Croatian Undersea World SI

Croatian Undersea World

Mediterranean violet aeolid 

Flabellina affinis


Mediterranean violet aeolid (lat. Flabellina affinis) is a marvellous intensively violet coloured, up to 5 centimetre long slug.  Through soft membrane of its body noticeable are his digestive organs.  Apart from the long feelers on the head, along its back there are numerous fibrous, intensively coloured and branched barbs.

How can this easily noticeable and slow slug survive in the sea full of hungry predator fishes?  His life strategy is just opposite to logic:  I look striking and recognisable - remember me! Once a predator bites the Mediterranean violet aeolid, it will remember very well how it looks. It will never commit the same mistake again and bite into this inedible and as food totally odious snail. But why is Mediterranean violet aeolid inedible?

Mediterranean violet aeolid feeds on symbiosis animals - Hydrozoans, a class of the phylum Cnidaria, generic to corrals, sea anemones and jellyfishes. All of them have in their tentacles cnidarian cells which contain poison and serve for hunting and defence. However, Mediterranean violet aeolid has developed a way how to eat the curb preventing him from launching its cnidarian cells. Mediterranean violet aeolid does not digest cnidarian cells but incorporates them into his long barbs on his back. Hungry predators bite first the barbs on the back and so activate cnidarian cells in them, which burn them.  In such a way the predator remembers forever that the Mediterranean violet aeolid in not at all a tasty bite.



European fan worm



European fan worm (lat. Sabella spallanzanii) is a worm like organism belonging to the class of polichaetes, permanently fixed on the sea bottom.  It lives in a skin-like up to forty centimetres long bendable tube, which it builds by excreting mucus. Brownish-yellow to white coloured and sometimes stripy, spirally bent European fan worms of the diameter of even more than 30 centimetres are gills used for extracting oxygen from the sea water. The gills are also used for catching small organic particles and plankton organisms on which it feeds. European fan worm lives on firm bottoms in protected areas, at the depth up to ten metres where sufficient organic particles can be found. They come in great numbers especially in river estuaries and in harbour areas.  

It is considered to live at least two years. It starts its life as male to become a female when grown up, which at the time of reproduction delivers into sea water more than 50 000 eggs.  Tender swaying of the European fan worm in the light sea current and the wavy movement of its fan has an almost hypnotic effect on diver. The most beautiful experience of watching the European fan worm can a diver experience at night, when with the submarine torch he can research the filigree complexity of its intensively coloured fan. But, tenderly! The gills are sensitive and the European fan worm - at the slightest signal of a peril – immediately withdraws them into the skinny tube.

The European fan worm has its original habitat in the Mediterranean Sea and in the eastern part of the Atlantic Ocean. However, it has also been discovered in Australia and in New Zealand, where it has probably been brought by means of ships' ballast waters. In these seas the European fan worm represents an extraneous species which has spread along harbour areas and has endangered their autochthonic organisms. So has our tender European fan worm in some seas become a problematic and undesirable extraneous species.



Ornate wrasse



Ornate wrasse a (lat. Thalassoma pavo) is among the most picturesque coloured species which we can encounter in the Adriatic Sea. You will rarely see this species at rest. It is characterised by haste movements, continuous fast swimming and sudden changes of direction. Females are most often found in groups of about ten specimens, while the males are most often alone and defend their territory. Like many other kinds of fishes the ornate wrasse changes gender during life, being first a female, to become a multi-coloured male when grown up. At daytime they keep close to shallow, rocky bottoms, where they catch tiny invertebrates. If you dive at night, you will not find them, because in twilight they all almost at the same moment dig themselves into sand.

Ornate wrasse is a warmwater fish.  It is often encountered in south part of the Adriatic Sea and along south sides of middle Dalmatian islands.  North from the islands of Kornati the ornate wrasse is rare, and the most north area of its finding is in the vicinity of the island Rab. Though, during last twenty years it has become more numerous in its most northern habitats, like the Kornati Islands. The spreading of such warmwater specimens toward colder waters of the north Adriatic Sea shows how the Adriatic Sea is changing, becoming warmer and so more apt for warmwater specimens, which do not enjoy cold winters.



Golden sponge / Yellow-tube sponge


The golden sponge (lat. Aplysina aerophoba) has a very characteristic sulphur yellow colour.  It is also called “changeable” in some languages because in the air its vivid yellow colour changes into black-green. The scientific name of the species, aerophoba, originates from the Greek word aerophobus, meaning fearful from air, thus pointing out the change of colour which takes place when it comes in contact with air. It lives in the Mediterranean Sea and along Spanish and Portuguese Atlantic coasts. It inhabits rocky and sunbathed sea bottoms up to the depth about twenty metres.

This sponge produces a series of poisonous compounds which are the reason why no algae or immobile small animals grow or live on it, thus leaving its surface always clean.

The golden sponge is built from numerous finger-like outgrowths 3/5 centimetres long which end in an opening through which the filtrated sea water runs out from the sponge. The sponge absorbs through numerous little holes on its whole body and filters through it organic particles and plankton on which it feeds.   

And who feeds on this sponge? One kind  of slugs, Tylodina perversa, that are of the colour  almost the same as that of the sponge, and that will spend all their life  on the sponge feeding on its tissue.  They will use also poisonous substances of the sponge to defend themselves from predators. The slug imbeds the yellow pigmentation from the sponge into his body and thus remains almost unnoticeable on it.  However, because of yellow pigmentation which make the sponge change the colour into darker colour the slug will, as well as the sponge in contact with air change its colour from yellow to black-green.


Ante Žuljević and Marija Despalatović

Technical Details

Issue Date: 09.04.2014
Designer: Alenka Lalić designer from Zagreb
Illustrator: Dalibor Andres, photographer from Sisak
Printer: ZRINSKI d.d., Čakovec
Process: Multicolor Offset Printing
Colours: 4 Colours
Size: 35.50 x 29.82 mm
Values: 5.80 HRK x 4