Freshening of Great Barrier Reef waters is deleterious for larval crown-of-thorns starfish, counter to the terrestrial runoff hypothesis

Abstract

Outbreak populations of crown-of-thorns starfish (COTS; Acanthaster spp.) cause widespread coral mortality. High nutrient river runoff generates phytoplankton blooms that potentially enhance larval food levels and success leading to outbreaks, a link posited by the terrestrial runoff hypothesis (TRH). Runoff plumes also lower salinity, although the parallel and potentially negative effects of freshening on COTS larvae have not been considered. The impact of decreased salinity across a range of food levels on larval development and survival was investigated in context with the TRH. Larval survival and incidence of bipinnaria larvae with normal morphology in 6-7 salinities and 3 food treatments were quantified, at 4 time points, to generate salinity performance curves. Salinity was the major factor determining bipinnaria larval survival. At 24 h of exposure, the optimal salinity (Sopt) (≥90% larval survival) was ~26-34‰, and the salinity with 50% mortality (LS50) was 21.8‰. By 168 h, the LS50 was 27.3‰, showing a narrowing of salinity tolerance over time. Low salinity impaired swimming, resulting in larvae sinking at <25-30‰. This was not due to halted ciliary beating and may be a mechanism to escape deleterious low-salinity surface water. The sharp onset of deleterious effects at 22-25‰ is commensurate with salinity levels that larvae would experience in runoff plumes. Counter to the hypothesis of eutrophic enhancement of larval success and outbreaks, a paradigm that has driven management regulations for decades, runoff plumes are likely to be deleterious for COTS bipinnaria larvae.