Opisthobranchia
Encyclopedia
Opisthobranchs 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. Euopisthobrancha is a collection of opisthobranchs that is monophyletic, but this group leaves out some "traditional" opisthobranchs.
The Opisthobranchia includes a number of species in the order Cephalaspidea
(bubble shells and headshield slugs), the sacoglossa
ns, the anaspidean sea hares, the pelagic sea angels, the sea butterflies, and many families of Nudibranchia.
Opisthobranch means gill
s 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 tentacle
s and a single gill
behind and to the right of the heart.
Opisthobranchia are known from as early as the Carboniferous
.
in 1931 Gastropoda were divided into the Prosobranchia
, Pulmonata
and Opisthobranchia. The later two were later combined into a single order.
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It is speculated that the Opisthobranchia may be paraphyletic, 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
, 1848 - sea slugs
A phylogenetic study published in November 2004, 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)
. Accordingly, recent articles do not use the term.
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 tentacle
s, situated close to the mouth, are used for orientation. Behind them are the rhinophore
s, 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
.
s. Opisthobranchs have a global distribution, but are restricted almost exclusively to marine habitats with the only exception being few freshwater acochlidians.
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 spider
s.
Opisthobranchs secrete irritants such as strong acid
s or accumulate toxins from their food. Aeolidioidea
pirate the stinging cells from their cnidaria
n prey and use them for their own defence.
Opisthobranchs may be herbivore
s, detritivores or carnivore
s. Being slow, the carnivores hunt sedentary prey. They may eat bryozoa
ns, 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 chloroplast
s of the algae they eat.
Like most lifeforms, they use chemical cues for much of their life cycle. The plankton
ic larva
e float until a pheremone alerts them to a suitable settling site, sometimes delaying metamorphosis
until favourable chemicals, such as prey pheromone
s, are detected. Some mating opisthobranchs chemicals to attract conspecifics.
Opisthobranchs are hermaphrodite
s 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.
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
gastropods previously united under Opisthobranchia within the Heterobranchia
Heterobranchia
Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....
, but no longer considered to represent a monophyletic grouping. Euopisthobrancha is a collection of opisthobranchs that is monophyletic, but this group leaves out some "traditional" opisthobranchs.
The Opisthobranchia includes a number of species in the order Cephalaspidea
Cephalaspidea
The suborder Cephalaspidea, also known as the headshield slugs and bubble snails, is a clade of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia....
(bubble shells and headshield slugs), the sacoglossa
Sacoglossa
Sacoglossa, commonly known as the sacoglossans or the "sap-sucking sea slugs", are a clade of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia...
ns, the anaspidean sea hares, the pelagic sea angels, the sea butterflies, and many families of Nudibranchia.
Opisthobranch means gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s behind (and to the right of the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
). In contrast Prosobranch means gills in front (of the heart). Opisthobranchs are characterized by two pairs of tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
s and a single gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
behind and to the right of the heart.
Opisthobranchia are known from as early as the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
.
Taxonomy
Under the old classification system by Johannes ThieleJohannes Thiele
Johannes Thiele, full name Karl Hermann Johannes Thiele was a German zoologist specialized in malacology. His Handbuch der systematischen Weichtierkunde is a standard work...
in 1931 Gastropoda were divided into the Prosobranchia
Prosobranchia
Not to be confused with the bivalve order Protobranchia.Prosobranchia was a large taxonomic subclass of sea snails, land snails and freshwater snails. This taxon of gastropods dates back to the 1920s...
, Pulmonata
Pulmonata
The Pulmonata, or "pulmonates", are an informal group of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills...
and Opisthobranchia. The later two were later combined into a single order.
Relation to pulmonates
Pulmonata
The Pulmonata, or "pulmonates", are an informal group of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills...
, although this is still disputed.
The Pulmonata may be a sibling group to an opisthobranch taxon
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
. The Opisthobranchia are therefore not a monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
group and can no longer be accepted as a valid taxon. They are now included within the subclass Orthogastropoda
Orthogastropoda
Orthogastropoda was a major taxonomic groupings of snails and slugs, an extremely large subclass within the huge class Gastropoda according to the older taxonomy of the Gastropoda ....
. 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
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
versus paraphyly
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
of the Opisthobranchia
Linnean taxonomy
Order Opisthobranchia Milne-EdwardsAlphonse Milne-Edwards
Alphonse Milne-Edwards was a French mammalologist, ornithologist and carcinologist. He was English in origin, the son of Henri Milne-Edwards and grandson of Bryan Edwards, a Jamaican planter who settled at Bruges .Milne-Edwards obtained a medical degree in 1859 and became assistant to his father...
, 1848 - sea slugs
- Suborder CephalaspideaCephalaspideaThe suborder Cephalaspidea, also known as the headshield slugs and bubble snails, is a clade of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia....
P. Fischer, 1883 - headshield slugs and bubble shells - Suborder SacoglossaSacoglossaSacoglossa, commonly known as the sacoglossans or the "sap-sucking sea slugs", are a clade of small sea slugs and sea snails, marine gastropod mollusks that belong to the clade Heterobranchia...
von Ihering, 1876 - sap-sucking slugs and bivalved gastropodsJuliidaeJuliidae, common name the bivalved gastropods, is a family of minute sea snails, marine gastropod mollusks or micromollusks in the superfamily Oxynooidea, an opisthobranch group.... - Suborder Aplysiomorpha P. Fischer, 1883 - sea hares
- Suborder NotaspideaNotaspideaNotaspidea, also known as the sidegill slugs, was a suborder which included both sea slugs and sea snails or false limpets, marine opisthobranch gastropod molluscs in the subclass Orthogastropoda....
P. Fischer, 1883 - sidegill slugs - Suborder Thecosomata BlainvilleHenri Marie Ducrotay de BlainvilleHenri Marie Ducrotay de Blainville was a French zoologist and anatomist.Blainville was born at Arques, near Dieppe. In about 1796 he went to Paris to study painting, but he ultimately devoted himself to natural history, and attracted the attention of Georges Cuvier, for whom he occasionally...
, 1824 sea butterflies with shells - Suborder Gymnosomata BlainvilleHenri Marie Ducrotay de BlainvilleHenri Marie Ducrotay de Blainville was a French zoologist and anatomist.Blainville was born at Arques, near Dieppe. In about 1796 he went to Paris to study painting, but he ultimately devoted himself to natural history, and attracted the attention of Georges Cuvier, for whom he occasionally...
, 1824 - sea angels, no shells - Suborder Nudibranchia BlainvilleHenri Marie Ducrotay de BlainvilleHenri Marie Ducrotay de Blainville was a French zoologist and anatomist.Blainville was born at Arques, near Dieppe. In about 1796 he went to Paris to study painting, but he ultimately devoted himself to natural history, and attracted the attention of Georges Cuvier, for whom he occasionally...
, 1814 - nudibranchs- Infraorder Anthobranchia FérussacAndré Étienne d'Audebert de FérussacBaron André Étienne Justin Pascal Joseph François d'Audebert de Férussac was a French naturalist best known for his studies of molluscs...
, 1819 - Infraorder Cladobranchia Willan & Morton, 1984
- Infraorder Anthobranchia Férussac
A phylogenetic study published in November 2004, gave new definitions of the seven main lineages of the Opisthobranchia. However, the grouping is no longer considered to represent a valid clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, and thus appears as an informal group within the Heterobranchia
Heterobranchia
Heterobranchia, the heterobranchs or Euthyneura, is a taxonomic clade of snails and slugs, which includes species from the sea, the land and freshwater; marine, aquatic and terrestrial gastropod mollusks....
in the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)
The taxonomy of the Gastropoda as it was revised by Philippe Bouchet and Jean-Pierre Rocroi is currently the most up-to-date overall system for classifying gastropod mollusks...
. Accordingly, recent articles do not use the term.
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.Opisthobranchs have undergone detorsion, an evolutionary reversal of the half revolution torsion
Torsion (gastropod)
Torsion is an anatomical event which takes place during the very early part of the life of snails and slugs of all kinds. In other words, torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle and shell 180˚...
of their immediate ancestors.
There is no marked distinction between head and mantle. The tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
s, situated close to the mouth, are used for orientation. Behind them are the rhinophore
Rhinophore
A rhinophore is one of a pair of club-shaped structures which are the most prominent part of the external head anatomy of a group of sea slugs, marine gastropod opisthobranch mollusks in the order Nudibranchia, the nudibranchs, specifically the dorid nudibranchs.- Etymology :The name relates to the...
s, 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
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
and cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
.
Ecology
Opisthobranchia represents a morphologically diverse group of gastropods occupying a great variety of ecological nicheEcological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s. Opisthobranchs have a global distribution, but are restricted almost exclusively to marine habitats with the only exception being few freshwater acochlidians.
Defence
Opisthobranchs are principally soft-bodied marine creatures with a reduced or absent shellMollusc shell
The mollusc shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes...
and no operculum
Operculum (gastropod)
The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...
and utilize other methods for protection. Due to a combination of outstanding camouflage
Camouflage
Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...
and aggressive toxicity they have few predators. However some utilize warning colouration
Aposematism
Aposematism , perhaps most commonly known in the context of warning colouration, describes a family of antipredator adaptations where a warning signal is associated with the unprofitability of a prey item to potential predators...
. Animals that do predate opisthobranchs include other opisthobranchs and toxin-resistant predators like sea spider
Sea spider
Sea spiders, also called Pantopoda or pycnogonids, are marine arthropods of class Pycnogonida. They are cosmopolitan, found especially in the Mediterranean and Caribbean Seas, as well as the Arctic and Antarctic Oceans. There are over 1300 known species, ranging in size from to over in some deep...
s.
Opisthobranchs secrete irritants such as strong acid
Strong acid
A strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...
s or accumulate toxins from their food. Aeolidioidea
Aeolidioidea
Aeolidioidea is a superfamily of small sea slugs, the aeolid nudibranchs. They are gastropod mollusks.-2005 taxonomy :The superfamily Aeolidioidea consists of the following four families :* Aeolidiidae Gray, 1827* Facelinidae Bergh, 1889* Glaucidae Gray, 1827* Piseinotecidae Edmunds, 1970Synonyms...
pirate the stinging cells from their cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
n prey and use them for their own defence.
Diet
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
s, detritivores or carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...
s. Being slow, the carnivores hunt sedentary prey. They may eat bryozoa
Bryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
ns, Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, 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 chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
s of the algae they eat.
Communication
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
ic larva
Larva
A larva 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 pheremone alerts them to a suitable settling site, sometimes delaying metamorphosis
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
until favourable chemicals, such as prey pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...
s, are detected. Some mating opisthobranchs chemicals to attract conspecifics.
Reproduction
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
s and have complex reproductive strategies, typically involving reciprocal sperm transfer and storage until the eggs are ready for fertilisation
Fertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
. 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.
Further reading
- Bieler R. (1990). "Haszprunar's "clado-evolutionary" classification of the Gastropoda—a critique". Malacologia 31(2): 371–380, 2 tabs. [28 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 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