| Opisthobranchia Temporal range: Carboniferous–Recent[1] |
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|---|---|
| Notodoris minor | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| (unranked): | clade Heterobranchia informal group "Opisthobranchia" |
| Included groups | |
|
clade Cephalaspidea |
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Opisthobranchs ( /ɵˈpɪsθəbræŋks/) are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping.[2] Euopisthobrancha is a collection of opisthobranchs that is monophyletic, but this group leaves out some "traditional" opisthobranchs.[3]
The Opisthobranchia includes a number of species in the order Cephalaspidea (bubble shells and headshield slugs), the sacoglossans, the anaspidean sea hares, the pelagic sea angels, the sea butterflies, and many families of Nudibranchia.[2]
Opisthobranch means "gills behind" (and to the right of the heart). In contrast, Prosobranch means gills in front (of the heart). Opisthobranchs are characterized by two pairs of tentacles and a single gill behind and to the right of the heart.
Opisthobranchia are known from as early as the Carboniferous.[4]
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Under the old classification system by Johannes Thiele in 1931 Gastropoda were divided into the Prosobranchia, Pulmonata and Opisthobranchia. The later two were later combined into a single order.[5]
It is speculated that the Opisthobranchia may be paraphyletic,[6] and may have given rise to the Pulmonata, although this is still disputed.
The Pulmonata may be a sibling group to an opisthobranch taxon. The Opisthobranchia are therefore not a monophyletic group and can no longer be accepted as a valid taxon. They are now included within the subclass Orthogastropoda. One can still encounter the old classification in many manuals and on most websites.
A new study of rRNA gene sequences, published in 2005, could not resolve monophyly versus paraphyly of the Opisthobranchia [7]
Order Opisthobranchia Milne-Edwards, 1848 - sea slugs
A phylogenetic study published in November 2004,[8] gave new definitions of the seven main lineages of the Opisthobranchia. However, the grouping is no longer considered to represent a valid clade, and thus appears as an informal group within the Heterobranchia in the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005).[2] Accordingly, recent articles do not use the term.[3]
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.[9]
Opisthobranchs have undergone detorsion, an evolutionary reversal of the half revolution torsion of their immediate ancestors.
There is no marked distinction between head and mantle. The tentacles, 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.
They have pit-cup eyes with a lens and cornea.[10]
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.[9]
Opisthobranchs are principally soft-bodied marine creatures with a reduced or absent shell and no operculum and utilize other methods for protection. Due to a combination of outstanding camouflage and aggressive toxicity they have few predators. However some utilize 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 defence.[11][12]
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 do the same with the chloroplasts of the algae they eat.[13]
Like most lifeforms, they use chemical cues for much of their life cycle. The planktonic larvae float until a pheremone alerts them to a suitable settling site, sometimes delaying metamorphosis until favourable chemicals, such as prey pheromones, are detected. Some mating opisthobranchs chemicals to attract conspecifics.
Opisthobranchs are hermaphrodites and have complex reproductive strategies, typically involving reciprocal sperm transfer and storage until the eggs are ready for fertilisation.[14] 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.[15]
This article incorporates CC-BY-2.0 text from reference.[9]