Opisthobranchs () is now an 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.
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,
anaspidea
The clade Anaspidea, commonly known as sea hares (''Aplysia'' species and related genera), are medium-sized to very large opisthobranch gastropod molluscs with a soft internal shell made of protein. These are marine gastropod mollusc
...
n
sea hares
The clade Anaspidea, commonly known as sea hares (''Aplysia'' species and related genera), are medium-sized to very large opisthobranch gastropod molluscs with a soft internal shell made of protein. These are marine gastropod molluscs in the ...
, pelagic
sea angels,
sea butterflies, and many families of the
Nudibranchia.
''Opisthobranch'' means "
gill
A gill () is a respiratory organ that many 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 respiration on land provided they ar ...
s behind" (and to the right) of the
heart
The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
. 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 period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...
.
Taxonomy
Under the 1931 classification system drawn up by
Johannes Thiele Johannes Thiele may refer to:
*Johannes Thiele (zoologist)
*Johannes Thiele (chemist)
Friedrich Karl Johannes Thiele (May 13, 1865 – April 17, 1918) was a German chemist and a prominent professor at several universities, including those in ...
, 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
It was speculated as far back as 1985 that the Opisthobranchia were
paraphyletic
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
, 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''; plural 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 n ...
, 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
Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribos ...
gene sequences could not resolve
monophyly
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic ...
versus
paraphyly of the Opisthobranchia
Subsequent
taxonomic classification in 2005 overturned the Opisthobranchia as a valid
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
, 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 Euthyneury is a plesiomorphic condition present in some gastropods which is a result of two evolutionary events. The first event, which was experienced by the ancestors of all extant gastropods, is known as torsion. Torsion describes the event in ...
(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
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, 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
In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; plural: parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed ...
, 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 and
cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...
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 resources and competitors (for ...
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 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
toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a sub ...
ity 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
Aeolidioidea is a superfamily of sea slugs, the aeolid nudibranchs. They are marine gastropod molluscs in the suborder Cladobranchia.
Taxonomy
As of 2019, the superfamily Aeolidioidea consisted of the following families:
* Aeolidiidae Gray, 1 ...
pirate the stinging cells from their
cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that ...
n prey and use them for their own defense.
Diet
Opisthobranchs may be
herbivores,
detritivores or
carnivores. 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 invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...
ns,
Cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that ...
, or
sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...
s, 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 known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...
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 found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
ic
larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
...
e 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
Opisthobranchs are
hermaphrodites and have complex reproductive strategies, typically involving reciprocal sperm transfer and storage until the eggs are ready for
fertilisation
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...
. 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 ]
See also
*
Symposia and workshops on opisthobranchs
*
Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
*
Changes in the taxonomy of gastropods since 2005
This overview lists proposed changes in the taxonomy of gastropods at the family level and above since 2005, when the taxonomy of the Gastropoda by Bouchet & Rocroi (2005) was published. In other words, these are recent updates in the way various ...
References
This article incorporates CC-BY-2.0 text from reference
Further reading
* Bieler R. (1990). "Haszprunar's "clado-evolutionary" classification of the Gastropoda—a critique". ''Malacologia'' 31(2): 371–380, 2 tabs.
8 May; G, Haszprunar's response published in ''Malacologia'', 1990, 32(1): 195–202
*
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