Tridacnids are the inhabitants of coral reefs’ shallow waters in the Indo-Pacific region and the largest mollusks not only among the modern but even among the fossil ones. Latypov (2018) notes that the weight of the largest tridacna – tridacna gigas, or giant clam – can exceed 300 kilograms, and its length – 1 meter. Tridacna deresa can measure as much as 50 cm, Hyppopus hyppopus – up to 45 cm, T. maxima and T. squamosa – as much as 40 cm, and T. crocea – up to 20 cm (Latypov, 2018). Tridacnids have a hole below the lock which connects the valves of their shells, through which passes a sturdy byssus. Molluscs use this byssus to attach themselves to the solid substrate. Moreover, tridacna crocea can bore into rocky terrains by rolling around its byssus. Such specimens as Hyppopus, T. gigas, and T. deresa, after losing their byssus, stay laying on the ground, fixed in place with their own weight.
During the day, tridacna lays its valves open and exposes its splendidly colored mantle to the light. Mantle tissues comprise a wealth of zooxanthellae cells – symbiotic algae which pass the products of their photosynthesis into the host cells. Additionally, there is an alternative mechanism for the host to use the products of algal photosynthesis. The surplus of the zooxanthellae cells constantly congregating in mantle tissues is absorbed and digested by the amoebocytes moving in the bloodstream. The undigested zooxanthellae density gets excreted by amoebocytes in the mollusk’s extremely large liver. Accordingly, a complementary digestive system excellently functions in tridacna’s bloodstream, which enhances the efficiency of the host’s use of algae’s photosynthetic production. Tridacna’s feces contain a slew of undigested zooxanthellae, which, for instance, in T. maxima, comprise up to four-fifths of their mass.
Reference
Latypov, Y. (2018). The ecology of coral reefs: Their nature and abundance. Cambridge Scholars Publisher.