New research from the University of Sydney suggests that our tropical coral populations are facing yet another obstacle as their keystone predator, the crown-of-thorns starfish (COTS), can withstand immense marine heatwaves. Byrne et al. 2023 exposed juvenile COTS to thermal stress scenarios at a tempo and temperature duration designed to reflect the Degree Heating Week (DHW) conditions that cause coral bleaching and mortality. The study found that juvenile COTS have a much greater thermal tolerance than our reef building corals.

They observed juveniles happily surviving extreme marine heatwave scenarios – more than 3x the level that triggers coral bleaching – and up to 2 weeks in acute temperature shocks of 34 °C. The juveniles’ tolerance of bleaching events is further exacerbated by their affinity for recruitment in coral-rubble habitats that allows them to rise up in numbers before turning to corallivory once available to sustain them.

Video illustration of how juvenile COTS are able to overcome mass bleaching events.
The University of Sydney / Matt Clements

This rings alarms bells for our corals as while adult COTS wreak havoc on our reefs by gorging themselves on corals, juveniles prefer a vegetarian diet of algae, meaning they can outlive bleaching events. Even more worrisome, COTS can control how quickly they grow up and can remain as herbivorous juveniles when there’s not enough coral to sustain them, an ability that the Byrne Lab liken to Peter Pan.

This means that our corals face another obstacle even post-bleaching, where young COTS can wait patiently feeding on algae covered rubble before developing into adults to devour recovered corals.

Adult Crown of Thorns Starfish.
Aisling Kelly