Croatian Undersea World II SI

Croatian Undersea World II

Long-snouted Seahorse

 

Long-snouted seahorse, lat. Hippocampus guttulatus (Cuvier, 1829), belongs to best known sea fish although at first sight it is not at all similar to a fish. His head and neck are similar to horse's head and neck and therefore, according to the Greek word hippos (horse) a part of its genus name is Hippocampus. The other part of the name originates from kampos in the meaning of sea monster. Though, while for us they are just sweet little fishes for even smaller animals that find themselves in their vicinity these small seahorses are dangerous predators. 

Seahorses live in shallow populations of seaweed and sea flowering plants, mostly without moving much, holding with their flexible tail to plants, to which they are alike. This makes them invisible even to larger predators but also for their tiny prey, most often small crabs or fish roe, which they catch by a jerk of head only when they get in front of their mouth.  

The seahorses are known for one unusual exception among fishes – a male seahorse gives birth to offspring. How this is possible, was a secret for a long time until it was discovered that during their love life a female carries over fertilised eggs into a special male’s pouch. In next few weeks the male seahorse holds eggs until from them appear fully developed small seahorses whom the father expulses from the pouch by a muscle convulsion. This usually happens during night, so that - protected by the darkness - the young and scarcely movable seahorses are protected from hungry predators in their first hours of life. After that the male's care about offspring ceases and with the arrival of new morning he is again ready for next pregnancy period. 

In world seas today there are more than 50 known kinds of seahorses, who live mainly in tropical and moderately warm seas. The smallest among them, pygmy seahorses, are smaller than 15 mm.

Long-snouted seahorse is a frequent species in the European part of Atlantic Ocean and in the Mediterranean and Black Sea. It grows fast and reaches up to 16 cm. It lives from four to seven years, and is believed to be able to give birth every year to about 900 seahorses. Young seahorses spend first weeks as plankton organisms, carried away by sea currents.   When they reach about 9 cm, they settle among seaweed and sea flowering plants.

In Croatia this species is considered as delicate and the decrease in its number is the consequence of habitat destruction and occasional getting caught in fishing nets.

 

Violescent Sea-whip

 

Violescent sea-whip, lat. Paramuricea clavata (Risso, 1826), is known in our country under the name of crvena gorgonija (red gorgonia) because of its dark red to violet colour which it assumes when illuminated under sea water. To the depths of 20 to 140 metres, where it lives, penetrates no red part of the sunlight spectrum, so that without the artificial light it looks dark blue to black.  

Violescent sea-whip is a colony of thousand polyps similar to a small fan-like, branchy tree. It grows up to one centimetre a year, so that some specimens that reach the height of one metre are probably more than hundred years old. It lives on vertical and slant rocks, in areas with stronger sea currents, which bring food and oxygen to polyps and where the temperatures rarely exceed twenty degrees Celsius. Polyps, similar to small, one centimetre high actiniaria, collect plankton organisms with their eight tentacles.

All the polyps in the colony are of the same sex. They multiply at the end of spring and beginning of summer, usually several days after the full moon. Male polyps let sperms in sea water, which brings them to female colonies. Female polyps emit mucus with eggs which fixes them to the surface of a female colony.  Only when they get fertilised and start to develop into a multicellular larvae, they let themselves go into the sea. The larva with cilia moves towards depth, reaches the sea bed and thus starts the life of a new colony.

Violescent sea-whip is spread out in the Mediterranean Sea and eastern Atlantic.  The species is considered endangered. It is often destroyed by fishing tools especially nets and also by anchoring. Global sea warming is probably the cause of the mass starvation of the violescent sea-whip, which happens more and more often in the Mediterranean Sea.

 

 

Cylinder Anemone

 

Cylinder Anemone, lat. Cerianthus membranaceus (Spallanzani, 1784), is an unusual organism which belongs to cnidarians, or more precisely to actiniaria. It lives partly implanted into the soft sea bed inside its waxen, wrinkled cylinder which she herself produces. The cylinder surrounds the whole animal except for the tentacles, but also tentacles are quickly drawn into the cylinder in case of danger.  Although it rarely happens, the animal can leave the cylinder, move to another place and build again its new protective cylinder. Earlier, it was believed that the cylinder  originated from the mucus,  however it was discovered that it is produced by weaving  of special  thread, emitted by cells very similar to stinging cells of cnidarians - those which cause  intensive skin irritation, like jelly fishes and snakelocks anemones.

The cylinder anemone is up to thirty centimetres high, while the diameter of tentacles is up to twenty centimetres. It has more than two hundred pointed tentacles of various colours, many of whom are often phosphorescent. It is believed that phosphorescent colours serve to scare away fishes which would like to bite on fleshy tentacles which do not irritate like those in a kin snakelocks anemone. However, the stinging cells are still sufficiently strong to daze small crabs or collect plankton organisms.

As well as in the case of many cnidarians it is not known how long a cylinder anemone can live. It is believed for many kinds of cnidarians that they do not die from getting old. One cylinder anemone in the aquarium in Naples is allegedly more than hundred years old. Maybe in future we shall find out more on the longevity from these animals.

 

Neptune’s Lace 

 

Neptune’s lace, lat. Reteporella beaniana (King, 1846), 'belongs to bryozoans. In spite of the fact that more than 200 species live in the Adriatic Sea and that more than 5000 species have been described living in world seas and several times more fossil species are known, the bryozoans are organism mostly unknown to majority of people.

Bryozoans are communities of animals that develop colonies of various forms and sizes, only rarely bigger than about ten centimetres.  By its form they can be crusty or elevated, very often of wonderful structures. A colony can be made of thousands of animals - zooids just about half a millimetre long. In some kinds of bryozoans some zooids are specialised for tasks like defence, feeding or reproduction. They feed by filtering the sea from which by means of whip-like extensions with cilia they catch organic particles or tiny plankton organisms. The tentacles serve also for respiration, since there are no respiratory organs. Zooids produce skeleton from organic matter or calcium carbonate which gives firmness to the colony.  

Defining the precise species of a bryozoan is very demanding and often possible only with help of microscope, whereat the look of a particular zooid is more important than the look of the entire colony.

Neptune’s lace is a tender bryozoan of a fanlike look and lacy structure with the diameter of up to about ten centimetres.  It is often found in posidonia meadows, at entrances to sea caves or in coralligen community. There are several similar species living in Adriatic and precise distinguishing is possible only by microscopic examination of zooids.

 

Ante Žuljević

Technical Details

Issue Date: 15.06.2015
Designer: Alenka Lalić designer from Zagreb
Illustrator: Photographs: Dalibor Andres, photographer from Sisak
Printer: AKD d.o.o., Zagreb
Process: Multicolor Offset Printing
Colours: 4 Colours
Size: 48.28 x 29.82 mm
Values: 5.80 HRK x 4