Carry-over effects of decreased salinity on larval metamorphosis and the early juvenile in context with eReefs hydrodynamic models

Abstract The environmental conditions experienced by marine invertebrate larvae can have carry-over effects on later life stage performance, affecting population dynamics. For crown-of-thorns sea stars (CoTS; Acanthaster spp.), the terrestrial runoff hypothesis (TRH) suggests that eutrophic runoff plumes boost larval phytoplankton food supply, increasing metamorphic success and seeding outbreaks. However, river plumes also decrease salinity which can be deleterious to sea star larvae. We investigated the impact of decreased salinity (17–30 PSU) across a range of exposure durations on survival, normal development, and the ability of settlement-ready brachiolaria larvae to produce a juvenile. Salinity performance curves identified salinity levels with deleterious effects and the tipping points for 50% normal development and survival. Decreased salinity and prolonged exposure reduced larval survival and morphology. To examine carry-over effects on metamorphosis and juvenile production, competent brachiolaria that had been exposed to decreased salinity were induced to settle. Juvenile production was highest in slightly reduced (30 PSU) and control (34 PSU) salinity treatments, with faster transition to the juvenile under extended 30 PSU exposures. However, by 5 days post-settlement juveniles produced by larvae from the 30 PSU treatment were smaller indicating the presence of negative carry-over effects that reduce juvenile quality. Larval salinity sensitivity and metamorphic success were evaluated against eReefs modelled low-salinity plumes affecting offshore reefs. Case studies from three runoff events indicate larvae may experience harmful conditions. Our results highlight the need to consider plume dynamics and COTS metamorphic transitions when assessing the TRH.