An opisthobranch is any member of the very large and diverse group of rather specialized, highly evolved marine slugs and snails (marine gastropod mollusks) known as Opisthobranchia, within the Heterobranchia.
This Opisthobranchia include a number of families of bubble snails which have shells, and other headshield slugs in the order Cephalaspidea. In addition it includes many other orders, including the saccoglossans, the anaspidean sea hares, the pelagic sea angels and sea butterflies, and a very large number of different families of nudibranchs.
This group was once a strict taxonomic order, but recent research in molecular phylogeny revealed Opisthobranchia's polyphyly (separate evolutions), and so the group no longer holds any strict taxonomic sense, being divided into nine different orders (and united with the so-called lower heterobranchs).
Their bodies have undergone detorsion, an evolutionary reversal of the 180° 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 you can find the rhinophores, olfactory organs often with 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.
Many have brilliant colors and carry stinging cells, so that predators may learn to avoid them as a food source.
The Pulmonata may be a sister group to a particular opisthobranch taxon. The Opisthobranchia are not therefore a monophyletic group and are no longer be accepted as a taxon. They are now included in the subclass Orthogastropoda. Note that one can still encounter the old classification in many manuals and on most websites. You can find more about these taxonomic issues at the gastropod page.
A new phylogentic study, published in November 2004, has given a new definition to the seven main lineages of the Opisthobranchia.
Serotonin-like immunoreactivity in the central and peripheral nervous systems of the interstitial acochlidean Asperspina sp. (Opisthobranchia).
Aug 01, 2007; Introduction Microscopic gastropods of the marine interstitial environment have long fascinated malacologists and...
Larval development and metamorphosis in Pleurobranchaea maculata, with a review of development in the Notaspidea (Opisthobranchia).
Oct 01, 2003; Introduction The Notaspidea is a small, specialized order of opisthobranchs that are considered to be phylogenetically...
Mating behaviour in the sea slug Elysia timida (Opisthobranchia, Sacoglossa): hypodermic injection, sperm transfer and balanced reciprocity.(Research)
Jul 04, 2007; Authors: Valerie Schmitt ; Nils Anthes ; Nico K Michiels (corresponding author) [1,2] Background Mating conflicts in...
The relationship between seaweed diet and purple ink production in Aplysia dactylomela rang, 1828 (gastropoda: opisthobranchia) from Northeastern Brazil.
Aug 01, 2004; ABSTRACT Aplysia dactylomela is a large marine opisthobranch gastropod, which inhabits shallow tropical shoreline regions, eats...