Undeterred by those spines, it seems that some sharks find nothing more irresistible than chowing down on a sea urchin snack – and researchers at Australia’s University of Newcastle reckon the discovery could prove vital in saving kelp forests from mass urchin depredation.
Native to the temperate sea off the south-east coast in New South Wales, long-spined sea urchins (Centrostephanus rodgersii) have been responding to ocean-warming in Australia by spreading south into the waters off Victoria and Tasmania.
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The seas in the region are warming at almost four times the global average rate, and the urchins are devouring the kelp and invertebrates in their path, drastically reducing kelp cover for other marine life in the process.
The urchins’ key predator had always been assumed to be the eastern rock lobster (Sagmariasus verreauxi), which had declined through over-fishing but is now recovering in the area. However, the lobsters’ presence did not seem to be stemming the urchin invasion, so the scientists decided to dig deeper into this predator-prey relationship.
They were astounded by what they found when they tethered 100 sea urchins to blocks outside a lobster hideout off Wollongong, leaving multiple video cameras to record what happened over each of the next 25 nights. A red-filtered light was used to illuminate the scene, because invertebrates dislike the white-light spectrum.
“We expected that our cameras would capture lobsters eating the urchins,” says PhD researcher Jeremy Day, “but in fact the lobsters showed little interest in the urchins and ate just 4% of them. They were often filmed walking straight past urchins in search of other food.”
Proving to be far more interested in the urchins were bullhead sharks. “Both crested horn sharks (Heterodontus galeatus) and Port Jackson sharks (H portusjacksonii) entered the den and ate 45% of the urchins,” reports Day. “This suggests that sharks have been overlooked as predators of sea urchins in New South Wales.”
The sharks appeared to have no trouble consuming the large urchins, which they did “sometimes in just a few gulps”, according to Day. “Our findings suggest the diversity of predators eating large sea urchins is broader than we thought – and that could prove to be good news for protecting our kelp forests.”
It turns out that there had in fact been little empirical evidence for the long-held belief that lobsters were an urchin’s worst nightmare.
While it is already established that kelp habitats can be boosted by protecting or reinstating urchin predators, the scientists now think conservationists might well have been backing the wrong predator. Their research has just been published in Frontiers.
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