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

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 ...

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

monophyletic 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 co ...

grouping. Euopisthobranchia 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 and the Acochlidiacea. The subclass Heterobranchia now contains all the species which used to be assigned to Opisthobranchia, plus all the species in the Pulmonata. The subclass Opisthobranchia included species in the order Cephalaspidea (bubble shells and headshield slugs), the sacoglossans, anaspidean sea hares, pelagic sea angels, sea butterflies, and many families of the Nudibranchia. ''Opisthobranch'' means "

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 behind" (and to the right) of the

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

. 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 time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

Taxonomy

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

Prosobranchia Prosobranchia was a large Taxonomy (biology), taxonomic Class (biology), 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 polyphyly, polyphyletic (consis ...

, Pulmonata 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 * 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, based on morphological evidence, and had given rise to the Pulmonata, 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''; : 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 name and ...

, 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 Heterobranchia, 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 gene sequences could not resolve

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 ...

versus

paraphyly Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping 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 In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

, reclassifying it as an informal group within the Heterobranchia. 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, Cephalaspidea, Runcinida, Aplysiida, Pteropoda and Sacoglossa.

Linnean taxonomy

Order Opisthobranchia Milne-Edwards, 1848 – sea slugs *Suborder Cephalaspidea P. Fischer, 1883 – headshield slugs and bubble shells *Suborder Sacoglossa H. von Ihering, 1876 – sap-sucking slugs and bivalved gastropods *Suborder Aplysiomorpha P. Fischer, 1883 – sea hares *Suborder Notaspidea P. Fischer, 1883 – sidegill slugs *Suborder Thecosomata Blainville, 1824 sea butterflies with shells *Suborder Gymnosomata 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 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 rhinophores, 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, fleshy winglike outgrowths. In several suborders, such as the Thecosomata and Gymnosomata, 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 that 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 transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

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 Resource (biology), resources an ...

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

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 Science Biology * Seashell, a hard outer layer of a marine ani ...

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 b ...

and aggressive toxicity 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 class (biology), class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). The class includes the only now-living order (biology), order P ...

s. Opisthobranchs secrete irritants such as strong acids or accumulate toxins from their food. Aeolidioidea pirate the stinging cells from their

cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

n prey and use them for their own defense.

Diet

Opisthobranchs may be

herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...

s, detritivores or

carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they ar ...

s. Being slow, the carnivores hunt sedentary prey. They may eat

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

ns,

Cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

, or sponges, absorbing the sponge toxin for defensive purposes. Opisthobranchs may maintain the zooxanthellae 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, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

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 that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

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 transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...

until favourable chemicals, such as prey

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

Reproduction

Opisthobranchs are

hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

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