Utilization of dissolved inorganic nutrients in growth and mariculture of the tridacnid clam Tridacna derasa

Abstract

The zooxanthellate clam Tridacna derasa takes up dissolved inorganic nitrogen (DIN) in the form of ammonia/ammonium (NH 3) or nitrate (NO 3), as shown by depletion from the medium. Rates of uptake were 5 to 18 times higher in the light than in the dark, and when measured separately the rate of NH 3 uptake was typically higher than the rate of NO 3 uptake. However, uptake of NO 3 was repressed in the presence of NH 3. A similar pattern was seen when ammonium nitrate (50 μM final concentration) was added to a 5000-l raceway containing 14 000 clams. Nitrate was depleted from the raceway only after the NH 3 concentration dropped below 2.5 μM. About half of the ammonia added to the raceway was calculated as taken up by clams in both light and dark, the other half was presumably removed by algae and other organisms in the raceway. Growth rates of small juvenile clams (2–3 mm) increased by about 75% upon addition of 50 μM NH 3 or NO 3, compared to control clams with no additions. Addition of 2 μM phosphate appeared to inhibit growth when added alone, but had little effect on clams maintained in 50 μM NH 3 or NO 3. Zooxanthellae in clams ‘fertilized’ with NH 3 or NO 3 had division rates (mitotic index, MI) a third to two times higher than those of zooxanthellae from control clams. The density of zooxanthellae in clams incubated with DIN for 10 days was approximately double that of controls, correlating positively with both increased MI and clam growth rate. The relative increase in linear shell growth of Tridacna derasa with addition of DIN increases with increasing clam size to at least 3.5 cm. The average time required for T. derasa to reach 1-year size in raceways in Palau (ca. 5 cm) has been reduced an average of 2 months with additions of DIN.