Aggressive mimicry lure polymorphisms in the parasitic mussel Lampsilis fasciola model fish or leech host prey and differ in morphology and pigmentation, but not in display behavior.

Unionoid freshwater mussels (Bivalvia: Unionidae) are free-living apart from a brief, obligately parasitic, larval stage that infects fish hosts, and gravid female mussels have evolved a spectrum of strategies to infect fish hosts with their larvae. In many North American species, this involves displaying a mantle lure: a pigmented fleshy extension that acts as an aggressive mimic of a host fish prey, thereby eliciting a feeding response that results in host infection. The mantle lure of Lampsilis fasciola is of particular interest because it is apparently polymorphic, with two distinct primary lure phenotypes. One, described as darter-like, has eyespots, a mottled body coloration, prominent marginal extensions, and a distinct tail. The other, described as worm-like, lacks those features and has an orange and black coloration. We investigated this phenomenon using genomics, captive rearing, biogeographic, and behavioral analyses. Within-brood lure variation and within-population phylogenomic (ddRAD-seq) analyses of individuals bearing different lures confirmed that this phenomenon is a true polymorphism. The relative abundance of the two morphs appear stable over ecological timeframes: the ratio of the two lure phenotypes in a River Raisin (MI) population in 2017 was consistent with that of museum samples collected at the same site 6 decades earlier. Within the River Raisin, four main darter-like lure motifs visually approximated four co-occurring darter species (Etheostoma blennioides, E. exile, E. microperca, and Percina maculata), and the worm-like lure resembled a widespread common leech, Macrobdella decora. Darters and leeches are typical prey of Micropterus dolomieui (smallmouth bass), the primary fish host of L. fasciola. In situ field recordings of the L. fasciola darter and leech lure display behaviors, and the non-polymorphic lure display of co-occurring L. cardium, were captured. Despite having putative models in distinct phyla, both L. fasciola lure morphs have similar display behaviors that differ significantly from that of sympatric L. cardium individuals, suggesting that the L. fasciola mantle lure polymorphism does not include a behavioral component. Discovery of discrete within-brood inheritance of the lure polymorphism implies potential control by a single genetic locus and identifies L. fasciola as a promising study system to identify regulatory genes controlling a key adaptive trait of freshwater mussels. ### Competing Interest Statement The authors have declared no competing interest.