Giant clams (tridacnine shellfishes) are large bivalves that inhabit tropical and subtropical waters and harbor the symbiotic microalgae zooxanthellae, which consist of diverse phylotypes (clades). Each clade exhibits unique physiological characteristics, and the cladal composition may influence the host's survival and its ability to tolerate environmental changes. Using quantitative PCR (qPCR) assays, we investigated the zooxanthellal genetic clades in Tridacna crocea (n = 93) and Tridacna squamosa (n = 93). These two clam species were artificially bred and maintained for an extended time period under an equivalent environment in an outdoor pond. Results showed that T. crocea had a simpler cladal composition and with an apparent dominance of clade A, whereas multiple clades were present in T. squamosa. The zooxanthellae clade A is known to occur in other zooxanthellae-bearing animals that inhabit shallow waters, which is consistent to the shallow water habitat preference of T. crocea. Interestingly, in larger individuals of T. squamosa, the main zooxanthellal clade was C rather than A. The mechanism underlying the dominance of clade C in the larger T. squamosa has not yet been clarified. However, the additional photosynthates supplied by clade C may be preferable for growing clams, as is observed in corals. The cladal composition of giant clams has previously been reported to be primarily controlled by environmental factors. However, our experiments subjected different clam species to the same environmental conditions, and our results suggested that species-intrinsic and/or growth-related processes may also influence the cladal composition.

Unlike most bivalve shellfishes, giant clams (tridacnines) harbor symbiotic microalgae (zooxanthellae) in their fleshy bodies. Zooxanthellae are not maternally inherited by tridacnine offspring, hence, the larvae must acquire zooxanthellae from external sources, although such algal populations or sources in the environment are currently unknown. It is well known that giant clams expel fecal pellets that contain viable zooxanthellae cells, but whether these cells are infectious or just an expelled overpopulation from the giant clams has not been investigated. In this study, we observed the ultrastructural and photosynthetic competencies of zooxanthellae in the fecal pellets of Tridacna crocea and further tested the ability of these cells to infect T . squamosa juveniles. The ultrastructure of the zooxanthellae cells showed that the cells were intact and had not undergone digestion. Additionally, these zooxanthellae cells showed a maximum quantum yield of photosystem II ( Fv/Fm ) as high as those retained in the mantle of the giant clam. Under the assumption that feces might provide symbionts to the larvae of other giant clams, fecal pellets from Tridacna squamosa and T . crocea were given to artificially hatched 1-day-old T . squamosa larvae. On the 9 th day, 15–34% of the larvae provided with the fecal pellets took up zooxanthellae in their stomach, and on the 14 th day, zooxanthellae cells reached the larval margin, indicating the establishment of symbiosis. The rate reaching this stage was highest, ca. 5.3%, in the larvae given whole (nonhomogenized) pellets from T . crocea . The composition of zooxanthellae genera contained in the larvae were similar to those in the fecal pellets, although the abundance ratios were significantly different. This study is the first to demonstrate the potential of giant clam fecal pellets as symbiont vectors to giant clam larvae. These results also demonstrate the possibility that fecal pellets are a source of zooxanthellae in coral reefs.