Bullina lineata from the Lady Jane Reef, NSW,
Australia. Picture: Richard Ling (Source).
Opisthobranchia or opisthobranchs are the third large group of snails, apart from prosobranchs and pulmonate snails. Opisthobranchs are special in that their gill is located behind the heart, other than the prosobranchs and pulmonate snails, where the respective respiratory organs still are located in front of the heart.
Starting from a situation of early snails with their pallial cavity (developed from a pallial groove running all the way around the body with many gill filaments), was located in the rear part of the body, the frontal pallial cavity of the so-called prosobranchs is the result of a procedure called torsion. During torsion the visceral hump turned a 180 degrees to the left and consequently, the visceral hump and also the shell coiled up in the characteristic snail spiral.
In opisthobranchs, the torsion developed backwards and after a new turning of the visceral hump to the right (Detorsion) the gill now is placed on the right side of the body (as can be seen especially well in side-gill slugs ('Notaspidea'), not existing anymore today as a systematic group. The gill can also be located in the rear part of the body but in any case behind the heart.
In nudibranchs (Nudibranchia) finally, pallial cavity and gills have been reduced and the snail breathes with secondary gills, tufts of gills at the rear end of the body.
A sea hare's (Aplysia dactylomela) rhinophores (the
eyes below them). Picture: Scott Muzlie (Source).
Opisthobranchs have at least a single pair of tentacles, which they cannot withdraw. It is, if present, located near the mouth. Above it, in many groups there are the rhinophores. Those tentacle-like chemosensory organs, apart from tactile information, mainly acquire olfactory information (hence the name "rhinophore", literally "nose-bearer). Sea hares (Aplysia) have their name from their large rhinophores looking like rabbits' ears.
The rhinophores are very highly developed and in some species can even be withdrawn into a special pouch. Their interior surface is strongly enlarged and covered with sense cells and hair (cilia). Some opisthobranchs also emit pheromones that can be detected with the rhinophores.
The actual tentacles, apart from being tactile organs, also serve to acquire information of taste, which means they serve about the same task as a terrestrial snail's lips. Especially obvious this is in some predatory groups, which have additional mouth tentacles helping them to follow their prey along its slime thread (see e.g. head-shield slugs, Cephalaspidea).
|Cummins, S. F.; Erpenbeck, D.; Zou, Z.; Claudianos, C.; Moroz, L. L.; Nagle, G. T.; Degnan, B. M. (2009): "Candidate chemoreceptor subfamilies differentially expressed in the chemosensory organs of the mollusc Aplysia". BMC Biology 2009, 7:28 (Link).|
Veliger larva of Aeolidiella stephaniae.
Source: Kristof, Klussmann-Kolb (2005).
Opisthobranchs are hermaphrodites, as are the pulmonate snails. Generally there is a reciprocal mating and eggs are laid. Larval development usually takes place passing a planktontic veliger larval stage.
Picture right: Veliger larva of the nudibranch Aeolidiella stephaniae. Size bar: 100 µm. v: Velum (sail); cil: Cilia; mp: Metapodium (end of foot); o: Operculum (!); s: Shell.
In the picture on the right it is very well visible that the opisthobranch veliger larval stage still bears a shell lid (operculum), which is always reduced in adult opisthobranchs. Besides, veliger larvae also still have a shell, which is either discarded during the veliger stage (sea angels, 'Pteropoda') or reduced during the metamorphosis.
|Kristof, A.; Klussmann-Kolb, A. (2010): "Neuromuscular development of Aeolidiella stephanieae Valdéz, 2005 (Mollusca, Gastropoda, Nudibranchia)". Frontiers in Zoology 7: 5. (Link).|
In some groups, mating chains may occur. In sea hares (Aplysiidae) this for example is the case when the population density is rather high. On snail functions exclusively as a female, on only as a male, and all snails in between function as females and as males. Many opisthobranchs also show an almost dance-like mating behaviour.
Bubble shell (Bulla ampliata) from the island of Moorea, French
Polynesia. Picture: Gustav Paulay (CalPhotos).
For the benefit of a better movability, a large number of opisthobranchs show a more or less extensive reduction of the shell. It can be thin walled and bubble-like, covered by the mantle, or even be completely absent.
Among opisthobranch snails still carrying a shell there are the bubble shells (Bullidae), a part of the head-shield snails (Cephalaspidea). With their thin-walled shell they resemble the terrestrial glass snails (Vitrinidae) and like among those, in bubble shells there are also all forms of species with shells, without shells are stages in between both.
The nudibranch Chromodoris collingwoodi, from New South
Wales (Australia). Picture: Doug Anderson (Source).
Probably best known among sea slugs are the very colourful forms of nudibranchs (Nudibranchia), the diversity of which keeps on astonishing divers in all parts of the world. Nudibranchs, like all gastropods cannot see colours. But many sea slugs have defensive or even toxic compounds at their disposition, mostly acquired with the food (such as blue algae, Cyanobacteria). Against this background, the colours of sea slugs can be understood as a warning pattern to prevent those slugs from being eaten by fish. Unpoisonous slugs profit from the general warning, as the fish has no possibility of knowing whether a colourful sea slug is poisonous or not.
Bill Rudman: Defensive colour in sea slugs.
Many sea slug species also have camouflage colours, which make them disappear in front of their habitat's background. Predators can also be confused by suddenly appearing colours or even light (bioluminescence). But by no means all sea slugs are nudibranchs; shell-less forms are present in all opisthobranch groups, and even in some groups that are not opisthobranchs.
The nutrition of opisthobranchs is extremely various. Some herbivorous species use chemical properties of algae they only eat, because they grow on their actual food. A very large portion of opisthobranch gastropods are predators (like a large part of the sea gastropods in general). Nudibranchs (Nudibranchia) for example, depending on which species, feed on jellyfish, marine worms, other molluscs and even other nudibranchs. Also very interesting are the feeding habits of sap-sucking slugs (Sacoglossa). Those suck the cytoplasm from plant cells and many species even are able to use the chloroplasts for their own nutrition. Their radula is reduced to one row of teeth and many species lack a jaw. The astonishing hooded nudibranch (Melibe leonina), a nudibranch species from the American west coast, does not have a radula left at all. Its mouth opening has developed into a large hood with marginal tentacles, so the slug can catch small sea creatures like a Venus fly trap.
Left: Sea angel (Clione limacina). Right: Sea butterfly (Limacina helicina).
Pictures: Russ Hopcroft (Arctic Ocean Diversity)
A pair of herbivore and predator from the world of opisthobranchs are the so-called pteropods: The sea butterfly (Limacina helicina) and the sea angel (Clione limacina). Apart from their quite similar scientific names, both are pelagic gastropods living in the open sea, "flying" through the water propelled by their parapodia, wing-like extensions of their foot.
But while Limacina still possesses a translucent shell and filters the water for plankton using a mucus net, the sea angel, whose name is due to its angel-like parapodia, does not have any shell left. And, very little angel-like, it hunts sea butterflies.
There appears to be little resemblance between the fairy-like sea angel and a terrestrial snail.
Similar to terrestrial snails, the sea-living opisthobranchs are also an important link in the marine food chain. Among the animals feeding on opisthobranchs, there are many fish, also sea spiders (Pantopoda, e. g. Pycnogonidae), sea stars, crabs and birds. The defence mechanisms protecting many species of sea slugs so effectively, only reach their maximal effectiveness when the slug is fully grown. So the juveniles often are a welcome prey. Last but not least, predators of many sea slugs are other sea slugs, such as head shield slugs (Cephalaspidea) or nudibranchs (Nudibranchia). Especially there, however, the predator may very fast become the prey.
Bill Rudman: What eats sea slugs? on seaslugforum.net.
According to Bouchet
et al. (2005).
Due to their thin-walled shell, of one is present at all, opisthobranchs do not make very good fossils. But still they have been known since the Carboniferous, 290 - 360 mill. years ago (see Geological timeline).
Opisthobranchia systematics still remains somewhat of a discussion. Originally, like still is found in many especially older text books, there was the division of gastropods in the three groups Prosobranchia, Pulmonata and Opisthobranchia or, after the form of the neural system caused by the degree of torsion, in crossed-nerve snails (Streptoneura - Prosobranchia) and straight-nerve snails (Euthyneura - Pulmonata und Opisthobranchia). But for nearly 30 years, especially after the discovery of new snail groups near submarine hot vents, it has been clear that the systematics of gastropods is neither that simple nor that clear:
The prosobranch snails had to be placed opposing opisthobranchs and pulmonate snails in several independent groups, the opisthobranchs and pulmonates collectively referred to as Heterobranchia.
Relationships between opisthobranchs (Opis-
thobranchia) and pulmonate snails (Pulmona-
ta). [RN] After: Jörger et al. (2010).
|Haszprunar, G. (1985): "The Heterobranchia – a new concept of the phylogeny of the higher Gastropoda". Z. f. zool. Systematik u. Evolutionsforschung, Bd. 23 H. 1: 15 - 37.|
What remained unclear was so far, if the pulmonate snails might have evolved from ancestors in the area of the opisthobranchs, are, as appears more probable, independently evolved from different ancestors in the area of the opisthobranchs and the prosobranchs respectively. Until this mystery is solved, in the modern systematics according to Bouchet, Rocroi et al. (2005) both groups, Opisthobranchia and Pulmonata, remain a so-called informal group.
In the meantime it could be researched (and published in 2010) that the Acochlidia group (or Acochlidiacea following Bouchet et al.) is closely related to the pulmonate snails. This group of opisthobranch snails comprises amphibious, brackish water and fresh water forms, apart from sea-living species. According to the results of Jörger et al. (2010) the Acochlidia are more closely related to terrestrial snails (Clade Eupulmonata), such as terrestrial pulmonate snails (Stylommatophora) and coast snails (Ellobioidea), than those are related to other, fresh water, pulmonate snails (Basommatophora, e.g. ram's horn snails and pond snails).
That would indicate that terrestrial and freshwater pulmonate snails have evolved from different ancestors together with today's opisthobranch snails. Also, this would render the Pulmonata, as well as the Opisthobranchia paraphyletic groups, in which descendants of the same common ancestor are missing.
|Jörger, K. M.; Stöger, I.; Kano, Y.; Fukuda, H.; Knebelsberger, T.; Schrödl, M. (2010): "On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia". BMC Evolutionary Biology 10: 323 (Link).|
Head shield slugs (Cephalaspidea).
Sea butterflies and sea angels (Thecosomata und Gymnosomata).
Sea hares' relatives (Aplysiomorpha).
Sap-sucking sea slugs (Sacoglossa).
|Wägele, H.; Klussmann-Kolb, A. (2005): "Opisthobranchia (Mollusca, Gastropoda) – more than just slimy slugs. Shell reduction and its implications on defence and foraging". - Frontiers in Zoology 2005, 2:3. (Link)|
Dr. Bill Rudman: http://www.seaslugforum.net - Sea slug forum of the Australian Museum, not only about sea slugs, but also about opisthobranchs with shells.