Common starfish may not survive extreme ocean conditions

Common starfish will struggle to survive the extreme marine heatwaves that are expected to arrive by the end of this century, according to a study from the British Ecological Society. The experts found that starfish are already experiencing long-term health impacts from current heatwaves.

Researchers at the GEOMAR Helmholtz-Centre for Ocean Research Kiel used experiments to simulate heatwaves of +8 °C, which are projected to occur by 2100. All of the starfish tested were killed. Furthermore, heatwaves of this magnitude are only one degree warmer than heatwaves that were recorded in 2018 in the Kiel Fjord, where the study was conducted.

The experts determined that present day heatwaves of +5 °C negatively affected starfish feeding. The starfish were able to recover if the event was short, but struggled to endure longer heatwaves and lost weight.

“Our findings showcase that extreme environmental conditions such as marine heatwaves may temporally exceed a species tolerance limit with potential implications for populations at these shallow coastal depths,” said study co-author Fabian Wolf.

The researchers also tested the compounding effects of other extreme events. Heatwaves were followed by a simulated hypoxic upwelling event, which is when coastal seawater becomes colder, more saline, more acidic, and higher in CO2.

“During spring and summer, plankton blooms (which often benefit from global warming) sink to the sea’s bottom as dead material. Here, bacteria degrade this material, consuming oxygen and producing carbon dioxide,” said Wolf. “In coastal areas, particularly strong and persistent winds may push the surface water away from land, while deep (acidified and hypoxic water) is shoaled up the shores.”

The experiments revealed that starfish were more affected by a hypoxic upwelling event if they had not experienced a heatwave first. This indicates that the initial stressor, the heatwave, may provide resilience for the starfish that enables them to better cope with the upwelling event.

“Our results emphasize that it is crucial to study different stressors in combination and not isolation as stressors naturally occur in succession, meaning that they are never independent of each other,” said Fabian.

Common starfish are abundant in the Northeast Atlantic, where their ecosystems depend on them as a keystone species. For example, common starfish prey on blue mussels and control their population size.

“If the common starfish is lost from a relatively species-poor system, blue mussels may grow uncontrolled and form monocultures,” said Wolf. “Other habitat-forming species like seagrass and seaweed could be lost in the process of blue mussel propagation.”

“It must also be considered that other, non-native species may come and fill these gaps, as can be seen by several recently introduced crab species to the Western Baltic Sea that have blue mussels as their preferred prey.”

Even before common starfish are killed by heatwaves, weight loss will alter their foraging behavior. This is because starfish select their prey based on their own size – when they are smaller, they prefer smaller prey. 

The research will be presented at British Ecological Society’s Festival of Ecology.

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By Chrissy Sexton, Earth.com Staff Writer