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Globetrotting Whelks

In 1946 near Novorossiysk, Russia, a strange gastropod shellfish was found. This was the first case of Rapana Venosa (whelks) to be found in the Black Sea. It was carried most probably on a ship’s hull, from the Japanese Sea, in the form of ‘lay mats”. Each lay-mat of eggs contains in the region of 50 – 500 egg cases and each case may contain 200 – 1,000 eggs. Between 14 and 21 days later the pelagic larvae hatch and eventually settle on the bottom of the sea where they develop into hard-shelled snails.

The larvae have a long planktonic phase which may last a maximum of 80 days. They grow quickly on a mixed algal diet and reach a shell length in excess of 0.5mm in 21 days. This arguably makes this ecological predator the most successful colonizer of the Black Sea.

The Rapana venosa exploits its habitat and practically annihilates every available prey within its path. It has successfully acclimatized and filled an empty ecological niche in the Black Sea. This has resulted in a devastating effect on the oyster, which no longer exists in the Black Sea and now the mussel banks are being affected in the same way. On the Caucasian Coast of the Black Sea at the end of September 2005 the young Rapana had eaten and had destroyed 2/3 of the wild mussel population to a depth of 35m.Shell fish farmers reported a decline in the mussel and oyster harvest by 50%.  

In a study of the food consumption and feeding periodicity of Rapana venosa undertaken in Turkey, it was discovered, that under laboratory conditions, Rapana weighing 50g were able to consume 0.17 – 0.30 g of mussel per day. This study also indicated that Rapana feed continuously.

Rapana has a high ecological fitness, as evidenced by its high fertility and fast growth rate. It also demonstrates high tolerance of low salinity, high and low temperatures, water pollution and oxygen deficiency.Most marine predatory snails like Rapana feed by drilling a hole into the shells of crabs and other shellfish. With their tongue the Rapana inject digestive enzymes through the drilled hole, and then eat the digested flesh from inside. The adult Rapana is also capable of smothering its prey by wrapping its strong muscled leg around the hinged region of the shell of the victim and feeding between the opened bivalves

Reaching adulthood, these carnivorous gastropods are voracious predators of commercial value, with the demand for Rapana meat increasing within the international market.The increase in value of this resource initially soared in Turkey and then progressed into Bulgaria

Rapana meat is very tasty and has a high pure biological protein hemocyanin, a bluish, copper-containing protein with an oxygen-carrying function similar to that of hemoglobin. It is rich in vitamins C, A, E, B6 & B12, and has a wholesome effect. Rapana is also believed to have aphrodisiac qualities.

The empty Rapana shells are used in jewelry production and are also supplied as homes for baby octopus for Octopus farming or catching. Due to the life cycle of the Rapana, the harvesting is seasonal and is takes place between April to October. The capacity of average Rapana production is 4 tons within a 24 hour period. Exported to Asia in 20’ and 40’ containers Rapana meat fetches, depending on the economic market, around 23.75 Euro per kilo.

The globetrotting Rapana has also been invading coastal waters of the Rio de la Plata estuary and Atlantic coast of Uruguay. During May 2005, 12 green sea turtles were found either stranded or entangled in gillnets with Rapana attached to their carapace (the shell structure).  The number of Rapana whelk attached to turtle’s carapaces varied from 2 to 49 individuals. This is the first documentation of this kind. This interaction constitutes a real threat to green turtles mainly because it affects the turtles buoyancy due to the extra weight, but also because these voracious gastropod cause severe injuries by drilling a hole into their prey.

 

Article Courtesy of Ian Wilkinson - Scuba Diving on the Black Sea Coast in Bulgaria - www.blackaqua.eu

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