Mate competition during pseudocopulation in shipworms

Shipworms are predominantly wood-eating bivalves that play fundamental roles in biodegradation, niche creation and nutrient cycling across a range of marine ecosystems. Shipworms remain confined to the wood they colonise as larvae; however, continual feeding and rapid growth to large sizes degrades both food source and habitat. This unique lifestyle has led to the evolution of a stunning diversity of reproductive strategies, from broadcast spawning to spermcasting, larval brooding, and extreme sexual size dimorphism with male dwarfism. Some species also engage in pseudocopulation, a form of direct fertilization where groups of neighbouring individuals simultaneously inseminate one another via their siphons - the only part of the animal extending beyond the burrow. Among the Bivalvia, this exceptionally rare behaviour is unique to shipworms and remains infrequently observed and poorly understood. Herein, we document pseudocopulation with video footage in the giant feathery shipworm (Bankia setacea), and novel competitive behaviours, including siphon wrestling, mate guarding, and the removal of a rival’s spermatozoa from the siphons of a recipient. As successful sperm transfer is likely greater for larger individuals with longer siphons, we suggest that these competitive behaviours are a factor selecting for rapid growth and large size in species that engage in pseudocopulation.