyle remanei'' (Er. Marcus, 1953) (Microhedylidae) # '' Ganitus evelinae'' Marcus, 1953 (Microhedylidae s.l. / Ganitidae) # '' Paraganitus ellynnae'' Challis, 1968 (Microhedylidae s.l. / Ganitidae) # '' Parhedyle cryptophthalma'' (Westheide & Wawra, 1974) (Microhedylidae) # '' Parhedyle odhneri'' (Ev. Marcus & Er. Marcus, 1955) - synonym: '' Microhedyle odhneri'' (Ev. Marcus & Er. Marcus, 1955) (Microhedylidae) # '' Parhedyle tyrtowii'' (Kowalevsky, 1900) (Microhedylidae) # '' Pontohedyle brasilensis'' (Rankin, 1979) (Microhedylidae) # '' Pontohedyle joni'' Jörger & Schrödl, 2013 # ''

Pontohedyle kepii ''Pontohedyle'' is a genus of sea slugs, acochlidians, shell-less marine (ocean), marine gastropod mollusks in the family Parhedylidae. Sea slugs in this genus are highly simplified and uniform. Distribution The genus ''Pontohedyle'' shows a cir ...

'' Jörger & Schrödl, 2013 # '' Pontohedyle liliae'' Jörger & Schrödl, 2013 # '' Pontohedyle martynovi'' Jörger & Schrödl, 2013 # '' Pontohedyle milaschewitchii'' (Kowalevsky, 1901) (Microhedylidae) # '' Pontohedyle neridae'' Jörger & Schrödl, 2013 # '' Pontohedyle peteryalli'' Jörger & Schrödl, 2013 # '' Pontohedyle verrucosa'' (Challis, 1970) (Microhedylidae) # '' Pontohedyle wenzli'' Jörger & Schrödl, 2013 # '' Pontohedyle wiggi'' Jörger & Schrödl, 2013 # '' Pontohedyle yurihookeri'' Jörger & Schrödl, 2013 # '' Helicohedyle dikiki'' Drainas, Carlson, Jörger, Schrödl & Neusser, 2017 (unassigned to family) – marine

References

This article incorporates CC-BY-2.0 text from referencesJörger K. M., Stöger I., Kano Y., Fukuda H., Knebelsberger T. & Schrödl M. (2010). "On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia". ''

BMC Evolutionary Biology ''BMC Ecology and Evolution'' (since January 2021), previously ''BMC Evolutionary Biology'' (2001–2020), is a peer-reviewed open access scientific journal covering all fields of evolutionary biology, including phylogenetics and palaeontology ...

'' 10: 323. .Jörger K. M., Norenburg J. L., Wilson N. G. & Schrödl M. (2012). "Barcoding against a paradox? Combined molecular species delineations reveal multiple cryptic lineages in elusive meiofaunal sea slugs". ''

'' 12: 245. . and CC-BY-SA-3.0 text from the reference.Gofas, S. (2010). Acochlidiacea. In: Bouchet, P.; Gofas, S.; Rosenberg, G. (2010) World Marine Mollusca database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=411937 on 2010-11-30

External links

Acochlidea on Sea Slug Forum
{{Taxonbar, from=Q1054947 Panpulmonata