Application of ammonium to enhance the growth of giant clams ( Tridacna maxima) in the land-based nursery: effects of size class, stocking density and nutrient concentration

Abstract

This study investigated the effects of ammonium (NH 4 +) enrichment and stocking density on the growth of juvenile Tridacna maxima throughout the land-based nursery stage. In four sequential experiments, clams of 5, 11, 16 and 18 mm shell length (SL) were maintained at low and high stocking densities in 60-l tanks with unfiltered seawater (8 l h −1) for 25 days. Throughout this period, clams received daily spikes of dissolved ammonium sulfate at concentrations of <1 (control), 20, 35 and 50 μM NH 4 + for the three smallest size classes of clams, and <1, 35, 50 and 80 μM NH 4 + for the largest size class. Our experiments simulated the conditions used to mass-produce clams of this species during the nursery phase. All individuals used in the four experiments were from the same cohort. The growth responses of T. maxima to ammonium enrichment were dependent on the size of the clams. For clams of 5 mm SL, both increase in mean wet weight and SL were significantly lower at 35 and 50 μM NH 4 + compared to control and 20 μM NH 4 + spikes. In contrast, increases in mean wet weight and SL of giant clams of 11, 16 and 18 mm SL were all significantly greater when ammonium was added, with the greatest increases in mean weight and SL occurring at the highest ammonium levels for the two largest size classes. The abundance of zooxanthellae per clam increased in response to addition of ammonium for clams of 11, 16 and 18 mm SL, but not for individuals of 5 mm SL. Stocking density had a variable effect on changes in mean weight, shell length and abundance of zooxanthellae. Values of all variables were significantly greater at low stocking densities for clams of 11 mm SL, but only marginally significant for most variables for clams of 16 and 18 mm SL. This study shows that addition of ammonium does not enhance growth of T. maxima during the early stages of rearing clams in the land-based nursery, but that relatively high levels of ammonium should be applied in the latter part of the nursery phase. This highlights the importance of identifying changes in nutrient requirements as juvenile giant clams grow.