Explosive expansion and functional specialization of SCP genes associated with "drifting" behavior in the hard clam...

Sarcoplasmic calcium-binding proteins (SCPs), key intracellular regulators of Ca²⁺ homeostasis in invertebrates, display evolutionary dynamics that are tightly associated with species-specific physiological functions. However, how SCP family expansion influences organismal physiological behaviors remains unclear. In this study, phylogenetic and genomic analyses revealed that the SCP gene family has undergone explosive expansions driven by extensive tandem duplications, particularly in lineages subject to extreme physiological demands, such as chitons and venerid clams. Transcriptomic profiling revealed that these expanded SCP genes have undergone significant subfunctionalization, with specialized expression across the adductor muscle, foot, mantle and hemolymph in Veneridae. This "functional amplification" may provide a general molecular basis for venerid clams to cope with intertidal environments, potentially facilitating traits such as rapid burrowing and immune defense. Against this background, the hard clam Meretrix meretrix exhibits a lineage-specific mechanism that supports its specialized "drifting" behavior. We identified a key paralog, MmerSCP14, which was specifically and highly expressed in drifting tissue and was significantly upregulated during the drifting stage (DI). Biochemical assays confirmed that the encoded protein binds Ca²⁺ but not Mg²⁺. Acting as a high-fidelity Ca²⁺-responsive switch, MmerSCP14 may function as an important regulator that helps sustain Ca²⁺-dependent mucus secretion during drifting. Together, these findings link genomic expansion, tissue-level subfunctionalization and behavior-associated neofunctionalization, and are consistent with the idea that large-scale gene duplications can provide a basis for novel regulatory mechanisms contributing to complex behavioral phenotypes in marine invertebrates.