Abstract
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 colonize as larvae; however, continual feeding and rapid growth to large sizes degrade 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.
Highlights
Most sessile marine invertebrates are broadcast spawners; fertilization success is predominantly determined by proximity to other reproductive individuals and success diminishes with increased distance owing to sperm dilution [1]
While many taxa exhibit sophisticated plastic responses when encountering rival mates [20,21], this is the first record of putatively competitive mating observed in shipworms and, to our knowledge, the first among the Bivalvia
It has been previously hypothesized that the rapid growth, swift development to maturity and large sizes of shipworms are traits associated with the ephemeral existence of life in wood [7,14], but are broadly characteristic of other taxa inhabiting temporary and unpredictable niches [22]
Summary
Most sessile marine invertebrates are broadcast spawners; fertilization success is predominantly determined by proximity to other reproductive individuals and success diminishes with increased distance owing to sperm dilution [1]. Shipworms (Teredinidae) are the principal degraders of wood in marine ecosystems and play a fundamental role in nutrient cycling, biodegradation and niche creation [5]. Several adaptations facilitate their xlyotrepetic (woodboring) and xyloptrophic (wood-eating) lifestyle, including highly adapted valves lined with rows of drilling teeth; a long, vermiform body and a calcareous tube that lines the inside of the burrow offering further protection to the animal; siphons (incurrent and excurrent) that extend beyond the burrow providing the only contact to the external environment; and calcareous pallets— paddle-like structures that seal the burrow entrance upon retraction of the siphons [6]
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