An uninvited guest in the Black Sea and elsewhere

The veined rapa whelk (Rapana venosa), also called the Asian rapa whelk, is a predatory marine snail with an original range in the Indian Ocean and the Western Pacific up to the Sea of Japan. In recent decades, however, the species has been introduced by people to other regions worldwide and today is classed as one of the worst invasive species. Nevertheless, it has gained a certain economic significance in the Black Sea.

Its high ecological fitness makes Rapana venosa very successful as an invasive species. The snail tolerates a wide range of different salinities and temperatures, can tolerate lack of oxygen and water pollution, can use a variety of different foods, grows very quickly, becomes sexually mature early and is characterised by tremendous fertility. Its larvae can also vary the duration of their pelagic swarming period and are not too selective when choosing a place to settle on the sea floor at the onset of their metamorphosis The presence of all of these features together means that this predatory marine snail has permanently established itself and penetrated into many marine regions into which it was almost without exception introduced by humans. This has frequently led to undesirable consequences, since the spread of Rapana causes profound changes in marine ecosystems in many locations In the Black Sea, for example, where this exotic intruder has long since become a widely distributed mollusc and an integral part of the sea floor bioceonosis

Rapana is smaller in the Black Sea due to the living conditions there, but otherwise does very well. While fully grown specimens in the Far East can reach snail shell lengths of up to 18 cm with weights up to 300 g, in the Black Sea these animals only reach two thirds of this size, at a length of 12 cm. The external appearance of the snails is however absolutely identical whether it is found in its original native marine region or in its newly conquered one. Rapana has a thick shell, a large body whorl as well as 6 or 7 right-turning spiral swirls on top of it. This creates a conical, tapered basic shape that the genus name Rapana is also derived from (Lat. rapa – beet). Just as characteristic are the deep umbilicus, the ribs running in a spiral direction and axial fins on the shell as well as the egg-shaped oval shell opening with its thin external lips and fluted edge. In large specimens, there are often hollow teeth on the edges of the opening. A horny operculum closes the oval opening of the shell like a lid, protecting the soft snail body from predators and from drying out, for example if its environment becomes dry during ebb tides.

The typical colouration of the snail shell varies from matt grey to orange-brown, usually with more or less conspicuous brownish or blackish lines and flecks on the spiral ribs, which form a vein-like pattern (hence the name “veined rapa whelk”).

In older animals, the surface of the shell can, however, be subject to shedding and erosion. The shell has a strongly coloured deep orange to cream yellow basic colouring in the opening Depending on the substrate, deviations in the body composition and colouration are possible. Populations on hard stone substrates have mainly darkcoloured shells, while population on bordering sand more often exhibit white or pale brown shells.

Enormous ecological plasticity promotes spread

The original distribution area of Rapana venosa is in the Japanese, Yellow and Bohai Seas as well as in the East China Sea from Peter the Great Bay off Vladivostok in the north to Taiwan in the south. From there, however, the species has been introduced almost worldwide to different European and American waters. It has spread particularly successfully in the Black Sea, where the first evidence for it was found in Novorossiysk Bay in Russia around 1947. It is generally suspected that snail larvae got here in the ballast water of ships, although introduction via oyster seed from the Pacific is also quite possible. Within just one decade, Rapana spread along the Caucasian and Crimean coast and up to the Azov Sea (1953) in the Black Sea. Then it penetrated via Bulgaria (first evidence found in 1955) and Romania (1961) to Turkish coastal areas (1959) and today can be found practically everywhere in the Black Sea on sandy and hard sea floor substrates up to 30 m depth. The lack of competition, rich range of food on offer and the enormous fertility of the invader were helpful in its rapid spread. It multiplied quickly even at low salinities around 18 parts per thousand. Rapana penetrated into the Mediterranean via the Sea of Marmara and was found in the Adriatic for the first time in 1974 before reaching Venice in 1983. Now the snail is present in abundance and well-established in the northern Adriatic. In 2002 it was also found off Mikhmoret Beach in Israel.

This was not the end of the expansion of this predatory snail, however, as it was spotted in Quiberon Bay off the French Atlantic coast for the first time in 1998. It is suspected that young snails were introduced as stowaways along with Japanese carpet shell (Ruditapes philippinarum) from the Adriatic. In the meantime, however, natural vectors for transport and spread were also found. In

Wassaw Sound (Georgia) off the East Coast of the US, juvenile Rapanas were found on sea turtles Currently there are five geographical regions worldwide outside of the natural distribution area of the species in Asia in which reproducing Rapana populations exist. These are, in addition to the Black Sea : the Adriatic and the Aegean, Chesapeake Bay in the North West Atlantic, the estuary of the Rio do la Plata in the South West Atlantic and the Bretagne coast in France in the North East Atlantic. Occasionally, however, individual animals are found in other areas, such as the Pacific coast of Canada and in Willapa Bay, Washington, USA , in the North Sea between the UK and the Netherlands as well as in Ría de Arousa, Spain. Genetic comparisons of Asian, Black Sea, Mediterranean, Atlantic-European and Chesapeake specimens show that all Atlantic populations of snails originally come from the Black Sea.

Rapana is a habitat generalist that can live in the subtidal water area on both sandy and rocky substrates at depths between 0.5 to around 30 m. Sandy sea floors are preferred, because the snails, which are primarily active at night, can almost completely bury themselves in the sand during the day, with only their siphon for respiratory water protruding above the sand. Just as unusual as their tolerance of a wide range of temperatures from 4 to 27°C (some authors even claim 35°C) is their tolerance for low salt contents, down to approximately 12 parts per thousand (apparently these animals can even survive for a few days at 9 parts per thousand). There is also a significant resistance to lack of oxygen (hypoxia) extending to 1.4 mg O2/l. At such low values, almost all shellfish that are part of the staple diet of the Rapana, are incapacitated.

A highly predatory animal that is responsible for significant changes in the ecology of many parts of the Black Sea, rapana is also a resource that is harvested and consumed either locally or shipped abroad.

A staple diet of shellfish and snails

When it comes to food, a distinction must of course be made between the individual life stages of these predatory snails The swarming veliger larvae that swim in the open water feed exclusively on plankton organisms and detritus, i.e. flagellates, diatoms and other organic particles. Upon their metamorphosis and their transition into a benthic way of life, their food spectrum changes and the young snails feed on thinwalled shellfish, oyster spat and young oysters To get to the flesh of their shellfish prey, the young snails scrape a hole in the shells with their toothed grating tongue (radula). From a size of about 35 mm, the predatory marine snails change their feeding style once again. Now they dispense with the time-consuming scraping and place their muscular foot over their prey to suffocate it. When the sphincter muscle of the shellfish goes limp, the snail opens the shell and pushes its proboscis into the soft innards in order to consume the nutritious contents Incidentally, this strategy does not make it easy for researchers to correctly estimate the extent of feeding damage caused by Rapana to shellfish cultures and beds, because the shells remain intact and do not have any holes in them. If available, Rapana will also feed on carrion, such as dead fish or crabs on the sea floor.

Rapana has hardly any predators of its own In its native region of origin, the species is mainly depleted by starfish, but these are not present in the nine bodies of water it has settled in In Chesapeake Bay, blue crabs (Callinectes sapidus) at least eat the smaller snails ; however, this does not seriously endanger this invasive snail species. Therefore, the most important factor in regulating stocks is man, who influences the number and spread of the snails both intentionally and unintentionally. The use of tributyltin (TBT) in antifouling paints, for example, negatively affects the fertility of Rapana (causing imposex) and therefore has an indirect regulating effect

Large animals produce several million offspring per year

Rapana is unisexual and fertilisation occurs internally. The animals usually reproduce in their first year at minimum lengths of 35 to 50 mm. In their native distribution area, they lay eggs from April to the end of July at temperatures of 13 to 26°C. In the Black Sea their reproduction time is somewhat later, from July to September (temperature window 19 to 25°C). Their clutches are comprised of numerous egg capsules that are around 9 to 40 mm high and 3 mm wide and taper to a broad, leaf-shaped tip. At the tip, the capsules end in a pore, through which the larvae leave the case after hatching Each of these capsules can contain 200 to 1,000 eggs The clutch, which is attached to the solid substrate, consists of a few dozen to 600 capsules (the average number in the Black Sea is 84). After the eggs are laid, the capsules are initially whitish, yellow or pale violet in colour. However they become increasingly grey over time and eventually black when the larvae are ready to hatch. The veliger larvae, which are around 1.3 mm long and 0.3 to 0.4 mm wide, usually leave the protective capsules after 12 to 17 days through the pores at the tip of the capsule and then swim using their doublelobed velum for nearly three weeks (at 25–29°C) among the plankton in order to find a suitable living space to settle on the sea floor. The enormous biological plasticity of these snails is also shown by the duration of their planktonic phase, which in some cases can extend to 80 days after hatching

The metamorphosis then occurs on the sea floor. This is defined by the loss of the larval velum, the organ responsible for swimming, gaseous exchange and the intake of plankton food. After their metamorphosis, the individuals grow more than a millimetre per week and can reach shell lengths of 40 to 50 mm in five months and over 60 mm after one year. Even the small snails feed as predators The length of their shell is a good 1 mm at the time of their metamorphosis (Rapana venosa). The lifelong fertility of this predatory snail is very high, since Rapana can live for up to 15 years. Both the size of the capsules and the number of eggs increase with the size of the females, as does fertility. Young snails of 35 mm in length lay on average twice per year, laying 100,000 eggs each time. Older animals with 160 mm or more shell length produce up to four clutches every year, each with 1,000,000 eggs

Despite the high mortality rates of the larvae, given these enormous fertility figures, it is understandable that the uncontrolled, invasive spread of Rapana means serious ecological consequences for many native species. Bivalves such as blue mussels, oysters, scallops, cockles and clams (Maierhofer gallina), inequivalve ark (Anadara inaequivalvis) and the Japanese littleneck clam (Venerupis philippinarum), and also some limpets and barnacles (Amphibalanus improvisus). In the Black Sea, where there are no natural predators of Rapana, this feeding opportunist has caused significant changes to shellfish banks that are seriously negatively affecting shellfish fishing Attempts to eradicate Rapana venosa off the Bulgarian coast were unsuccessful.

Important target species for small-scale artisanal fishing

However, the arrival of the snails also has an economic upside, as Rapana is an esteemed delicacy in some Asian countries, particularly Japan and South Korea For this reason, export-oriented Rapana fisheries with dredges, beam trawls and divers have developed in the Black Sea, particularly in Turkey and Bulgaria. In Turkey alone, almost 800 boats were licensed for Rapana fishing in 2018. According to TURKSTAT (2020) the usually small snail fishing operations generated revenue of USD 5.7 million in 2019. Almost 5,000 people are employed in fishing, processing and marketing Rapana has therefore achieved a certain economic importance, in particular for smallscale artisanal fishing operations. Since there are signs of regional overexploitation, some countries have even introduced protective measures. In Turkey a licence is required for fishing that is issued by the Ministry for Agriculture and Forestry and there are size restrictions for catches To prevent damage being caused by dredging in benthic habitats, more sustainable catch methods are currently being tried in Turkey. Attempts involving baited traps similar to lobster pots are particularly promising These involve limited bycatch and enable better control of Rapana catches. A management model for all Black Sea countries is also currently being considered.

Their nutritional value and decorative snail shells make Rapana a valuable biological resource in the Black Sea. In Turkey alone, the annual harvest is approximately 1,000 tonnes of meat only, of which more than two-thirds has been exported to Asia since early 2000s. In the meantime, the muscular foot of this mollusc has also become an in-demand product in many restaurants on the Black Sea coast. Demand has significantly increased since the animals have been marketed with the claim that Rapana meat has aphrodisiac properties.

The attractive shape of the snail shell with its intense orangecoloured interior makes Rapana a popular product with souvenir retailers Craftspeople make a variety of jewellery and decorative products from the shells In addition, the spread of the marine snails in the Black Sea has also had an interesting side-effect for at least one species from the animal world there. The shells of Rapana are used by the hermit crab Diogenes pugilator as a house. Earlier, prior to the introduction of Rapana, these crabs usually hid their soft and fragile rear ends in shells of the netted dog whelk Nassarius reticulatus, which however reaches significantly smaller sizes of around 3 cm. This necessarily limited the size of the crabs, because it was much too dangerous to become bigger than their available accommodation in which they can hide in case of danger. Today, however, biologists are observing that the Diogenes hermit crab is becoming significantly larger than before. Because Rapana shells are significantly more spacious, even large, sizeable crabs can always find suitable housing

The question of whether the arrival of foreign and invasive species such as Rapana is more of an advantage or a disadvantage can hardly be answered in a general sense. It is really a moot point anyway, because there is no real room for action and all attempts to combat the species, i.e. to get rid of it, have been unsuccessful to date. Under these conditions, commercial fishing is one of the few options available to somewhat limit the spread of the species and to exploit its economic potential. In Chesapeake Bay off the coast of the US, attempts are being made to push back the Rapana through a comprehensive public information programme. This is an attempt to increase public awareness of the risks associated with the Rapana invasions. A reward is being paid for specimens delivered to the Virginia Institute of Marine Science. There are guidelines on how to correctly destroy any clutches and snail eggs found. Whether these measures and the public information programme will finally be enough to effectively stem the damaging effects of these invaders on native marine ecosystems, however, remains doubtful.

Manfred Klinkhardt