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Nudibranchs
Two nudibranch species common to B.C. waters. Hermissenda crassicornis (left photo) grows to 50 mm and is found along the west coast from Alaska to Mexico. Dendronotus iris (right photo) is one of the larger species, growing up to 200 mm long. It is seen here heading towards its prey species, Pachycerianthus finbriatus, the burrowing anemone. (Photos courtesy of the Bamfield Marine Station)
External Morphology
Nudibranchs are essentially snails without shells, and their name
literally means "naked gill". In most species, the gills
are prominently displayed on their dorsal surface. Some
kinds of nudibranchs have a tuft of gills located on their backs
at the posterior end, which are used only for respiration. Other
kinds of nudibranchs have many finger-like structures on their backs
called cerata, which function for gas exchange and often defense
(see Defense below).
Sea slugs have a pair of tentacles (called rhinophores) located on
top of their heads, which biologists believe are used as sensory organs
to assist in finding food and seeking a mate. Many
nudibranchs are brilliantly coloured while others are subtly coloured
to match their background, thereby providing camouflage.Habitat and Distribution Nudibranchs
are benthic organisms, meaning they live on the ocean bottom. They
can be found crawling over rocks, seaweeds, sponges, corals and
many other substrates.
Sea slugs range from the lower intertidal zone to depths of over
700 m. They
are found all over the world, including the British
Isles, the tropics
and even the Antarctic.Diet All known nudibranchs are grazing carnivores and as a whole they feed on a wide variety of animals including sponges, hydroids, tunicates, anemones, corals, sea pens, bryozoans, barnacles, and sometimes other nudibranchs! Each species of nudibranch tend to be very selective feeders, and may only eat one certain prey species (e.g. the giant swimming nudibranch, Dendronotus iris, only feeds on the tube-dwelling anemone, Pachycerianthus fimbriatus - see photo above). Most nudibranchs have a ribbon of teeth called a radula, the structure of which is usually adapted to the animal's particular prey items. For example, many dorid nudibranchs (Suborder Doridacea) have broad radulae with numerous teeth for grazing on sponges, while most aeolid nudibranchs (Suborder Aolidacea) have narrow radulae and strong jaws for feeding on hydroids and bryozoans.Reproduction and Development Nudibranchs are simultaneous hermaphrodites, which means that they possess both male and female sex organs at the same time. This strategy increases the probability of finding a mate, since every mature individual of the same species is a potential partner (self-fertilization is very rare). After mating, nudibranchs lay their egg masses either on or near the organism on which they feed. These egg masses vary in shape, size and colour depending on species. Some sea slugs lay single coils of eggs, while others are in the shape of a thick ribbon wound into a spiral. The egg masses are often white, but they can also be red, pink, orange or any other colour depending on the species. Egg development can take between 5 and 50 days, and is strongly influenced by temperature. Warmer waters generally result in a shorter embryonic period. Usually
the eggs develop first into a larval form called a veliger, which
drifts in the ocean currents as plankton. Specific environmental conditions
trigger the larvae to settle and metamorphose into the adult form.
This larval dispersal is important in the successful exploitation
of new areas, since adult nudibranchs move very slowly and cannot
travel long distances.Defense Since nudibranchs have lost their protective shells, they require alternative means of defense. The bright colouration seen in many species is believed to warn potential predators that the nudibranchs contain distasteful or even toxic compounds. Many of the dorid nudibranchs (Suborder Doridacea) are strongly scented, which is thought to be a chemical anti-predator warning. Many sea slugs feed on sea anemones and hydroids (Phylum Cnidaria), which contain stinging cells called nematocysts. The nudibranchs are able to block the discharge of these nematocysts during digestion, and they are then passed to special storage sacs in the cerata. The adopted stinging cells form part of a defensive mechanism against any predators that attempt to prey on the nudibranchs.
Some nudibranchs can also swim short distances when disturbed by predators
- they contract their body muscles and undulate through the water
while flapping their cerata.Interesting Facts Some nudibranchs in tropical waters feed on corals (Phylum Cnidaria), which contain photosynthetic single-celled algal symbionts called zooxanthellae. These nudibranchs are able to remove the algae intact from the coral's tissues and store them in their cerata, where the algae photosynthesize and provide the host nudibranch with a continuous supply of manufactured sugars. Beachwalkers can search for nudibranchs in tidal pools at low tide - try looking under loose rocks or near animals that they are known to feed on (e.g. sponges and bryozoans). One of the best ways to locate nudibranchs is by spotting their egg masses, which are usually quite visible - the parents are likely nearby! If you are lucky enough to find nudibranchs in tide pools or while diving, please do not attempt to pick-up or touch them. These creatures are very soft and easily damaged, and you may hurt them inadvertently.Useful Internet SitesThe Slug Site - This site features a "Nudibranch of the Week", with lots of great pictures and links to other nudibranch sites all over the world. The Sea Slug Forum - An Australian Museum site where you can ask questions, post information and find out all about nudibranchs, and their close relatives. Check out OceanLink's Questions and Answers about Marine Molluscs ReferencesBehrens, D.W. 1991. Pacific Coast nudibranchs: a guide to the opisthobranchs, Alaska to Baja California. 2nd ed. Sea Challengers, California, U.S.A. Nicol, J.A. 1968. The Biology of Marine Animals. 2nd ed. Pitman Publishing, New York, U.S.A. Pechenik, J.A. 1996. Biology of the Invertebrates. 3rd ed. WCB Publishers, Iowa, U.S.A. Sumich, J.L. 1980. An Introduction to the Biology of Marine Life. WCB Publishers, Iowa, U.S.A. Thompson, T.E. 1976. Nudibranchs. T.F.H. Publications, New Jersey, U.S.A. |
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