Giant clams are representative bivalves in coral reef ecosystems that host photosynthetic dinoflagellates extracellularly and rely on their photosynthates, functioning as "solar-powered animals." Unlike corals, which harbor intracellular dinoflagellates, the molecular mechanisms and evolutionary history underlying this symbiosis remain largely unknown. In the present study, we integrated chromosome-scale genome assembly, transcriptome profiling, and bleaching experiments involving giant clams, Tridacna crocea, to explore the genetic basis of extracellular symbiosis. Signals associated with sterol transport by Niemann-Pick type C2 (NPC2) proteins and carbon-concentrating mechanisms suggest that giant clams share some nutrient exchange strategies with corals. Strikingly, the nitrate transporter NRT2, a "plant-like" gene previously thought to be absent in animals, represents an unexpected evolutionary retention that enables nitrate-based nutrient supply, highlighting a fundamental difference from coral symbiosis. Our findings reveal both conserved and distinct molecular strategies of photosymbiosis in reef-dwelling marine invertebrates and provide insights into evolution and ecological resilience of coral reef ecosystems.
Uchida et al. (Wed,) studied this question.