Opisthobranchs () is now an informal name for a large and diverse group of specialized complex

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

s which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

grouping.

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 ...

is a taxon containing a revised collection of opisthobranchs, and that taxon is considered monophyletic. Euopisthobranchia does not include some "traditional" opisthobranchs such as the

Sacoglossa Sacoglossa, commonly known as the sacoglossans or the "solar-powered sea slugs", are a superorder of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia. Sacoglossans live by ingesting the cellular ...

and the

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. Description These a ...

. The subclass

Heterobranchia Heterobranchia, the ''heterobranchs'' (meaning "different-gilled snails"), is a taxonomic clade of snails and slugs, which includes marine, aquatic and terrestrial gastropod mollusks. Heterobranchia is one of the main clades of gastropods. Curr ...

now contains all the species which used to be assigned to Opisthobranchia, plus all the species in the

Pulmonata Pulmonata or pulmonates, is an informal group (previously an order, and before that a subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group includ ...

. The subclass Opisthobranchia included species in the order

Cephalaspidea The order Cephalaspidea, also known as the headshield slugs and bubble snails, is a major taxon of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia.Jörger K. M., Stöger I., Kano Y., Fukuda H., ...

(bubble shells and headshield slugs), the

sacoglossa Sacoglossa, commonly known as the sacoglossans or the "solar-powered sea slugs", are a superorder of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia. Sacoglossans live by ingesting the cellular ...

ns, anaspidean

sea hares The clade Anaspidea, commonly known as sea hares (''Aplysia'' species and related genera), are medium-sized to very large opisthobranch gastropod molluscs with a soft internal shell made of protein. These are marine gastropod molluscs in the s ...

, pelagic

sea angels Sea angels (clade Gymnosomata) are a large group of small free-swimming sea slugs, not to be confused with Cnidarians (Jellyfish and other similar creatures), classified into six different families. They are pelagic opisthobranchs in the clade Gy ...

sea butterflies Sea butterflies, scientific name Thecosomata (thecosomes, "case / shell-body"), are a taxonomic suborder of small pelagic swimming sea snails. They are holoplanktonic opisthobranch gastropod mollusks. Most Thecosomata have some form of calcifi ...

, and many families of the Nudibranchia. ''Opisthobranch'' means "

gill A gill () is a respiratory organ that many 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 respiration on land provided they are ...

s behind" (and to the right) of the

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...

. In contrast, ''Prosobranch'' means ''gills in front'' (of the heart). Opisthobranchs are characterized by two pairs of

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s and a single gill behind and to the right of the heart. With the lack of a heavily mineralized shell, there has been very little fossil record of the group. However, molecular clock studies have suggested that Opisthobranchia emerged as early as the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

Taxonomy

Under the 1931 classification system drawn up by Johannes Thiele, the class Gastropoda was divided into three subclasses;

Prosobranchia Prosobranchia was a large taxonomic subclass of sea snails, land snails and freshwater snails. This taxon of gastropods dates back to the 1920s. It has however been proven to be polyphyletic (consisting of more than one lineage of descent). Gen ...

and Opisthobranchia. The latter two were later combined into a single order. The current classification of the gastropods is more nuanced, with the following subclasses: * Subclass Caenogastropoda * Subclass Cocculiniformia * Subclass Heterobranchia * Subclass Neomphalina * Subclass

Neritimorpha Neritimorpha is a taxonomic grouping, an unranked major clade of snails, gastropod mollusks. This grouping includes land snails, sea snails, slugs, some deepwater limpets, and also freshwater snails. Neritimorpha contains around 2,000 extant s ...

* Subclass Patellogastropoda * Subclass Vetigastropoda * Subclass Gastropoda incertae sedis

As a non-monophyletic taxon

It was speculated as far back as 1985 that the Opisthobranchia were

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

, based on morphological evidence, and had given rise to the

, a group also of subclass rank. Because the Pulmonata are a sibling group to an opisthobranch

taxon In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

, some authors argued that the Opisthobranchia are therefore not a

group and can no longer be accepted as a valid taxon. The opisthobranchs are now included within the subclass

, although many manuals and websites still use the old classification. A phylogenetic study published in November 2004, gave new definitions of the seven main lineages of the Opisthobranchia. However, in 2005, a study of

rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosoma ...

gene sequences could not resolve

monophyly In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...

versus

paraphyly In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

of the Opisthobranchia Subsequent taxonomic classification in 2005 overturned the Opisthobranchia as a valid

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

, reclassifying it as an informal group within the

. Accordingly, articles no longer use the term Opisthobranchia, replacing it with Heterobranchia to emphasise a different concept. By 2011, the "Opisthobranchia" were declared artificial and obsolete and were replaced by new phylogenetic hypotheses. The concept was abandoned. It now includes including "Lower Heterobranchia", Acteonimorpha, Ringipleura,

Umbraculida Umbraculoidea is a superfamily of unusual false limpets with a thin soft patelliform shell, marine gastropod molluscs in the clade Umbraculida, within the clade Euopisthobranchia. There are two families in this superfamily, which is listed as ...

, Runcinida, Aplysiida,

Pteropoda Pteropoda (common name pteropods, from the Greek meaning "wing-foot") are specialized free-swimming pelagic sea snails and sea slugs, marine opisthobranch gastropods. Most live in the top 10 m of the ocean and are less than 1 cm long. The monop ...

and

Linnean taxonomy

Order Opisthobranchia Milne-Edwards, 1848 – sea slugs *Suborder

P. Fischer, 1883 – headshield slugs and bubble shells *Suborder

H. von Ihering, 1876 – sap-sucking slugs and bivalved gastropods *Suborder

Aplysiomorpha The clade Anaspidea, commonly known as sea hares (''Aplysia'' species and related genera), are medium-sized to very large opisthobranch gastropod molluscs with a soft internal shell made of protein. These are marine gastropod molluscs in the s ...

P. Fischer, 1883 – sea hares *Suborder

Notaspidea Notaspidea, also known as the sidegill slugs, is an artificial grouping of sea slugs which is now split into two unrelated groups, the Umbraculida and the Pleurobranchomorpha. Taxonomic history Notaspidea, also known as the sidegill slugs, was a ...

P. Fischer, 1883 – sidegill slugs *Suborder

Thecosomata Sea butterflies, scientific name Thecosomata (thecosomes, "case / shell-body"), are a taxonomic suborder of small pelagic swimming sea snails. They are holoplanktonic opisthobranch gastropod mollusks. Most Thecosomata have some form of calcifi ...

Blainville, 1824 sea butterflies with shells *Suborder

Gymnosomata Sea angels ( clade Gymnosomata) are a large group of small free-swimming sea slugs, not to be confused with Cnidarians (Jellyfish and other similar creatures), classified into six different families. They are pelagic opisthobranchs in the clade ...

Blainville, 1824 – sea angels, no shells *Suborder Nudibranchia Blainville, 1814 – nudibranchs **Infraorder Anthobranchia Férussac, 1819 **Infraorder Cladobranchia Willan & Morton, 1984

Description

The reduction or loss of the shell, the elaboration of the head, foot or mantle, and the acquisition of chemical defences are evolutionary trends shared by most opisthobranch taxa. The loss of shell in the group is an example of parallel evolution and has occurred on multiple independent occasions. Opisthobranchs have undergone detorsion, an evolutionary reversal of the half revolution

torsion Torsion may refer to: Science * Torsion (mechanics), the twisting of an object due to an applied torque * Torsion of spacetime, the field used in Einstein–Cartan theory and ** Alternatives to general relativity * Torsion angle, in chemistry Bi ...

of their immediate ancestors. As a result of this detorsion, the visceral ganglia no longer overlap and are described as euthyneurous (as opposed to streptoneurous, the more common condition among gastropods, in which these ganglia form a distinct twist within the animal's body). There is no marked distinction between head and mantle. The

s, situated close to the mouth, are used for orientation. Behind them are the

rhinophore A rhinophore is one of a pair of chemosensory club-shaped, rod-shaped or ear-like structures which are the most prominent part of the external head anatomy in sea slugs, marine gastropod opisthobranch mollusks such as the nudibranchs, sea har ...

s, olfactory organs which often have complex forms. The middle part of the foot is the sole, used for locomotion. The sides of the foot have evolved into

parapodia In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; plural: parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed ...

, fleshy winglike outgrowths. In several suborders, such as the

and

, these parapodia are used to move in a swimming motion. Their eyes are simple pit-cup eyes with a

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

and

cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...

capable of detecting light and the passage of shadows but not of producing a coherent image.

Ecology

Opisthobranchia represents a morphologically diverse group of gastropods occupying a great variety of

ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...

s. Opisthobranchs have a global distribution, but are restricted almost exclusively to marine habitats with the only exception being few freshwater

acochlidian 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. Description These a ...

Defense

Principally soft-bodied marine creatures with a reduced or absent

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 ** Thin-shell structure Science Biology * Seashell, a hard o ...

and no operculum, opisthobranchs use other methods for protection. The group has a diverse variety of highly specialized auxiliary defenses. Due to a combination of outstanding

camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...

and aggressive

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

ity they have few predators. However, some use warning colouration. Animals that do predate opisthobranchs include other opisthobranchs and toxin-resistant predators like

sea spider Sea spiders are marine arthropods of the order Pantopoda ( ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). They are cosmopolitan, fou ...

s. Opisthobranchs secrete irritants such as

strong acid Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a proton, H+, and an anion, A-. The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions. ...

s or accumulate toxins from their food.

Aeolidioidea Aeolidioidea is a superfamily of sea slugs, the aeolid nudibranchs. They are marine gastropod molluscs in the suborder Cladobranchia. Taxonomy As of 2019, the superfamily Aeolidioidea consisted of the following families: * Aeolidiidae Gray, ...

pirate the stinging cells from their

cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

n prey and use them for their own defense.

Diet

Opisthobranchs may be

herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...

detritivores Detritivores (also known as detrivores, detritophages, detritus feeders, or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...

carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...

s. Being slow, the carnivores hunt

sedentary Sedentary lifestyle is a lifestyle type, in which one is physically inactive and does little or no physical movement and or exercise. A person living a sedentary lifestyle is often sitting or lying down while engaged in an activity like soci ...

prey. They may eat

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

ns,

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

, or

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

s, absorbing the sponge toxin for defensive purposes. Opisthobranchs may maintain the

zooxanthellae Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus ''Sy ...

of their coral prey and use their metabolic products for themselves. Some herbivorous slugs of the sub-order Sacoglossa do the same with the

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

s of the algae they eat.

Communication

Like most lifeforms, they use chemical cues for much of their life cycle. The

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

e float until a

pheromone A pheromone () is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting like hormones outside the body of the secreting individual, to affect the behavio ...

alerts them to a suitable settling site, sometimes delaying

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

until favourable chemicals, such as prey

s, are detected. Some mating opisthobranchs release chemicals to attract

conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organ ...

Reproduction

Opisthobranchs are

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...

s and have complex reproductive strategies, typically involving reciprocal sperm transfer and storage until the eggs are ready for

fertilisation Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

. Eggs are commonly laid in ribbons of varying structure. The egg ribbons are usually unique to each species and in some cases are the only means of differentiating them.Gosliner, Terrence (1987) ''Nudibranchs of Southern Africa'' p. 11

References

This article incorporates CC-BY-2.0 text from reference

External links

The Sea Slug Forum
a resource from the Australian Museum.
Sea slugs of HawaiiBorn to be Wild: Sea Slugs
retrieved 30.4.2008

* ttp://www.nudipixel.net/ Nudi PixelNudi Pixel is a web-based identification tool for opisthobranchs worldwide {{Authority control Obsolete gastropod taxa Carboniferous first appearances