168
Reef Zoobenthos
The degree of autotrophy of tridacnids could be evaluated from the ratio of
their daytime photosynthesis to respiration (Pt/Mt). It varies within 1.2 to
3.4 in clams of 1 to 10 kg weight depending mainly upon the size of the
Tridacna. In 24h this balance corresponds to Pt/Mt ratios of 0.6-1.7 (Munro
and Gwyther 1981). In the medium-sized specimens of T. maxima the Pt/Mt
ratio in 24-h balance was 1 down to the depth 10m (Jaubert 1977). Thus the
tridacnids can cover a large portion of their daily energy expenditures with
the aid of their algal symbions, the same as the hermatypic corals. The
calcification function in Tridacna and the very ability of growing of their
gigantic shells are provided by its coupling with the photosynthesis, again
the same as in corals. Besides, the tridacnids have also a normal filtering
apparatus, which separates even small phytoplankton cells of 1O-12l! size as
well as the zooxanthellae, which often could be found in plankton over the
coral reefs (Goreau et al. 1973; Ricard and Salvat 1972). But there is
evidence that the plankton and the particulated matter, taken by tridacna by
filtration, are poorly digested by the mollusc. The latter authors concluded
that these molluses did not need heterotrophic sources of feeding at all,
being actually autotrophs by their total photosynthesis - respiration balance.
This thesis does not seem to be acceptable, not only in relation to the
tridacnids, but for the hermatypic corals either (Sorokin 1984a; d. 9.3). It is
highly possible also that among the functions of the filtering apparatus in
tridacna besides respiration there is also an inorganic nutrients supply from
the water. Because of their filtration activity the tridacnids readily
accumulate heavy metals in their kidneys, e.g., Zn, Cu, Cd, especially in
areas of water pollution. Therefore, they could be used as a very sensitive
indicator of such kinds of pollution of coral reef waters (Khristoforova et al.
1979).
The tridacnids are hermaphrodites. The eggs and the sperm are excreted
into the water column where the fertilization soon proceeds. The spawning
of one specimen within their population triggers the spawning of neighbors,
thus increasing the possibility of cross-fertilization, which gives healthier
generations. So the success of their breeding depends upon the density of
their populations. On the untouched reefs their populations are usually
dense and often are composed by one or two species (Ricard 1981). In
places of their extensive collection their population became rare, because of
the above-mentioned features of their propagation. The catch leads often to
their gradual extermination. The veligers of Tridacna settle after 1-2 weeks
of planktonic life and become tiny clams of 0.3 mm size with a welldeveloped leg. With its aid they crawl until they find an appropriate place
for the attachment. The young tridacnids have a relatively high growth rate
(Mc Michael 1974). Tridacna gigas during the first year of life reaches a size
of 10 cm and attains the weight of 40- 50 g. After 10 years this species grows
up to 60cm. Another Tridacna T. deresa, has a growth rate of nearly
5 cm year-I (Munro and Gwyther 1981). T. maxima attains its maximum
weight at the age of 11-13 years (Ricard 1981). The annual coefficient of
Reef Zoobenthos
The degree of autotrophy of tridacnids could be evaluated from the ratio of
their daytime photosynthesis to respiration (Pt/Mt). It varies within 1.2 to
3.4 in clams of 1 to 10 kg weight depending mainly upon the size of the
Tridacna. In 24h this balance corresponds to Pt/Mt ratios of 0.6-1.7 (Munro
and Gwyther 1981). In the medium-sized specimens of T. maxima the Pt/Mt
ratio in 24-h balance was 1 down to the depth 10m (Jaubert 1977). Thus the
tridacnids can cover a large portion of their daily energy expenditures with
the aid of their algal symbions, the same as the hermatypic corals. The
calcification function in Tridacna and the very ability of growing of their
gigantic shells are provided by its coupling with the photosynthesis, again
the same as in corals. Besides, the tridacnids have also a normal filtering
apparatus, which separates even small phytoplankton cells of 1O-12l! size as
well as the zooxanthellae, which often could be found in plankton over the
coral reefs (Goreau et al. 1973; Ricard and Salvat 1972). But there is
evidence that the plankton and the particulated matter, taken by tridacna by
filtration, are poorly digested by the mollusc. The latter authors concluded
that these molluses did not need heterotrophic sources of feeding at all,
being actually autotrophs by their total photosynthesis - respiration balance.
This thesis does not seem to be acceptable, not only in relation to the
tridacnids, but for the hermatypic corals either (Sorokin 1984a; d. 9.3). It is
highly possible also that among the functions of the filtering apparatus in
tridacna besides respiration there is also an inorganic nutrients supply from
the water. Because of their filtration activity the tridacnids readily
accumulate heavy metals in their kidneys, e.g., Zn, Cu, Cd, especially in
areas of water pollution. Therefore, they could be used as a very sensitive
indicator of such kinds of pollution of coral reef waters (Khristoforova et al.
1979).
The tridacnids are hermaphrodites. The eggs and the sperm are excreted
into the water column where the fertilization soon proceeds. The spawning
of one specimen within their population triggers the spawning of neighbors,
thus increasing the possibility of cross-fertilization, which gives healthier
generations. So the success of their breeding depends upon the density of
their populations. On the untouched reefs their populations are usually
dense and often are composed by one or two species (Ricard 1981). In
places of their extensive collection their population became rare, because of
the above-mentioned features of their propagation. The catch leads often to
their gradual extermination. The veligers of Tridacna settle after 1-2 weeks
of planktonic life and become tiny clams of 0.3 mm size with a welldeveloped leg. With its aid they crawl until they find an appropriate place
for the attachment. The young tridacnids have a relatively high growth rate
(Mc Michael 1974). Tridacna gigas during the first year of life reaches a size
of 10 cm and attains the weight of 40- 50 g. After 10 years this species grows
up to 60cm. Another Tridacna T. deresa, has a growth rate of nearly
5 cm year-I (Munro and Gwyther 1981). T. maxima attains its maximum
weight at the age of 11-13 years (Ricard 1981). The annual coefficient of