Dive to the bottom of the icy ocean that envelops Antarctica and the critter you’re most likely to encounter are echinoderms: a family of marine animals that includes the star fish, sea urchin and the warty, vegetable-like sea cucumber.
Many species of Antarctic echinoderm are unique to the region, having lived untouched for millions of years in the frigid and relatively stable conditions on the seabed. But climate change is altering this precious environment, and, as the mercury soars, warmer sea temperatures aren’t the only thing making echinoderms uncomfortable. Melting ice sheets are pouring freshwater into the polar oceans, diluting the saltwater these animals are finely attuned to.
“We wanted to know whether Antarctic echinoderms can cope with extreme influxes of meltwater, something that might happen after a short-lived polar heatwave,” said Nick Barrett, a PhD student jointly based at Cambridge Earth Sciences and the British Antarctic Survey.
In a series of experiments at BAS’ polar aquarium, Barrett exposed six common species of Antarctic echinoderms to dilute seawater over a 24-hour period and measured their physical responses.
The new study helps pinpoint the most at-risk species of Antarctic echinoderm, “knowing how these animals respond to low-salinity conditions is an important step in protecting them and pushing conservation efforts toward vulnerable species,” said Barrett.
BAS diver collecting Antarctic echinoderms: Credit: Simon Brockington.
Echinoderms are found in all climate zones, but in Antarctica they make up a larger proportion of the seabed community. “Echinoderms underpin the Antarctic marine ecosystem,” said Barrett, “the worry is that changes in their abundance could upset the delicate balance of energy flowing through food webs here.”
In the new study, Barrett monitored how a range of echinoderms, including one species of sea urchin, a starfish, a brittle star, and three sea cucumbers, reacted to low salinity. He measured their blood salt levels and checked their metabolic rate and activity levels before and after exposure.
To gauge the animals’ activity levels, Barrett measured how long it took for the echinoderms to manoeuvre into an upright position if placed upside down. However, he couldn’t use this method for sea cucumbers because their barrel-shape body doesn’t have a clear ‘top’ and ‘bottom’. Instead, he counted the number of wave-like muscle contractions that rippled along the sea cucumber’s body—the same action that earthworms use to move through soil.
Barrett was surprised to find that sea cucumbers were most resilient to low salinity levels, “when you look at them, it’s easy to liken them with slugs,” he said, “because they are soft-bodied, I had thought they’d soak up the surrounding water and be vulnerable to changes in their surroundings. But that wasn’t the case at all.”
The long-armed and spiny brittle stars, an iconic Antarctic species, were, however, least able to cope in low salinities. “We already knew that brittle stars are vulnerable to warming,” said Barrett, “previous studies have shown they can’t survive in ocean temperatures of +2oC, which is within the range predicted due to climate change. And now we know this species can’t survive in low salinity.” Barrett said that, in the real world, the combined climate impacts of warming sea temperatures and lower salinity will likely amplify the stress placed on echinoderms.
Barrett is also exploring the longer-term effects of seawater freshening on Antarctic echinoderms in experiments over several months, “the short-term experiments give us an idea of which species are vulnerable, but we also working on understanding whether these animals can acclimatize to protracted environmental perturbations, which is key to a species survival.”
Reference: Barrett, N, Harper, E and Peck, L (2024), “The impact of acute low salinity stress on Antarctic echinoderms”, Proceedings of the Royal Society B.
Feature image: Antarctic brittle stars by Nick Barrett.