Enhanced performance of juvenile crown of thorns starfish in a warm-high CO2 ocean exacerbates poor growth and survival of their coral prey

Declines in coral cover across the Indo - Pacific are associated with multiple stressors of global ocean change, and this is exacerbated by population outbreaks of the coral-eating crown of thorns starfish (COTS), Acanthaster spp. We investigated the growth and feeding responses of the corallivorous juvenile stage of COTS and their coral food, Acropora spp., to near-future ocean warming and acidification. Juvenile COTS and Acropora spp. were exposed to three temperatures (26, 28, 30 °C) and three pH levels (NIST scale: 8.0, 7.8, 7.6) in a flow-through cross-factorial experiment. Increased temperature and acidification had independent positive effects on growth and feeding of juvenile COTS. They grew faster and were larger under these conditions, but weighed less at low pH. Accelerated growth of juvenile COTS under ocean acidification conditions was decoupled from skeletogenesis with increased size largely due to development of soft tissue. Juveniles reared at 30 °C also had the greatest number of spines. To assess the recovery of Acropora spp. following sublethal COTS predation in warm and acidified conditions, corals that had been preyed on by the juveniles for 1 week were removed and held in experimental conditions to monitor their growth and survival. The stress from warming and predation had an interactive effect in reduction in coral growth. Mortality of corals was highest following predation and in the 30 °C treatments. There was no direct effect of acidification on growth or survival of Acropora spp. following predation. It appears that COTS juveniles may benefit from near-future warming and acidification. Thus, changing climate may increase the threat of this predatory starfish to coral reefs.