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Black Sea Mollusca and sea
shells
Shell is both the outer skeleton and the house of
Bivalve and Gastropod Molluscs. Molluscs grow, and their
shells grow with them. Special cells in the edge of molluscan
mantle (outer skin) form a crystalline limestone
deposit, which adds to the growing shell, making it wider and
thicker. Molluscs grow more slowly in winter than in summer,
so the rate of shell growth is not continuous, and we can
therefore identify annual growth rings on shell surface - just
like in the trunk of a tree (growth rings are usually more
coarse, less sharp than the normal concentric patterns of
certain molluscan shells, e.g. the shells of Veneridae
species). Growth rings are better discernible on clam valves,
but even the age of a gastropod such as the common Black Sea
marine snail Tritia or Rapana can be counted by
growth rings.
On normally smooth shells, such as valves of the Black Sea
mussel Mytilus galloprovincialis, annual growth rings
are not the only things to be seen: every disturbance of the
near-shore marine habitat (sea storms, sudden cooling of
water, water pollution event etc.) leaves its own ring on the
shell because it affects the animal's feeding rate, and -
correspondingly - the shell growth rate. Studying the chemical
composition of shells - growth rings of the same shell, or
shells of the same species from different years or centuries -
can provide investigators with information on the chemical
composition of the seawater during the corresponding period of
time.
metamorphoses
of Venus gallina shells:
right - normal coloration,
black shells
that have been buried in anoxic sediments where
they blackened by interaction with sulfuric hydride;
yellow shells traveled up
through the sediment to its surface by the action of
waves or currents; black metal sulfides turned into
yellow salts after interaction with oxygen;
white shells -
protective upper layer of shell rubbed off by
sand |
Venus gallina and Donax
trunculus are the most common clams living on the
shallow sandy bottom at the Black Sea coast. A lot of
their shells are scattered on the beach. Triangular
shells of Spisula triangula are also common
here. Everywhere on sandy beach are tiny transparent
shells of Lucinella divaricata and Lentidium
mediterraneum. These are fast growing, short-living
annual bivalves. Heavy shells of Scapharca
inaequivalis: this tropical clam of Arcidae family
colonised the Black Sea in late 20th century.
Left to right: most
common Black Sea beach shells
Venus
gallina
Cerastoderma
glaucum
Donax
trunculus
Scapharca
inaequivalis |
Most clams (bivalve molluscs) inhabit the soft bottom -
sand or silt. They hide inside the sediments leaving
only siphons - two mantle pipes - sticking out: one for
sucking water in, another one for letting it out. Co-ordinated
movement of the thousands of cilia (like those of Infusorian)
on the surface of ciliate cells on the mollusc's gills make
the water flow through the animal; the gills first take oxygen
from the water - clam breathing; then the water flow is
directed to the mouth of the bivalve. The food of bivalves,
which they filter from seawater, is seston - that is,
both live plankton
and detritus - dead organic particles in suspension.
See also Bivalves of the Black Sea - soft
bottom habitats
A few species of Bivalvia in
Black Sea live on hard substrate:
mussels Mytilus galloprovincialis and
Mytilaster lineatus fasten themselves with bissal threads
to underwater stones, macroalgal stems or pier columns,
whereas oyster valves grow into the stone or other oyster
shells. Black Sea oyster shells used to be white with a
light green or pink tint; now we are finding mostly black
oyster shells on the beach because they have spent some time
buried in bottom sediments, where they were blackened by
sulfuric hydride produced by anaerobic bacteria living in the
sediment. The stone drill Pholas dactylus has
found another way of settling on hard substrate - when it's
not too hard: Pholas makes holes in soft stones, mainly
in marly limestone, with its toothed drill-shell. Black Sea Shells - Molluscan
species of hard substrates
Pitar rudis lives on sandy bottom at depths greater
than 10m. Because of that we seldom find its red-speckled
shells on the beach. For the same reason we rarely see
Modiolus phaseolinus shells - despite the fact that it is
the most abundant molluscan species in the Black Sea;
phaseoline silt consisting of disintegrated Modiolus
shells is a major type of sediment in the Black Sea shelf
bottom.
Life cycle of a marine gastropod,
Rapana venosa
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Marine bivalves and
gastropods are bottom dwellers for the most part of
their life. But the first weeks after hatching from
eggs, molluscan larvae spend within water column; they
are microscopic zooplankton animals.
The most common gastropod at Black Sea coasts is Rapana
venosa; during the spawning period (June to August),
white brushes of its its egg cases can be found on any
hard underwater substrate. In sandy bottom habitats
it is most usual that female Rapana glues fertilized
eggs to their mate's shells. Plankton larvae of marine
gastropods called veliger swims using cilia,
and feed on detritus and smaller plankton; it forms
tiny shell, grows, and settles on the bottom two weeks
after hatching from egg.
Life cycles and planktonic larvae of other benthic
animals - Black Sea
zooplankton page
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Marine gastropods prefer
crawling on hard substrates. A green Gibbula snail
scrapes periphyton off rock surfaces with its grater-tongue -
radula. Very small and beautiful marine snails -
Tricolia, Bittium, Nana, Rissoa - live on macroalgal
branches scraping periphyton and dead crust cells from
them.
Some marine gastropods in the Black Sea such
as Rapana venosa and
Trophonopsis breviata are predators: they drill into other
shellfish shells with their tongue, inject digestive enzymes
through the drilled hole, and then eat the flesh digested
inside its shell. The large adult Rapana usually simply
prises open a bivalve using its strong muscled leg.
Rapana venosa eats any large shellfish in the Black Sea:
bivalves, other gastropods, crabs; the smaller
Trophonopsis predates mostly Modiolus phaseolinus
bivalves.
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Bivalvian diversity in the Black Sea has declined about
two-fold since the Rapana invasion in 1947. (About the Rapana venosa Black
Sea invasion: Evolution of the Black Sea Ecosystem).
Due to ecological pressure from Rapana, many
Black Sea bivalves like Cerastoderma spp.,
Donacilla cornea, Gouldia minima, Loripes lucinalis,
Gastrana fragilis, Mactra corallina, and others,
as well as some Black Sea gastropods e.g. Patella
tarentina have become very rare or completely disappeared.
The most beautiful bivalve of Black Sea - Black Sea
scallop Flexopecten ponticus is considered extinct:
no live specimens have been found since the late 1990s
(fresh 0.5 cm scallop shell was found in 2008 near Tuapse,
so that mollusc was alive in 2007 at least). Oyster
banks no longer exist in Black Sea; a very few live
edible oysters Ostrea edulis survive
in the Black Sea in narrow crevices inside underwater
rocks, or growing on downward rock surfaces where
Rapana can not reach them. We still are finding
all those shells on Black Sea beaches.
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Oyster (Ostrea edulis) at
Crimean coast, 2005
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Spring 2005 was
marked by massive settling of Rapana venosa
larvae at Caucasian Coast of Black Sea - unprecedented
in at least 30 past years. In May, any hard underwater
surface including farmed mussels and oysters shells to
the 30m depth was covered by tiny Rapanas, 1-3mm
size. Settling density reached 5 gastropods per
cm2 at some sites from Tuapse to Anapa.
To the end of
September, young Rapana venosa had eaten almost
all bivalves (mainly Donax trunculus) on the
shallow sandy bottom of the coast strip, and had
destroyed 2/3 of wild mussel population to a depth of
35m - rough visual estimate made by mussel farmers
and Rapana hunters. Shellfish farmers reported a
ca. 50% decline in the mussel and oyster harvest. |
Pacific oyster
Crassostrea gigas shells drilled by young Rapana
venosa, generation 2005. Utrish shellfish farm,
Caucasian coast of Black
Sea |
Below, and in the two following pages are pictures of
seashells from the beaches of Caucasian and Crimean coasts of
the Black Sea. They do not represent all the species of
Mollusca in the Black Sea , still shells of most of them are
there.
Gastropods -
Gastropoda - soft bottom - Black Sea
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Rapana
venosa, veined Rapa whelk <15 cm, the
largest marine gastropod, and one of most ruthless
predators in the Black Sea; it eats bivalves so
efficiently that bivalvian diversity in the Black Sea
reduced about two-fold since Rapana invasion in
1947. Small-sized Rapana drill bivalvian shells with
its radula, inject digestive enzymes
inside, and then suck the digested flesh out; they even
do the same to crabs! Adult Rapanas just open bivalves
with their large versatile leg. There are no predators
for adult Rapana venosa in the Black
Sea the starfish that normally prey on
this gastropod can't tolerate the low salinity of the
Black Sea
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Calyptraea
chinensis
- Chinese hat <4 cm, one of few
gastropods living on the soft sediment bottom;
the mollusc is housed in a tiny half-volute of the
shell, the rest of the wide shell-cap is needed for
staying on a soft surface - like a ski on snow
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Clathrus
turtonis <4 cm, a rare species in the
Black Sea
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Trophonopsis breviata
<2cm Small predator eating bivalves, living deeper
than 20m. It preys mostly on
Modiolus
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