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22 April 1997 - Commemorative Postage Stamps: Fauna of the Croatian Area
Motifs: Kn 1.40 - The Pen Shell; Kn 2.40 - The Cave Beetle; Kn 3.60 - The Tun Shell |
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The Pen Shell (Pinna nobilis)
It is an endemic species of the Mediterranean, belonging to the family of Pinnidae. The valves of the pen shell are thin and breakable, and with the young specimen they are decorated by scaled lamellas - spiky formations. The inner part of the valves is coloured in a sort of reddish-brown and shiny because of the characteristic layer of mother-of-pearl.The pen shell is the largest shellfish of the Adriatic sea, with a possible length of 1200 mm. The most frequent specimen reach the length between 300 and 500 mm. Its habitat is in the coastal region, at the depth between 2 and 30 metres, on the shoals that are either sandy or overgrown by sea-flowering plants, Posidonia oceanica and Cymodocea nodosa. The pen shell is dug into the oozy-sandy base with a third of its valve by its sharp pointed end. It can manage to keep attached to the base by the threads of the byssus glands. Like many other organisms it remains anached to the same substratum in the same place, almost stationary, to the end of its life. It spawns in the summer months. During the larva stadium it spends its life swimming freely. After a certain period the larva descends to the bonom of the sea and attaches itself to the sandy shoal by the byssus glands. During the period of growing up many small pen shells fall victim to octopuses and fish, especially if they had not anached themselves to a densely overgrown shoal. The valves of the pen shell are a very favourable substratum for the settlement of various organisms like algae, sponges, moss animals, tube worms, snails and smaller shellfish. By the presence of these organisms overgrowing the pen shell it loses some of its original beauty. The pen shell lives in a symbiosis with small crabs, the Pinnotheres pinnotheres and the Pinotheres pisum, who act as the pen shell's guardians.
Many shellfish, the pen shell including, have a byssus gland that secretes byssus, an organic maner that hardens in contact with water and forms threads that help fasten the valves to the base. With the pen shell the byssus consists of tender, fine thin threads, like silk, and it was well known to the ancient Romans. They used byssus to manufacture very light and transparent materials. In the twenties some places in Sycily also manufactured linen items, stockings and gloves.
The pen shell used to be a widely spread species in the sea of the eastern part of the Adriatic, but has recently become rather rare. It is considered to be an appropriate souvenir owing to its beautiful large valves. Many of the once rich regions are almost completely devastated nowadays. Though it is a protected species in the Republic of Croatia (Official gazette 23/1977) it continues to be endangered.
The pen shell is spread all over the Mediterranean.
Radziella Styx - The Recently Discovered Endemic Cave Beetle
Owing to its geographic position and geologic structure, Croatia is known to the world as the country with a great number of rare, endemic and relict species of the flora and fauna. The great amount of Karst in the regions of our country made Croatia abound in caves and deep pits that often become tourist anractions owing to their beauty. At the times when the climate was undergoing many changes on the surface of the earth in the last million years, some animal species found their shelter underground and have adapted to the new environment in the course of time.
During the speleological research of the mountain Biokovo on August 31, 1983, the well-known malacologist and speleologist from Split, Tonci Rada, found severa lblind beetles, coleoptera, in a pit that was unknown at the time. The pit was later called Pretner's Pit, in honour of the Slovene biospeleologist Egon Pretner, who deserves credit for the research of cave beetles in Croatia.
The scientific analysis of the beetles found in 1988 showed that it goes here for a quite new genus and species of cave beetles. This is what makes the discovery exceptionally important. The scientists have named the new genus Radziella in honour of T. Rada who had found the beetles.
The specific structure of the mouth apparatus of this species is particularly interesting because of the way it feeds; it was noted that this insect finds its food in the trickling waters by filtering them through a line of fine fluff on its head above the mouth. As the beetle must move vertically up the walls of the pits, down which water keeps pouring, its body has developed into a lenghtened form, with strong talons on its feet that enable it to move safely. Owing to the fact that the beetle is anached to water, the genus received the name of the mythological river Styx. Before this discovery science had knowledge of only one other highly specialized underground beetle, whose life style resembles the described one - this makes our discovery all the more important. Biokovo, and with it the Republic of Croatia, have thus got another new endemic fauna genus.
The Tun Shell (Tonna galea)
The tun shell belongs to the group of Prosobranchiata and the family of Tonnidae. The colours of the shell range from light yellow to brown and it has seven spirals. The top, or apex, is usually darker. The animal is lightly coloured with darkly toned spots. It moves by the help of a wide and very strong foot. It has no lid or opeculum. It can reach the length between 150 and 250 mm, rarely 270 mm. It keeps to the sandy muddy grounds and also visits the shoals where shellfish live. We can find its habitat in depths between 10 to 150 m. It is a night animal. During the day it is dug into the movable substratum which it leaves during the night. Being a predator it feeds on shellfish, crabs, sea-urchins, sea cucumbers and other smaller animals. It dissolves the shells of other animals by an acid secretion from its salivary glands and then sucks the soft contents. It spawns from August to October.
The densest settlements of tun shells can be found in the shallow channels of central Dalmatia. In the region of the eastern coast of northern Adriatic it could be oflen found in the shallow zones of the channels of the islands Unije and Losinj. The Natural History Museum in Rijeka houses specimen from the bay Studencic from the island of Losinj. Live specimen were caught at the depth between 7 and 10 metres in August 1965. According to the local fishermen, those years could boast of frequent catches. The seas of the island Pag in the years between the wars were another well-known habitation of the tun shell. Owing to the development of the tourist trade, catching tun shells has become very frequent because their fine large shells can be well sold, and this is why their population has suddenly declined in numbers. They are a rare species today and protected with reason in the Republic of Croatia (Official gazette 52/1979).
The tun shell is also a rare species in the Italian seas, except in the lonian sea where it is densely populated.
The tun shell is a cosmopolitan species. It can be found in the east Atlantic, from Portugal to Angola, the Mediterranean, then in the west Atlantic - North Carolina to Florida and Texas, the Caribbean down to the de la Plata river, and also in the Indian Pacific, from eastern Africa to Indonesia and Japan.
The x-ray shows an interesting multi-layered structure of the shell, within which it is easy to detect the spiral. Such pictures can help specialists in their studies of the structure of shells, their determining of the species, and in some cases also in determining the age of the shell.
Source: The Croatian Post and Telecommunications, Jurisiceva 13, 10001 Zagreb, Croatia
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