Genomic insights into photosymbiosis in giant clams and comparisons with coral strategies

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.