Sea slugs belong to the phylum Mollusca, class Gastropoda, subclass Opisthobranchia. Some opisthobranchs have a small shell, not large enough to protect the whole animal (as in the order Prosobranchia). Most opisthobranchs are small size, soft bodied, often very colourful animals, loved by scuba divers.
Usually we identify the sea slugs with the nudibranchs. Actually this is the more interesting group, with a very high biodiversity, and amazing colours and shapes, making them very popular with divers and great subjects for macro photography. Nudibranchia is only an order of the Opistobranchia, so not all sea slugs are Nudibranchs.
Opistobranchs move by crawling on a muscular foot, with a flat sole, moving forward via a wave of contraction from the head to the tail. A few sea slugs like the sea hares, e.g. Aplysia fasciata and Aplysia depilans, two failrly common species on the Costa Brava, can swim for short tracts, undulating the fleshy mantle to glide through the water in an ungainly fashion reminiscent of the Disney character “Dumbo” when first learning to fly!
The main sense organs are located on the head: The eyes are very small in most sea slugs, the antennas being a very important sensory organ for touch and smell. Nudibranch antennas are called rhinophores. The skin covering the back is called a mantle. The mantle produces the shell in shelled species. Gills are external in most nudibranches (but they can be retracted) and have a respiratory function. Some opistobranchs have no gills, but breath through their skin.
Like in the other molluscs, the radula is the organ used for feeding, a sort of tongue covered with small teeth, whose number and shape vary according to the feeding habits. Sea slug distribution follows the availability of food and they are usually found close to their favourite food source. Amongst sea slugs we find many different feeding strategies. There are herbivorous and carnivorous species, mollusc predators or species feeding on sessile animals (hydrozoans, corals, sea squirts, sponges, bryozoans). Many species have an extremely specialized diet, feeding on only one prey.
There are diurnal and nocturnal sea slugs.
The majority of sea slugs accumulate or secrete toxic or repellent substances (self produced or derived from the diet), making them unpalatable to predators. The striking colour patterns, especially in nudibranches, are warning colours.
Most species are simultaneous hermaphrodites, usually cross fertilizating. The sexual papilla is on the right side of the animal. Eggs are benthic, the larva spend a variable time in the plankton before settling and metamorphosing to become a small replica of the adult. Their life span can be 1 year or more for the species feeding on slow growing sessile animals (sponges, sea squirts). Nudibranchs feeding on transitory species like hydrozoans, sometimes have an adult life of only few weeks.