Opisthobranchia

Opisthobranchs () is a now informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping.

thumb|[[Bullina lineata]]

Taxonomy

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

thumb|[[Aplysia californica, a sea hare.]]

It was speculated as far back as 1985 that the Opisthobranchia were paraphyletic, 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, some authors argued that the Opisthobranchia are therefore not a monophyletic 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 versus paraphyly of the Opisthobranchia

Subsequent taxonomic classification in 2005 overturned the Opisthobranchia as a valid clade, 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.

Ecology

Opisthobranchia represents a morphologically diverse group of gastropods occupying a great variety of ecological niches. Opisthobranchs have a global distribution, but are restricted almost exclusively to marine habitats with the only exception being few freshwater acochlidians. Due to a combination of outstanding camouflage 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 spiders.

Opisthobranchs secrete irritants such as strong acids or accumulate toxins from their food. Aeolidioidea pirate the stinging cells from their cnidarian prey and use them for their own defense.

Diet

thumb|[[Oxynoe olivacea]]

Opisthobranchs may be herbivores, detritivores or carnivores. Being slow, the carnivores hunt sedentary prey. They may eat bryozoans, Cnidaria, 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 chloroplasts of the algae they eat.

Communication

thumb|[[Pleurobranchus mamillatus]]

Like most lifeforms, they use chemical cues for much of their life cycle. The planktonic larvae float until a pheromone alerts them to a suitable settling site, sometimes delaying metamorphosis until favourable chemicals, such as prey pheromones, are detected. Some mating opisthobranchs release chemicals to attract conspecifics.

Reproduction

thumb|Winged snail [[Clione limacina, a sea angel.]]

Opisthobranchs are hermaphrodites 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.

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 Hawaii
Born to be Wild: Sea Slugs, retrieved 30.4.2008
Various opisthobranch species from Indonesia, Philippines and Thailand
Nudi Pixel Nudi Pixel is a web-based identification tool for opisthobranchs worldwide