Nudibranch

Nudibranch
Berghia coerulescens
A pair of Chromodoris lochi from Puerto Galera, the Philippines.
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
(unranked): clade Heterobranchia

informal group Opisthobranchia
clade Nudipleura
clade Nudibranchia
Clades
  • Euctenidiacea
  • Dexiarchia

See text for superfamilies

A nudibranch (pronounced /ˈnjuːdɨbræŋk/)[1] is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine opisthobranch gastropod mollusks which shed their shell after their larval stage[2]. They are noted for their often extraordinary colors and striking forms. The clade Nudibranchia is the largest clade within the heterobranchs, with more than 3,000 described species.

The word "nudibranch" comes from the Latin nudus, naked, and the Greek brankhia, gills.

Nudibranchs are often casually called "sea slugs", a non-scientific term. This has led some people to assume that every sea slug must be a nudibranch. Nudibranchs are very numerous in terms of species, and are often very attractive and noticeable, but there are a wide variety of other kinds of sea slugs, and these belong to several taxonomic groups that are not very closely related to nudibranchs. A fair number of these other sea slugs are colorful, and can be confused with nudibranchs.

These other marine shell-less gastropods or "sea slug" groups include additional heterobranch shell-less gastropod groups such as the Cephalaspidea sea slugs including the colorful Aglajidae, and other heterobranchs such as the Sacoglossa, the sea butterflies, the sea angels, and the often rather large sea hares. The term sea slug is also sometimes loosely applied to the only very distantly related pelagic caenogastropods within the superfamily Carinarioidea, and may also be casually used for the even more distantly related pulmonate sea slugs, the Onchidiidae.

Contents

Distribution

Nudibranchs occur in seas worldwide.

Habitat

Nudibranchs live at virtually all depths of salt water, but reach their greatest size and variation in warm, shallow waters.

Aeolidiella stephanieae. Typical body of a nudibranch with cerata consist of:
ot = oral tentacles,
ft = foot tentacles,
e = eye,
r = rhinophores,
c = cerata. This species has cnidosacs at the cerata tips.
Chromodoris willani is a dorid nudibranch. Dorids breathe with branchial plumes on their back.
Spanish Dancer taken at night, Red Sea.

Description

The body forms of nudibranchs vary enormously, but because they are opisthobranchs, unlike most other gastropods they are bilaterally symmetrical because they have undergone secondary detorsion. Some species have venomous appendages on their sides. These are used to deter predators. Many also have a simple gut and a mouth with a radula.

They lack a mantle cavity.

Solar powered Pteraeolidia ianthina have adapted cerata to contain zooxanthellae which continue to photosynthesize and provide energy to the nudibranch.

Their eyes are simple and able to discern little more than light and dark.[3] The eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors.[4]

They vary in adult size from 20 to 600 millimetres (0.79 to 24 in).

The adult form is without a shell or operculum (a bony or horny plate covering the opening of the shell, when the body is withdrawn).

The name nudibranch is appropriate, since the dorids (infraclass Anthobranchia) breathe through a "naked gill shaped" like branchial plumes of bushy extremities on their back, near their tail rather than using gills.[5] By contrast, on the back of the aeolids in the clade Cladobranchia there are brightly colored sets of protruding organs called cerata.

Nudibranchs have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect odors.

Life habits

Reproduction

Nudibranch mating behavior in Nembrotha purpureolineata.
Dorid nudibranch egg ribbon in Moss Beach, California
Acanthodoris lutea laying eggs

Nudibranchs are hermaphroditic, and thus have a set of reproductive organs for both sexes, but they can rarely fertilize themselves.

Nudibranchs typically deposit their eggs within a gelatinous spiral.[6]

Feeding

Most nudibranchs are carnivorous. Some feed on sponges, others on hydroids, others on bryozoans, and some eat other sea slugs, or, on some occasions, are cannibals and prey on members of their own species. Other groups feed on tunicates, barnacles, or anemones.

Nudibranchs (Nembrotha kubaryana) eating clavelina tunicate colonies. They also take the stinging cells off jellyfish when they eat them.

The surface dwelling nudibranch, Glaucus atlanticus is a specialist predator of siphonophores, such as the Portuguese Man O' War. This predatory mollusk sucks air into its stomach to keep it afloat and using its muscular foot it clings to the surface film. If it finds a small victim Glaucus simply envelopes it with its capacious mouth, but if the prey is a larger siphonophore the mollusk nibbles off its fishing tentacles, the ones carrying the most potent nematocysts. Like some others of its kind Glaucus does not digest the nematocysts; instead, it uses them to defend itself by passing them from its gut to the surface of its skin.[7]

Colors and camouflage

Among this group can be found the most colorful creatures on earth. In the course of evolution, sea slugs have lost their shell because they have developed other defense mechanisms. Their anatomy may resemble the texture and color of the surrounding plants, allowing them to camouflage (cryptic behavior). Others, as seen especially well on chromodorids, have an intense and bright coloring, which warns that they are distasteful or poisonous (aposematic coloration).

Nudibranchs that feed on hydroids can store the hydroids' nematocysts (stinging cells) in the dorsal body wall, the cerata.[8] The nematocysts wander through the alimentary tract without harming the nudibranch. Once further into the organ, the cells are brought to specific placements on the creature's hind body via intestinal protuberances. Nudibranches can protect themselves from the hydroids and their nematocysts. It is not yet clear how, but special cells with large vacuoles probably play an important role. They can also take in plants' chloroplasts (plant cell organelles used for photosynthesis) and use them to make food for themselves.

Some sponge-eating nudibranch concentrate the toxins from their prey sponge in their bodies, rendering themselves toxic to predators[9].

Another method of protection is the release of an acid from the skin. Once the specimen is physically irritated or touched by another creature, it will release the slime automatically.

Taxonomy

The taxonomy of the Nudibranchia is still under investigation. Many taxonomists in the past treated the Nudibranchia as an order, based on the authoritative work of Johannes Thiele (1931), who built on the concepts of Henri Milne-Edwards (1848).

Newer insights derived from morphological data and gene-sequence research, have confirmed these ideas. On the basis of investigation of 18S rDNA sequence data, there is strong evidence for support of the monophyly of the Nudibranchia and its two major groups, the Anthobranchia/Doridoidea and Cladobranchia.[10] A study published in May 2001, has again revised the taxonomy of the Nudibranchia.[11] They are thus divided into two major clades:

The dorids (infraorder Anthobranchia) have the following characteristics: the branchial plume forms a cluster on the posterior part of the neck, around the eyes. Fringes on the mantle do not contain any intestines.

The aeolids (infraorder Cladobranchia) have the following characteristics: Instead of the branchial plume, they have cerata. They lack a mantle. Only species of the Cladobranchia are reported to house zooxanthellae.

According to the taxonomy by Bouchet & Rocroi (2005), currently the most up-to-date system of classifying the gastropods , the Nudibranchia are a subclade within the clade of the Nudipleura. The Nudibranchia are then divided into two clades :

Nembrotha cristata, in Bunaken National Park
Clown nudibranch Triopha catalinae, Northern California
Chromodoris annae from Lembeh Straits, Indonesia

References

  1. ↑ Longman Pronunciation Dictionary (2nd edition), ISBN 0582364671
  2. ↑ doi:10.1017/S0025315400006044
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  3. ↑ http://ngm.nationalgeographic.com/2008/06/nudibranchs/holland-text/2
  4. ↑ CHASE, RONALD (June 1, 1974). "The Electrophysiology of Photoreceptors in the Nudibranch Mollusc, Tritonia Diomedia". Journal of experimental biology 60 (3): 707. PMID 4847278. http://jeb.biologists.org/cgi/content/abstract/60/3/707. 
  5. ↑ Dayrat, B. (2005). "Advantages of naming species under the PhyloCode: An example of how a new species of Discodorididae (Mollusca, Gastropoda, Euthyneura, Nudibranchia, Doridina) may be named" (PDF). Marine Biology Research 1: 216–232. doi:10.1080/17451000510019141. https://campillos.ucmerced.edu/~bdayrat/PDF%20of%20Papers/Dayrat-MBR-2005.pdf. Retrieved 2009-06-14. 
  6. ↑ Klussmann-Kolb A (2001). "The Reproductive Systems of the Nudibranchia (Gastropoda, Opisthobranchia): Comparative Histology and Ultrastructure of the Nidamental Glands with Aspects of Functional Morphology". Zoologischer Anzeiger 240 (2): 119–136. doi:10.1078/0044-5231-00011. http://www.ingentaconnect.com/content/urban/351/2001/00000240/00000002/art00011. 
  7. ↑ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
  8. ↑ Frick, K (2003). "Predator Suites and Flabellinid Nudibranch Nematocyst Complements in the Gulf of Maine.". In: SF Norton (ed). Diving for Science...2003. Proceedings of the American Academy of Underwater Sciences (22nd Annual Scientific Diving Symposium). http://archive.rubicon-foundation.org/4744. Retrieved 2008-07-03. 
  9. ↑ Gosliner, T.M. 1987. Nudibranchs of Southern Africa ISBN 0-930118-13-8
  10. ↑ Wägele H. & Willan R. C. (September 2000). "Phylogeny of the Nudibranchia". Zoological Journal of the Linnean Society 1 (1): 83–181. doi:10.1111/j.1096-3642.2000.tb02196.x. 
  11. ↑ Schrödl M., Wägele H. & Willan R. C. (2001). "Taxonomic Redescription of the Doridoxidae(Gastropoda: Opisthobranchia), an Enigmatic Family of Deep Water Nudibranchs, with Discussion of Basal Nudibranch Phylogeny". Zoologischer Anzeiger, 240 (1): 83. doi:10.1078/0044-5231-00008. http://www.ingentaconnect.com/content/urban/351/2001/00000240/00000001/art00008. 

Further reading

External links

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