Shiny purple with yellow dots, deep blue with golden stripes, pink with orange lines, endless colours and designs in combination and beauty. This is the dazzling world of the nudibranchs of the Greek seas. Nudibranchs are one of the least known groups of molluscs with relatives such as the famous mussels, tuns, tritons, cuttlefish, octopuses, etc. Molluscs are one of the most successful phylum in the marine environment, as they are represented by the largest number of species than any other group. Nudibranchs, or more commonly the sea slugs, are a subgroup of gastropods molluscs distinguished by the absence of a shell that protects the soft parts of the animal. The truth is that they have a shell in the early stages of their lives, which they lose as soon as they grow up. A few of them do have a shell in a rudimentary form even in their adult life. Their name in Greek (gymnovranchia) is derived from the fact that their respiratory organs (gills, vranchia in Greek) are exposed to the environment (naked, gymno in Greek). Their classification seems a little confusing, as they belong to the opisthobranchs, a family to which some other sea slugs do not belong. So while naturalists and divers call each naked sea slug a nudibranch, this is scientifically not the case.
More than 3,000 different species are found in the world’s seas, from the cold waters of the Arctic Ocean and Antarctica to the warm equatorial waters. A few of them enter brackish waters and it seems surprising that they are absent from freshwaters. They live almost from the surface of the sea up to 2,500 meters deep with the vast majority of them preferring the warm, calm and relatively shallow waters of the reefs. Others are planktonic organisms carried away by the currents of the sea and others, the larger ones, live on rocks, sand and on other organisms. Their size varies from a few millimetres to 40 centimetres while their lifespan is short and ranges from a few weeks to a year. Nudibranchs are hermaphrodite animals that cannot reproduce themselves. During courtship many of them give a dance performance before they embrace in reproduction. They produce eggs in numbers ranging from a few tens to 25 million, such as the Aplysia fasciata, the well-known “sea hare”. Their eggs are usually deposited in a circular arrangement under stones and cavities, while some species lay solid masses of eggs that resemble a lacy ribbon.
Most nudibranchs have a close parasitic or symbiotic relationship with other species, such as sponges, hydrozoans, bryozoans and seaweeds, from which they feed or use as a shelter. For many of them this relationship is exclusive, for example Tylodina perversa that lives only on sponges of the species Aplysina aerophoba. They are typically carnivorous animals and eat, crawling on the seabed, various species such as sponges, anemones, corals, hydrozoans, but also smaller nudibranchs or their eggs. Usually each species limits its food to a type of prey that may include a family of animals or a single species. Typical examples are Discodoris atromaculata, the well-known sea cow, which feeds on the Petrosia ficiformis sponge and the impressive Flabellina affinis that hunts the colonies of the hydrozoan Eudendrium capillare. Some nudibranchs like, the common in Greece Elysia timida, have the ability of kleptoplasty. They feed on algae and seaweed, then they incorporate the chloroplasts in their body and reuse them to make food for themselves.
Nudibranchs have a lot of defence mechanisms. Their special feature is the colour and design of their bodies that are impressive to the extent that many regard them as the most beautiful animals of the world. But these colours, apart from playing a role in their camouflage on the colourful seabed, are also a warning to their aspiring predator that their taste is from bad to dangerous. Indeed most of them collect and produce chemical secretions that prevent predators. Many nudibranchs feeding on hydrozoans have the ability to store nematocysts and use them as a defence mechanism. Others produce some kind of a deterrent mucus once they come into contact with another animal.
Over 180 species of nudibranchs have been recorded in the Greek seas, while it is important that about 600 species have been recorded in the Mediterranean. This means that there is a good chance, even for the amateur naturalist, that he will discover a species that has not yet been reported from Greek seas. Most of them live from the surface up to about 100 meters deep and prefer rocks or other habitats as long there is an intense presence of life. The truth is that there is no part of the greek seabed that does not host even one species of these animals but most of the time they escape our attention because of their small size. The most common of them are Aplysia fasciata, Elysia timida, Thuridilla hopei, Umbraculum umbraculum, Tylodina perversa, Berthella aurantiaca, Felimare picta, Felimare villafranca, Dendrodoris grandiflora, Phyllidia flava, Platydoris argo, Calmella cavolini, Caloria elegans, Cratena peregrina, Flabellina affinis and Janolus cristatus. Other less common but rather impressive are Philinopsis depicta, Bursatella leachii, Notarchus punctatus, Berthella ocellata, Pleurobranchus testudinarius, Felimida luteorosea, Felimida purpurea, Diagorodoris papillata, Trapania lineata, Berghia coerulescens, Dondice banyulensis, Flabellina babai and Spurilla neapolitana. In recent years a lot of nudibranchs, mainly from the Red Sea, have entered Greek seas, like Chromodoris annulata, Hypselodoris infucata, Haminoea cyanomarginata and Polycerella emertoni which constitute the absolute “pearls” for every naturalist. Their observation is rather difficult as most of them reach a length of 2-3 cm. The best way is to limit the search to a small area of half to one square meter full of marine life (sponges, plants). Soon you will notice the minuscule slow motion of a colourful little jewel.