Analysis of inbreeding depression on performance traits of three giant clams (Tridacna derasa, T. squamosa, and T. crocea) in the South China Sea

Abstract Inbreeding depression is common, especially in hermaphroditic animals, and causes a decline in biological fitness. To evaluate the effects of inbreeding on giant clams, two by two factorial cross experiments between individuals were conducted on three species, Tridacna derasa, T. squamosa, T. crocea. Thus, self-fertilized progeny (f = 0.5) and cross-fertilized progeny (f = 0) were produced for each species. The effects of inbreeding depression on fertilization rate, hatching rate, progeny growth and survival were slight for all three species. However, heavy inbreeding depression was observed on the larval metamorphosis rates of T. squamosa and T. crocea, and very few self-fertilized juveniles from these two species were successfully obtained. Surprisingly, inbreeding depression of performance traits was not found in T. derasa. Our results show that medium (T. squamosa) and small type (T. crocea) giant clam are prone to serious inbreeding depression (δ>0.99) at the larval metamorphic stage, while little inbreeding depression occurred in the large type giant clam (T. derasa). Two-factor ANOVA of survival rates for the whole experimental period showed that progeny survival of medium and small type giant clams was always affected by the mating strategy. Almost no inbreeding depression on growth traits was found these species, which may be due to the nutritional strategy of these autotrophic species. Our study proposes a theory regarding the magnitude of inbreeding depression in different giant clam species based on the evolution of the wild populations and provides a practical account of a mating strategy suitable for seed production for giant clam aquaculture.