Led Astray: Ascidian Larvae Preferentially Settle on Low-quality Sites with Resident Conspecifics

Abstract

Colonial ascidians are abundant encrusting organisms on boat hulls, dock pilings and marinas worldwide. In the Salish Sea of northern Washington, U.S.A. and British Columbia, Canada, Botrylloides violaceus is an invasive, seasonally abundant ascidian that threatens regional ecosystem health with recurrent fouling events. Thorough characterisation of its recruitment patterns and population demographics may inform mitigation efforts. Using a factorial laboratory experiment, we determined that the positive density-dependent gregarious settling behaviour of the species’ tadpole larvae may be responsible for B. violaceus’s locally patchy distribution and rapid population growth. In an all-else-equal scenario, pre-settlement by conspecifics on a substratum otherwise perceived as low-quality or undesirable promotes subsequent larval settling. Our study further identified that B. violaceus larvae are able to respond readily to adult chemical cues as a trigger for settlement.