Environmental Engineering Reference
In-Depth Information
reproduction and for better tolerance to variations of temperature, salinity and quality
substrate that it presents with respect to local species.
2. Biological features and life cycle of Manila clam
Manila clam
(Tapes philippinarum
or
Venerupis philippinarum
) is an edible species of saltwater
clam, a marine bivalve mollusk in the family
Veneridae
, the Venus clams. The common
names of the species include "Manila clam", "Japanese littleneck", "steamer clam", "Filipino
Venus", "Japanese cockle", and "Japanese carpet shell". Its shell is solid, equivalve,
inequilateral, beaks in the anterior half, somewhat broadly oval in outline. Ligament inset,
not concealed, a thick brown elliptical arched body extending almost half-way back to the
posterior margin. Lunule elongate heart-shaped, clear though not particularly well defined,
with light and dark brown fine radiating ridges. Escutcheon reduced to a mere border of the
posterior region of the ligament. Sculpture of radiating ribs and concentric grooves, the
latter becoming particularly sharp over the anterior and posterior parts of the shell, making
the surface pronounced decussate. It has three cardinal teeth in each valve; centre tooth in
left valve and centre and posterior in right, bifid. No lateral teeth. Pallial sinus relatively
deep though not extending beyond the centre of the shell; it leaves a wedge-shaped space
between its lower limb and the pallial line. Margins are smooth. Extremely variable in color
and pattern, white, yellow or light brown, sometimes with rays, steaks, blotches or zig-zags
of a darker brown, slightly polished; inside of shell polished white with an orange tint,
sometimes with purple over a wide area below the umbones. The features most diagnostic
for the identification of this species are the following: the inner ventral margin of the shell is
smooth; the ligament is prominent and elevated above the dorsal margin. In the living
animal, the siphons are separated at the tips. Water is drawn in and out of a clam by siphons
that protrude from the posterior end of the shell. In this species, the siphons are mostly
fused, and are only separate at the tips. The siphons are short relative to some other clams,
which means that the clam lives burrowed only a shallow distance under the surface of the
substrate. Short siphons are what inspire the common name “littleneck clam” (Bourne,
1982).
T. philippinarum
is a dioecious animal (Bardach et al., 1972; Chew, 1989; Eversole, 1989;
Devauchelle, 1990). In natural populations
T. philippinarum
becomes sexually mature in the
first to the third year of age.
T. philippinarum
are strictly gonochoric and their gonads are
represented by a diffused tissue closely linked to the digestive system. The period of
reproduction varies, according to the geographical area; spawning usually occurs between
20-25 °C. A period of sexual rest is observed from late autumn to early winter.
Gametogenesis in the wild lasts 2-5 months, followed by the spawning. A second spawning
event may occur in the same season, 2-3 months later. The pre-winter recovery phase
facilitates energy build up, by filtering seawater still rich in organic matter and
phytoplankton. Temperature and feeding are the two main parameters affecting
gametogenesis, which can be initiated at 8-10 °C and is accelerated by rising seawater
temperature. Its duration decreases from 5 to 2 months between 14 and 24 °C. Within this
temperature range,
T. philippinarum
are ready to spawn. Although the optimal temperature
is between 20 and 22 °C, 12 °C is the minimum threshold below which this species cannot
spawn efficiently. Food availability influences the amount of gametes produced. Larval
development lasts 2 to 4 weeks before spatfall. Settlement size is between 190 and 235 µm in
shell length. Many external factors regulate spatfall success in the wild, such as temperature,
Search WWH ::
Custom Search