Fecal pellets of giant clams, as vectors to transport zooxanthellae.

Coral reefs provide habitats for more than one quarter of marine species. Giant clams (tridacnines or Tridacna spp.) live closely associated with coral reefs in tropical shallow waters. Both corals and giant clams harbor symbiotic microalgae (zooxanthellae) in their bodies and acquire photosynthates from the symbionts. Most of these host animals must acquire their own zooxanthellae from surrounding environments, however; whether such zooxanthellal population sufficiently exists in the surroundings and whether they are alive and infectious remain unclear.

We found that fecal pellets expelled from Tridacna crocea contained numerous red fluoresced granules, namely photosynthetically active zooxanthellal cells. Maximum quantum yield of photosystem II (Fv/Fm) of the zooxanthellae in the pellets were as high as those retained in the mantle tissue of the giant clam. These facts suggested the zooxanthellal population in the fecal pellet were physiologically competent and might potentially be the source of symbionts. Under this assumption, fecal pellets from Tridacna squamosa and T. crocea were given to artificially hatched T. squamosa larvae, and it was confirmed that the larvae acquired and established symbiosis with zooxanthellae via the fecal pellets. Furthermore, fecal pellets obtained from T. crocea were supplied to planula larvae of a coral (Acropora tenuis) once a day until 3^rd day, and the larvae were further incubated without zooxanthellal source for 4 days. After total 7 days incubation, the A. tenuis larvae established symbiosis with the zooxanthellal cells with high success rate; ratios of the infected larvae reached 80±0 %, 66.7±15.3 %, 96.7±5.7 % (mean±SD of triplicated experiments), each given with the fecal pellets from different T. crocea individuals.

These results strongly suggest fecal pellets from giant clams could convey symbionts to the zooxanthella-bearing animals.