Nudibranchs

Laura Pepi

They’re small marine organisms with tentacles on their head, and they’re found in the lower intertidal and subtidal regions of the ocean. What are they? They’re Nudibranchs. Their name, if broken down to nudi-branchs, literally means naked gills. The marine Nudibranchs are bilaterally symmetrical externally, and they are quite small. They can be circular or elongated, and they usually have two antennae, called rhinopores, attached to their head. These antennae are used to smell each other and to locate food. Toward their backs they have gills, which are delicate and flower-like. Nudibranchs occur in a wide variety of shapes and are often brightly colored. "They have awesome colors, bizarre shapes, and weird behavior…To them, these colors, shapes and behavior are evolutionary adaptations that enable them to survive"(Bertsch 1999). These organisms can either be brightly colored or well camouflaged. One special characteristic that these organisms have is that since they lack shells for defense, most of them are good swimmers and many of the species have skin glands that secrete sulfuric acid to deter would be predators.

Nudibranchs are from the kingdom Animalia, the phylum Mollusca, and the class Gastropoda. They come from the subclass of Opisthobranchia, and the order Nudibranchia. There are four suborders - Doridoidea, Aeolidoidea, Dendronotoidea, and Arminoidea. These contain forty-eight families, and one thousand different species (Grzimek 1970).

Nudibranchs are related to other shell-less Opisthobranchiate snails mostly because they don’t have shells, and because most of the species resemble what they feed on. This is similar to some of the Opisthobranchiate snails. The four suborders mostly differ either in how they breathe, or the way in which they interact with their environment. All Nudibranchs are flesh eaters, or carnivores. They feed on other invertebrates such as sponges, soft corals, anemones, sea pens, Portuguese man-of-wars, bryozoa, ascidians, and hydroids.

Nudibranchs have feathery gills that surround the anus. These gills are used in gas exchange, and can also be used in moving and feeding. During feeding the gills can help to sweep the food toward the mouth so it is easier to get. Some Nudibranchs are good swimmers. All species of Nudibranchs have a foot that extends for the length of its body. Through strong muscular actions the organism is able to send waves along the foot in the opposite direction that the organism is moving. This allows them to get away from predators, or to find food.

Nudibranchs are hermaphrodites, meaning that they have male and female gonads, but they cannot self fertilize. In order to mate, two Nudibranchs come together side by side and pass sperm sacs though a tube in their "neck" to each other. The sperm is stored until the eggs develop, when fertilization occurs. They then both go their own ways and lay egg masses, which can contain millions of eggs (Ellis 1999).

Chemical defense is used to protect the spawn against predation. Nudibranchs don’t play any part in raising their young. Development can take two forms. In direct development, the eggs are laid in a firm mass on or near a food source and the young hatch fully developed. The other form of development is called larval development. In this form the eggs are laid in a filamentous mass on any surface. When the young hatch they are carried in water currents. They then eventually settle onto a food surface and continue to develop in to adults. The reason for laying such a large quantity of eggs in this form is because many of the eggs don’t make it to adulthood. Nudibranchs usually live up to one year depending on food and suitable conditions. Like many other organisms, there are times that there are population explosions and droughts. Water temperature may also play a role in their survival chances. Nudibranchs that feed on Hydroids can be seen more than once throughout the year. And those that eat sponges can live longer than a year. Most other Nudibranchs only live for a year. In a study done that related life cycles and dietary needs, researchers found that species that have more than one generation each year were strongly associated with their dietary species and that their distribution was dependent of their dietary species (Aerts 1994).

Nudibranchs can be found in most of the world’s seas and oceans. They are mainly found along the bottom. There aren’t really any special habitat requirements. It depends on the species and whether they mimic other species or whether they camouflage into the scenery around them. There have been studies done that looked at the coloring patterns of some of the species. Researchers have developed a theory that many species mimic each other. So depending on the species, they could be required to be around other species to be able to mimic others. Some are camouflaged, blending into their environment, whereas others are brightly colored. Some species also have the interesting characteristic of mimicry. Mimicry allows one species of Nudibranchs change to look more like another species of Nudibranchs. Some mimic another species that may have toxic glands. This can be useful, and results in a higher rate of survival because predators have learned not to eat certain species of Nudibranchs. The coloration of a Nudibranch can also be changed in order to mimic the color of their food to be able to feed on that source.

Some Nudibranchs have the ability to move around almost unnoticed to the naked eye. Some species are very small and cannot be seen, and they are able move through grains of sand. While others are larger, but have the ability to blend into their surroundings. Their sizes range from 1-30cm.

These fascinating creatures are both clever as to staying alive, and very beautiful due to their color patterns. And the best part of it all is that they live just about everywhere from Antarctica to the tropics.

Literature Cited

Aerts, L.A.M. 1994. "Seasonal distribution of nudibranchs in the southern Delta

Area, S.W. Netherlands. Journal of Molluscan Studies. Vol. 60, no. 2, pg. 129 –

139.

Allaby, M. 1992. "The Oxford Dictionary of Zoology." Oxford University Press,

Pg. 318-319.

Bertsch, H. 1999. "Nudibranchs: Marine Slugs with Verve." Online. The slug site.

Available: http://siolibrary.ucsd.edu/slugsite/nudi_han.htm Last updated:

2/6/99.

Ellis, W. "Nudibranch: an Introduction." Online. Nudis site. Available:

http://ozemail.com.au/~glaskin/nudibr.htm#whatarendwx. Last updated:

8/17/99.

Grzimek. 1970. "Animal Life Encyclopedia." Van Nostrand Reinhold Company,

New York. Vol. 3, Mollusks and Echinoderms. Pg. 110-113.