Nudibranchs, also known as sea slugs, are some of the most beautiful and diverse creatures in the ocean.

There are over 3000 described species worldwide.

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

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.

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.