Sea slugs

Scientific name : Opisthobranchia

Sea slugs belong to the phylum Mollusca, class Gastropoda, subclass Opisthobranchia. Some opisthobranchs have a small shell, not fit to protect all the animal (like in the order Prosobranchia), but most of them are naked. Most opisthobranchs are small size, soft bodied, often very colourful animals.  Usually we identify the sea slugs with the nudibranchs. Actually this is the more interesting group, with a very high biodiversity, amazing colours and shapes, very popular amongst divers, but Nudibranchia is only a order of the Opistobranchia. When we find a slug in the sea, it is not always a nudibranch.
In this section we will deal also with other "non nudibranchs" sea slugs.
Opistobranchs are soft bodied, small size, mobile animals. They move crawling on a muscular foot (1), with a flat sole, moving forward thanks to a contraction wave proceeding from the head to the tail. Few sea slugs can swim for short tracts, undulating the foot (sea hares) or the mantle (nudibranchs).  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 (touch and smell). Nudibranch antennas are called rhinophores (2). The skin covering the back is named mantle (3). The mantle produces the shell in shelled species. Gills (4) 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 mollusks, the radula is the organ used for feeding, a sort of tongue covered with small teeth, whose number and shape changes according with the feeding habits.
Sea slugs live in all the reef parts. Their distribution follows the availability of food: usually they are found close to their favourite food. Amongst sea slugs we can find many different feeding strategies. We have herbivorous and carnivorous species, mollusk predators or species feeding on sessile animals (hydrozoans, corals, sea squirts, sponges, bryozoans). Many species have extremely specialized diet, feeding on only one prey. There are diurnal and nocturnal sea slugs.
The maximum diversity of Anaspidea (sea hares) and Sacoglossa (sap-sucking slugs) are in the lagoon and bay areas, and on the reef front. Cephalaspidea (headshield slugs) and Notaspidea (sidegills slugs) are common on sandy bottoms in lagoon and bay. Nudibranches have the maximum diversity in the areas of reef front, external reef and deep reef, but also in lagoon and bay.
Only appearently harmless, the majority of sea slugs accumulates or secretes toxic or repellent substances (self produced or derived from the diet), and is not palatable for 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, therefore the typical position during mating is as in the pictures.
Eggs are benthic, the larva spend a variable time in the plankton before settling and metamorphosing, becoming a small snail.
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 few weeks. 
Shelled species can be confused with prosobranchs (sea shells). Usually the shelled opistobranchs have small shell, not covering the whole animal.
Nudibranchs sometimes can be confused with flatworms, especially for the brilliant colour pattern. The main difference is that flatworms are completely flat, and they have no gills neither rhinophores (even if thay can have head folds that recall the antennas).