Global warming could threaten Antarctic sea life with a re-invasion by powerful, fast-moving predators that deserted the area 35 million years ago, researchers announced this week at a meeting in London.
Throughout the world’s oceans, small crustaceans, starfish and molluscs are nimble, armour-plated or remain hidden to avoid being eaten by fast-moving fish, sharks, rays, crabs and lobsters with powerful jaws or claws.
Death on the Antarctic seafloor comes more softly. Here the top predators are starfish, which catch and gnaw only the slowest-moving species. Fleshy ribbon-worms lance the mud for buried shellfish and giant woodlice nibble on sponges.
This slower pace results from the departure of “fast-moving, bone-crushing predators”, explains palaeobiologist Richard Aronson of the Dauphin Island Sea Lab near Mobile, Alabama. They abandoned the Southern Ocean when it suddenly cooled down. “Things went a little crazy,” says Aronson.
Cooler water is thought to have made predators too sluggish to catch fast-moving food, forcing them gradually north. There is no direct evidence for this theory, however.
Aronson’s team studies the fossil remains of brittle stars, a type of starfish and favourite food item for fish, crabs and lobsters. Fossils from before 35 million years ago often have one or more arms missing – like brittle stars in the warmer oceans of today. Later Antarctic brittle stars are usually fully armed.
Warming, Aronson argues, could see the bone-crushers return, and with it a complete change in the Southern Ocean’s ecosystem. Marauding crabs would quickly polish off the area’s slow-moving brittle stars and sea urchins. Their sedentary cousins, the sea lilies, would quickly be stripped bare by fish.
“It’s not an impossible scenario,” says marine palaeontologist Alistair Crame of the British Antarctic Survey in Cambridge. “There are some very big crabs at the tip of South America that could migrate south.”
The threat of global warming will be difficult to gauge until researchers can work out why the banished predators haven’t returned before. Fossils from the northern Arctic Ocean show that ice ages sent predators packing there too, but they adapted and returned. Also, the larvae of the South American crabs are commonly found floating in Antarctic waters, yet they don’t seem to settle there, Crame points out.
Aronson hopes that more detailed fossil studies will settle this question. His best guess is that the Antarctic circumpolar current – a carousel of freezing seawater ringing the icy continent – stops banished species returning. “It could be some kind of isolating mechanism,” he speculates.
This current could be the first thing to change as the Southern Ocean warms – which it is doing at twice the global average rate.