Marine heatwaves could wipe out all common sea stars by 2100

Increasingly hot and long marine heatwaves could kill all common sea stars by the end of the century. The loss of this key ocean predator could lead to cascading ecological impacts, including an overabundance of their primary prey, mussels.

Fabian Wolf at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany and his colleagues tested how these orange-hued Atlantic sea stars or “starfish” (Asterias rubens) would fare during marine heatwaves – short periods when the ocean becomes unusually warm, usually driven by pockets of hot air above.

Using 10 saltwater tanks the size of large bathtubs, the team subjected 60 sea stars to five temperature scenarios: current average temperatures in the sea star habitat range, a hypothetical condition without marine heat waves and temperatures expected in marine heatwaves by the end of the century under three warming scenarios. The coldest condition included no heatwaves as a baseline – a steady temperature of 18.4°C (65°F) – while the hottest peaked at 26.4°C (79°F), a temperature the researchers say is possible given the most extreme warming scenario.

They kept the heat steady for 13 days, the projected length of severe marine heatwaves by 2100, followed by a few days of cold, low-oxygen water that mimicked the upwelling of deeper water, which often follows heatwaves in coastal areas. Throughout the two-month study, the researchers fed the sea stars a diet of blue mussels and measured their size and weight regularly. They also recorded the time it took each sea star to right itself after being flipped on its back, an ability critical for feeding.

In the most severe warming scenario, 100 per cent of sea stars died before the 13-day heatwave had concluded. In all three future warming scenarios, the sea stars ate fewer mussels, though animals in the no-heatwave and present-day conditions maintained a healthy appetite and weight. Sea stars in the two warmest scenarios also took the longest to right themselves after being flipped over. “The longer the heatwave lasted, the stronger the effect became,” says Wolf.

The sea stars used in the study were collected off the coast of Germany, so it is possible some members of their species from warmer areas of the Atlantic may have higher heat tolerance, says Lloyd Peck at the British Antarctic Survey, who wasn’t involved in the work.

Surprisingly, the sea stars that endured heatwaves in each scenario were more likely to survive the subsequent shock of cold water that mimicked upwelling, which can stress the animals by depleting them of oxygen. “We thought that there would be an accumulation of stress, but actually the opposite was the case,” says Wolf.

He doesn’t yet know the mechanism behind this ability but suspects it might be that animals that survive increased temperatures have a higher expression of so-called heat shock proteins, which help guard existing proteins against damage from stress.