Common names Veined rapa whelk, Asian rapa whelk (USA, UK); Thomas’ whelk
(English common name for Black Sea whelks identified as R. thomasiana);
Cocozza, Bobolone (IT); Geaderde stekelhoorn (NL); Bai top shell (commercial
name for Bulgarian Black Sea specimens); Akanishi (JP).
Taxonomic note The genus Rapana is sometimes referred to the family
Muricidae (e.g., DAISIE, 2006), sometimes to Thaididae (e.g., Koutsoubas &
Voultsiadou-Koukoura, 1991; Mann & Harding, 2000). According to World Register
of Marine Species (WoRMS) the family should be Muricidae, and the Thaididae is
considered a synonym of Rapaninae, a subfamily of Muricidae (see Bouchet &
Rapana venosa is a large, 9-16 cm shell length,
species with a heavy, strongly sculptured shell. Specimens up to 17.5 cm have
been recorded in the Black Sea (Micu et al., 2008). The spire is relatively
short, less than half the height of the aperture. Colour varies from dark to
light brown, often with darker spiral lines. Specimens collected on sandy
bottoms tend to be more lightly coloured than specimens from rocky habitats. The
inside of the aperture is often dark orange or yellowish brown. There is no
other gastropod in European waters that can be misidentified for R. venosa.
The large size, the pigmented inside of the aperture and the short but wide
siphonal canal are unique characters for this species. In its native region
there are a couple of other species whose range may overlap slightly (see
below). Juvenile specimens do not have the orange aperture and the spire is
relatively higher. This means that juveniles can be mistaken for some of the
native whelks, e.g. Ocenebra erinacea (Linnaeus, 1758), Buccinum
undatum Linnaeus, 1758 or the invasive Ocenebra inornata (Recluz,
1851) (=Ocinebrellus inornatus) and Urosalpinx cinerea (Say,
1822), which have not yet invaded Nordic waters. However, sculpture is
Rapana venosaeating a mussel. Photo by Dario Savini, University of Pavia, Italy.
Rapana venosa is a native of the Yellow Sea, Bohai Sea, East China Sea, and
the Sea of Japan. Molecular studies have shown that there is high genetic
diversity among populations and that this is not related to geographic distance
between populations. Populations in the native area have been overexploited and
have severely declined (Yang et al., 2008). Other species of Rapana from
the western Pacific are R. bezoar (Linnaeus, 1767) is common in the South
China Sea, but also occurs in and R. rapiformis (Von Born, 1778) from
the Indian Ocean and the Red Sea.
This species was introduced to the Black Sea in the 1940s, the first record
being from 1946 (Micu et al., 2008), and between 1959 and 1972 it spread to most
of the Black and Azov Seas (Zolotarev, 1996). It was originally misidentified as
R. bezoar and later as R. thomasiana, which is now considered a
synonym of R. venosa (ICES, 2004; Gollasch, 2006). From the Black Sea it
has spread to the Marmara Sea in the 1960s and to the Mediterranean, where it
was found in the Adriatic Sea in 1973 (Savini et al., 2004) and in the Aegean
Sea in 1986 (Koutsoubas & Voultsiadou-Koukoura, 1991). There are a few records
from the Tyrrhenian Sea (Savini & Occhipinti-Ambrogi, 2006). In 1997 it was
found in Brittany on the Atlantic coast of France (Goulletquer et al., 2002;
ICES, 2004). There is a single record from the North Sea south of the Dogger
Bank in 1992 (Mann & Harding, 2000). In 2005 it was recorded from the southern
North Sea (Kerckhof et al., 2006). In 2007 it was found on the Atlantic coast of
Spain (Rolán & Bañón Díaz, 2007).
Records from the Red Sea (e.g., Seyhan et al., 2003) most
likely refers to R. rapiformis. The record of shells inhabited by hermit
crabs in New Zealand is probably shells thrown overboard from Asian fishing
vessels (Pastorino et al., 2000). Also, a few records from the Pacific coast of
North America are assumed to have been introduced with seed oysters, but unable
to establish (ICES, 2004).
Rapana venosa has also been
introduced to Chesapeake Bay on the east coast of the USA where it was first
found in 1998 (Mann & Harding, 2000), and in 1998 it was found in La Plata
estuary between Uruguay and Argentina (Pastorino et al., 2000; Giberto et al.,
2006; Lanfranconi et al., 2009; Carranza et al., in press). Size of specimens
first collected in Chesapeake Bay indicated that it may have been present for up
to 10 years prior to detection (Mann & Harding, 2000).
Molecular studies have shown that only one haplotype occurs
in the introduced areas, and this occurs in only two native populations, Japan
and Korea, indicating that one of these must be the source population for the
Black Sea introduction (Chandler et al., 2008). This also supports the idea that
introduction was probably with oysters, Crassostrea gigas, imported from
Japan. It is not possible to say whether the Adriatic population has been
established by natural dispersal from the Black Sea or by human introduction,
also from the Black Sea. (Chandler et al., 2008).
The original vector for its introduction to the Black Sea is unknown, though
association with import of oyster spat is the most likely since vessel operation
was hampered by World War II (ICES, 2004). Ballast water has also been suggested
(Gomoiu et al., 2002) as has hull fouling (egg cases attached to hull) (Uyan &
Aral, 2003). Introduction to the Adriatic Sea may also have been with bivalves
imported for culture. Ballast water is suspected to be the vector for
introducing R. venosa to the USA, either from the Black Sea or from the
Mediterranean (Mann & Harding, 2000).
Rapana venosa is a predatory gastropod feeding on
bivalves. Small snails feed by drilling through the bivalve shell, whereas large
snails can attack and consume bivalves without leaving a drill-hole (Harding et
al., 2007a). They grasp the shell of e.g. Mercenaria mercenaria
(Linnaeus, 1758) along the margin, cover it in mucus and, when the valves gape,
they insert the proboscis (Harding & Mann, 1999).
Drilling and edge-chipping leave scars that can be identified, but the
“suffocating” method of killing prey does not (Harding et al., 2007a). In the
USA it includes the large clams, Mercenaria mercenaria and Mya
arenaria Linnaeus, 1758, which do not have native predators in the area, as
well as Crassostrea virginica (Gmelin, 1791), Geukensia demissa
(Dillwyn, 1817) and Mytilus edulis Linnaeus, 1758 in its diet (Harding et
al., 2007a), and Mercenaria is a preferred prey item (Harding & Mann,
1999). In the Adriatic Sea R. venosa feeds on Mytilus
galloprovincialis (Lamarck, 1819), Ruditapes philippinarum (Adams &
Reeve, 1850) and Anadara (Scapharca) inaequivalvis (Bruguière, 1789). The
latter two species are both introduced (Savini & Occhipinti-Ambrogi, 2006).
Growth has only been studied in the Black Sea and in the laboratory for
Chesapeake Bay specimens. The first year rapa whelks from the Black Sea grew to
between 20 and 40 mm, and mean shell size for the second year was 65 mm and 92
mm for the sixth year. Laboratory experiments with specimens from Chesapeake Bay
yielded much higher growth rates, 40-50 mm in 5 months and more than 60 mm in
one year (ICES, 2004). R. venosa may have a life-span of 12-18 years.
Juveniles are very rarely found in the wild (Savini et al., 2004). They are said
to feed on barnacles (ICES, 2004).
Rapana venosa has broad
ecological tolerances of temperature, low salinity, oxygen depletion and
pollution (Zolotarev, 1996). In its native range it tolerates temperatures
between 4° and 27° C, and in the Black Sea between 7° and 24° C. In the Sea of
Azov it is absent from those parts that are ice-covered during winter, but
survives continuous salinity below 12 ppt (ICES, 2004). In Chesapeake Bay R.
venosa occurs at salinities from 18 to 28 ppt, in the Black Sea from 25 to
32 ppt (Mann & Harding, 2000). It seems to prefer sandy or sandy-muddy bottoms
in the introduced areas (Koutsoubas & Voultsiadou-Koukoura, 1991; Harding &
Mann, 1999; Seyhan et al., 2003). Rapa whelks are active burrowers and may
remain completely buried in the sediment with only the tip of the siphon
sticking out (Harding & Mann, 1999).
The shells of R. venosa
from the Adriatic Sea are often bored by the sponge Cliona lobata, which is an
Atlantic species not observed in the Mediterranean prior to the appearance of
R. venosa. Barnacles,
serpulid polychaetes, oyster and mussel spat are also commonly found as epifauna
(Savini et al., 2004). In Chesapeake Bay shells are often bored by the
polychaete Polydora websteri
(Mann & Harding, 2000), and in the Black Sea the
Polydora ciliata is a shell borer (ICES, 2004).
Epifauna is most abundant in whelks living on hard substrates (Mann & Harding,
Rapana venosa has been shown to
accumulate toxins responsible for paralytic shellfish poisoning (PSP). The
toxins are produced by phytoplankton species
Alexandrium tamarense, which also occurs in Nordic
waters. The algae are ingested by suspension-feeding bivalves, and these are
then consumed by the rapa whelk (Ito et al., 2004). On the other hand, high
mortality of R. venosa has been recorded during a bloom of the toxic alga
Alexandrium monilatum in Chesapeake Bay (Harding et al., 2009). The
potent toxin tetrodotoxin has been found in R.
venosa from Taiwan, but the source of the toxin
is unknown (Hwang et al., 1991). Whelks also produce their own toxins for
paralyzing their prey (Seyhan et al., 2003; Savini & Occhipinti-Ambrogi, 2006).
R. venosa accumulates various environmental pollutants, such as heavy
metals (Bat et al., 2000; Liang et al., 2004), DDT and other organic pesticides
(Yang et al., 2004) as well as butyltin compounds such as TBT (Yang et al.,
2006), and imposex, although only early stages, has been detected in Chesapeake
Bay whelks (Mann et al., 2006).
In its native area R. venosa
is eaten by octopus, but these predators do not
occur in the introduced areas. No field observations are available for predation
on R. venosa in the
introduced areas. However, laboratory experiments have shown that crabs are
capable of feeding on small specimens (Harding, 2003).
Rapana venosa reaches sexual maturity at a shell
size of 50-70 mm, corresponding to an age of 1-3 years (Savini et al. 2004). In
its native area R. venosa
spawns between June and August (Yang et al., 2008), and similar periods, May to
September, have been observed in the introduced range (ICES, 2004; Harding et
al., 2007b). Eggs are deposited in characteristic elongate 30-40 mm tall, 2-4 mm
wide egg cases, bent at the tip and each containing 200-1000 eggs (Mann &
Harding, 2000; Pastorino et al., 2000; ICES, 2004; Saglam & Duzgunes, 2007). Egg
cases are usually found in clusters of 50-500 cases (ICES, 2004). The size of
egg cases and the number of eggs increase with size of the female whelk (Harding
et al., 2007b; Saglam & Duzgunes, 2007). Egg cases change colour as the embryos
develop, from pale yellow to almost black (Harding & Mann, 1999). Uncleaved eggs
are about 150 to 275 µm in diameter (Uyan & Aral, 2003; Saglam & Duzgunes,
2007). Veligers hatch from the egg capsule after about 17 (ICES, 2004) to 25
days (Uyan & Aral, 2003). There is a free swimming larval stage lasting from 14
days to about one month (Mann & Harding, 2000; Yang et al., 2008), occasionally
up to 80 days (Savini et al., 2004). At hatching veligers are about 0.4 mm long
and have a bilobed velum. After 6-9 days they have a 4-lobed velum, and shell
length has increased to 1.2 mm at day 24 (Harding, 2006).
Rapana venosa is a predator of bivalves, including
commercial species as oysters. Its introduction to the Black Sea has been
correlated with the collapse of local oyster production as well as seriously
impacting mussel populations (Zolotarev, 1996). It also interferes with
cuttlefish fishing in the Adriatic Sea. They use the nets for attaching spawn,
and they add a lot of weight to the nets (Savini et al., 2004). R. venosa
has been suspected of impacting populations of commercially important bivalves
in several areas, e.g. Mercenaria mercenaria in Chesapeake Bay and
Ruditapes philippinarum in the Adriatic Sea. However, in most cases
calculated consumption rates are very low compared to existing bivalve stocks.
Mussels cultured on hanging ropes not touching the bottom, are considered out of
reach of rapa whelks (Savini & Occhipinti-Ambrogi, 2006). Competition with other
predatory gastropods, such as oyster drills and moon snails, has also been
suggested as a likely impact (ICES, 2004). The long planktonic stage is suitable
for transfer to new regions in ballast water, and the epifauna on shells may be
transferred with live whelks exported for food. Empty shells may be used by
hermit crabs (ICES, 2004).
The shells of R. venosa were used for tourist
souvenirs shortly after its introduction to the Black Sea, and after 1980 also
the meat has been exploited (Uyan & Aral, 2003). Fishery is now primarily
operated from Turkey and Bulgaria. Overfishing may have occurred as the Turkish
catch decreased from 10,000 tonnes in 1988 to 2000 tonnes in 1997 (Uyhan & Aral,
2003). In its native area R. venosa is a valuable fisheries resource.
Commercial size is 90-160 mm, and market price in China is 7-9 USD per kg (Yang
et al., 2008).
Scientists from Virginia Institute of Marine Science have had a “bounty” program
where local clam and crab fishers were paid a token fee for each rapa whelk they
donated to the scientists (Harding & Mann, 1999). However, the rapa whelks are