sea slug

(Sea slugs)

Phylum: Mollusca

Class: Gastropoda

Subclass: Opisthobranchia

Number of families: 110

Thumbnail description
Marine snails found in oceans throughout the world; the diversity encompasses species that range in habit from pelagic to burrowing, in color from transparent to vividly colored, and in structure from those with protective shells to soft-bodied forms with flowing movements

Evolution and systematics

Opisthobranchia is a subclass of gastropod mollusks whose evolutionary ancestors date back to the Paleozoic period (543–248 million years ago). The earliest ancestors were probably all benthic, burrowing animals with rigid shells covering their soft body tissues. As members of this group evolved, a major trend was towards reduction, internalization, or loss of the shell. Numerous lineages have arisen independently that have exhibited this same evolutionary trend. In place of the shell, opisthobranchs have evolved other defensive mechanisms, including cryptic or warning coloration, secretion of toxic chemicals, and the ability to fire stinging cells obtained from animals on which they feed. The subclass, Opisthobranchia, includes a great diversity of species. More than 3,000 species are recognized, and these are classified into more than 110 families and eight orders.

Physical characteristics

Physical appearance varies greatly among members of the Opisthobranchia. Generally, their bodies are divided into three sections: head, foot, and visceral mass. In some species, these components are easily visible; in others, the sections may have merged in various ways. The head is flattened in some species, but in others, it may bear up to four types of sensory tentacles: oral tentacles, which project from the sides of the mouth; rhinophores, which are located on top of the head; propodial tentacles, which occur on the front of the foot; or posterior cephalic tentacles, which project from the posterior portion of the head.

The foot has a flattened sole that is used for creeping over substrates. Some species have wide lateral extensions of the foot that appear winglike; these structures are called parapodial lobes and are used for swimming. The visceral mass contains the digestive and reproductive organs. It is covered by the mantle, which generates a shell in some species and hangs like a skirt around the body in others. The shell has become reduced, internalized, or lost in some of the more evolved species. The more primitive species have a gill located between the mantle and visceral mass near the head. In place of the gill, some of the more advanced species have cerata, small projections from their bodies, or a ruffled mantle edge to facilitate gas exchange.

Species range in size from some that are so minute that they can move between grains of sand to others that reach lengths of more than 17.7 in (45 cm). They vary in shape from rounded, shell-covered forms to elongate, often ornate, bodies. Some species are cryptically colored to blend in with their natural surroundings, while others are brightly colored, perhaps as a warning signal to predators; some of the pelagic species are nearly transparent.

Distribution

Opisthobranchs occur worldwide. The greatest diversity is in the tropical seas, but several species have been reported from the polar waters of Antarctic.

Habitat

Sea slugs are found in marine habitats such as reefs, intertidal areas, and the deep ocean. Some species live on the substrate, while others remain in the water column.

Behavior

Most species of sea slugs are benthic and live on some type of substrate as adults; a portion of these species can swim for short periods of time as well. Benthic sea slugs are slow-moving organisms. They live relatively sedentary lives, and some species spend their entire life on one prey organism such as a sponge or coral reef. Almost all dispersal to new areas occurs during the veliger larval stage; veligers will settle out of the water column only when suitable substrate is present. Some larvae have crossed entire ocean basins because of lack of suitable settlement substrate.

A smaller portion of species remain planktonic and float in the water column throughout their lives. Most pelagic species migrate up and down the water column on a daily cycle. They move closer to the surface at night and to deeper waters during the day; however, in some species, this pattern is reversed.

Sea slugs exhibit several defensive behaviors. Some species secrete toxic chemicals or retain stinging cells from animals they eat; these defenses can be used to ward off or harm potential predators. Other species, typically benthic, swim to escape predators, while pelagic species sink in the water column to avoid predation.

Feeding ecology and diet

Sea slugs feed on a wide variety of organisms. Some are herbivores that eat algae, while others are filter feeders that take in particles from the water. Most are carnivores that eat many types of animals, including hydra, sponges, corals, barnacles, worms, other mollusks, and even the eggs of other sea slugs, cephalopods, or fishes. Despite the diversity of organisms consumed by sea slugs as a group, many species are highly specialized feeders that consume only a single type of food item; some will only prey upon a single genus or species.

Most sea slugs have a pair of jaws and a radula, a rasping, tonguelike organ with rows of small teeth, that are used for feeding. The radula is capable of scraping, piercing, tearing, or cutting food particles. Species of sea slugs that lack a radula may suck in whole prey like a vacuum, pry tissues off of their prey, or have special adaptations for trapping food items. To overcome defensive mechanisms of their prey, such as spines, exoskeletons, or stinging cells, many sea slugs cover their food with mucus.

Few organisms prey on sea slugs; some sea slugs use chemical defenses to keep predation to a minimum. The sea slugs often take up distasteful or toxic chemicals from their own prey and, in turn, use them to deter predators; some nudibranchs even eat the stinging cells off of jellyfish and Portuguese man-of-war and use them to ward off potential predators. One of the most frequently reported acts of predation occurs between two different types of opisthobranch mollusks. Navanax inermis, a species of the order Cephalaspidea, feeds on nudibranchs as it crawls along the substrate; it follows the trail of slime left by a nudibranch, sneaks up onto the prey, and sucks the organisms in with a sort of suction tube from inside its body.

Reproductive biology

Sea slugs are hermaphrodites; each individual has both male and female sexual organs and produces sperm and eggs. Some simultaneous hermaphrodites possess male and female organs at the same time, while others begin life as one sex and then transform to the other gender. Individuals do not exchange their own sperm and eggs; the reproductive system of sea slugs keeps these products separate within an individual.

Mating behaviors are highly variable in sea slugs. Some species mate as pairs; others form long chains of individuals. In some species, particularly those that are simultaneous hermaphrodites, both sperm and eggs are reciprocally exchanged between partners at the same time. Individuals in other species behave as either male or female during a mating session. Mating may last from minutes to days, depending on the species. Fertilization may not take place immediately; an organ called the seminal receptacle can store sperm for several months until the eggs they will fertilize have reached maturity.

Eggs are laid in masses ranging from hundreds to millions of individual eggs; the masses are sometimes formed into chains or globules on the substrate or in the water column, and some species protect their egg masses with a mucous covering.

When eggs hatch, planktonic larvae emerge; these larvae, called veligers, soon develop a shell around them. A few species retain their eggs inside their bodies, and young emerge as juveniles, thereby eliminating the planktonic larval stage.

Conservation status

Although little is known about their populations, sea slugs are not considered threatened or endangered. No species are listed by the IUCN.

Significance to humans

Many sea slugs are admired by humans who are fortunate enough to view them during snorkeling or diving activities. Beyond their aesthetic appeal, they are of little significance to humans.

Species accounts

Resources

Lalli, Carol M., and Ronald W. Gilmore. Pelagic Snails: The Biology of Holoplanktonic Gastropod Mollusks. Stanford, CA: Stanford University Press, 1989.

"Sea Slug Forum." July 10, 2003 [July 27, 2003].

[Article by: Katherine E. Mills, MS]

A subclass in the class Gastropoda containing about 4000 living species, arranged in nine orders, including the herbivorous Aplysiomorpha (sea hares) and Sacoglossa and the carnivorous Thecosomata (sea butterflies) and Nudibranchia (sea slugs). Primitive members of many of the orders show adaptations for burrowing beneath sand or mud; more advanced members are always active surface-living or pelagic forms. This trend is accompanied by a decrease in the importance of the shell and operculum for passive defense. These are replaced by more dynamic chemical (some species secrete decinormal sulfuric acid through the skin if annoyed), physical (daggerlike calcareous epidermal spicules), or biological (redirected nematocysts derived from coelenterate prey) defensive mechanisms.

The adult shells of the primitive opisthobranchs living today are often strongly developed and sometimes colorful; a more typical opisthobranch shell is fragile, inflated, and egg-shaped. In rather more advanced forms, the external shell has a very wide gape, and in animals like Berthella the widely gaping shell is wholly internal, covered by the mantle. The most varied shells are found in the Sacoglossa. In the highest sacoglossans, some bullomorphs and aplysiomorphs, and in all the nudibranchs, the true shell is completely lost after larval metamorphosis. See also Gastropoda; Mollusca; Nudibranchia; Sacoglossa.

Opisthobranchia Fossil range: Carboniferous–Recent PreЄ Є O S D C P T J K Pg N
Notodoris minor
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
(unranked):	clade Heterobranchia informal group Opisthobranchia
Clades and groups
clade Cephalaspidea clade Thecosomata clade Gymnosomata clade Aplysiomorpha group Acochlidiacea clade Sacoglossa group Cylindrobullida clade Umbraculida clade Nudipleura

Bullina lineata

Opisthobranchs, pronounced /ɵˈpɪsθəbræŋks/, are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia. The group has also been called Euthyneura referring to the untorted visceral nerve cord.^[1]:223

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.

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^[2]. In light of recent research, this grouping is not a valid clade, and thus Opisthobranchia appears as an informal group within the Heterobranchia in the most recent taxonomy of the Gastropoda (Bouchet & Rocroi, 2005).

The Opisthobranchia currently 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.

Opisthobranchia are known from as early as the Carboniferous.^[3]

Contents 1 Description 2 Defence 3 Diet 4 Communication 5 Reproduction 6 Relation to pulmonates 7 An older taxonomy 8 References 9 External links

Description

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^[ambiguous] with a lens and cornea.^[4]

Defence

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.^[5][6]

Diet

Oxynoe olivacea

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^[7].

Communication

Pleurobranchus mamillatus

Like most lifeforms, they 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.

Reproduction

Winged snail Clione limacina, a sea angel.

Opisthobranchs are hermaphrodites and have complex reproductive strategies, typically involving reciprocal sperm transfer and storage until the eggs are ready for fertilisation^[8]. 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^[9].

Relation to pulmonates

Aplysia californica, a sea hare.

It is speculated that the Opisthobranchia may be paraphyletic^[10], 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 ^[11]

An older taxonomy

Order Opisthobranchia Milne-Edwards, 1848 (sea slugs)

• Suborder Cephalaspidea P. Fischer, 1883 (headshield slugs and bubble shells)
• Suborder Sacoglossa von Ihering, 1876 (sap-sucking slugs and bivalved gastropods)
• Suborder Anaspidea 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

A phylogenetic study published in November 2004, ^[12] gave new definitions of the seven main lineages of the Opisthobranchia.

References

1. ^ Wilbur, Karl M.; Trueman, E.R.; Clarke, M.R., eds. (1985), "5. Aspects of evolution within the Opisthobranchs", The Mollusca, 10. Evolution, New York: Academic Press, ISBN 0-12-728702-7 
2. ^ Knudsen, B.; Kohn, B.; Nahir, B.; Mcfadden, S.; Moroz, L. (Feb 2006). "Complete DNA sequence of the mitochondrial genome of the sea-slug, Aplysia californica: conservation of the gene order in Euthyneura". Molecular phylogenetics and evolution 38 (2): 459–469. doi:10.1016/j.ympev.2005.08.017. ISSN 1055-7903. PMID 16230032.  edit
3. ^ (in Czech) Pek I., Vašíček Z., Roček Z., Hajn. V. & Mikuláš R.: Základy zoopaleontologie. - Olomouc, 1996. 264 pp., ISBN 80-7067-599-3.
4. ^ Hughes, H. P. I. (1970). "A light and electron microscope study of some opisthobranch eyes". Zeitschrift für Zellforschung und Mikroskopische Anatomie 106: 79–98. doi:10.1007/BF01027719.  edit
5. ^ Gosliner, Terrence (1987) Nudibranchs of Southern Africa p.7 ISBN 0-930118-13-8
6. ^ Heike Wägele, Annette Klussmann-Kolb (2005). "Opisthobranchia (Mollusca, Gastropoda) – more than just slimy slugs. Shell reduction and its implications on defence and foraging". PubMed Central - Frontiers in Zoology 2 (3): 3. doi:10.1186/1742-9994-2-3. PMID 15715915. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=554092. 
7. ^ Gosliner, Terrence (1987) Nudibranchs of Southern Africa p.9 ISBN 0-930118-13-8
8. ^ Debelius, Helmut (2001) Nudibranchs and Sea Snails Indo-Pacific Field Guide p.7 Ikan, Frankfurt
9. ^ Gosliner, Terrence (1987) Nudibranchs of Southern Africa p.11 ISBN 0-930118-13-8
10. ^ (Haszprunar, 1985)
11. ^ Verena Vonnemann, Michael Schrödl, Annette Klussmann-Kolb and Heike Wägele (2005). "Reconstruction of the phylogeny of the Opisthobranchia (Mollusca: Gastropoda) by means of 18s and 28s rRNA gene sequences". Journal of Molluscan Studies 71 (2): 113-125. doi:10.1093. 
12. ^ Cristina Grandea, Josè Templadoa, J. Lucas Cerverab and Rafael Zardoya (2004). "Phylogenetic relationships among Opisthobranchia (Mollusca: Gastropoda) based on mitochondrial cox 1, trnV, and rrnL genes". Molecular Phylogenetics and Evolution 33 (2): 378–388. doi:10.1016/j.ympev.2004.06.008. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WNH-4D2WMC3-2&_user=10&_coverDate=11%2F01%2F2004&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8d8d0d6571bfce984ca4c209669ba07c. 

This article includes a list of references or external links, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations where appropriate. (July 2009)

• The Sea Slug Forum, a resource from the Australian Museum.
• Sea slugs of Hawaii
• Haszprunar G., 1985 The Heterobranchia – a new concept of the phylogeny of the higher Gastropoda. Z. f. zool. Systematik u. Evolutionforschung Bd. 23 H. 1:15–37
• 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].
• Bieler, R., 1992. Gastropod phylogeny and systematics. Annual Review of Ecology and Systematics, 23: 311–338.
• Verena Vonnemann, Michael Schrödl, Annette Klussmann-Kolb and Heike Wägele (2005). "Reconstruction of the phylogeny of the Opisthobranchia (Mollusca: Gastropoda) by means of 18s and 28s rRNA gene sequences". Journal of Molluscan Studies 71 (2): 113–125. doi:10.1093/mollus/eyi014. 

External links

Wikimedia Commons has media related to: Opisthobranchia

Wikispecies has information related to: Opisthobranchia

• Born to be Wild: Sea Slugs, retrieved 30.4.2008
• Various opisthobranch species from Indonesia, Philippines and Thailand

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