Abstract
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.
Highlights
Giant clams are large bivalves that inhabit tropical and subtropical seas
The nutritional requirements of the giant clams are largely supplied by these symbiotic microalgae, and reports have indicated that more than 50% of the carbon requirements of these animals are supplied through the photosynthetic products of the symbionts [6, 7]
Symbiodinium clade compositions within the two giant clam species Positive quantitative PCR (qPCR) results were obtained for 186 tridacnine clam individuals, 93 T. crocea and 93 T. squamosa specimens, and clear differences in the trend of clade compositions were observed between these two species (Table 1)
Summary
Giant clams (tridacnine shellfishes) are large bivalves that inhabit tropical and subtropical seas. Similar to other animals living in tropical shallow waters, such as corals, hydrozoans, and foraminifer protozoans, giant clams harbor the symbiotic microalgae zooxanthellae, the dinoflagellate genus Symbiodinium, in their bodies [2, 3]. These clams are unique and significantly different from other Symbiodinium-bearing animals in terms of their method of hosting the symbiotic microalgae, where the algae do not live intracellularly but rather intercellularly in a special tubular system generated from the clams’ stomach [4]. Giant clams might not need to filter-feed because the algae can provide sufficient nutrients for survival [8], some authors have reported direct ingestion of algal cells within the clams’ stomach [9, 10]
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