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Can deep reefs rescue shallow ones?

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Science  03 Mar 2017:
Vol. 355, Issue 6328, pp. 903
DOI: 10.1126/science.355.6328.903

Scientists test the idea that species squeezed out of shallow coral reefs may find a refuge in deeper habitats.

Big, red, delicious “kumu” goatfish are tough to find in Hawaii's shallow coral reefs because they are so heavily hunted by spear fishermen. But the fish are still common in deeper coral gardens. “If I want to get a fish for a party, I will bomb to 200 feet and get my fish in 10 minutes,” says deep diver Randy Kosaki, who is also deputy superintendent of the Papahānaumokuākea Marine National Monument in Hawaii.

Kosaki's fish story supports a hopeful notion: that the little-studied, dimly lit deep coral reefs (see main story, p. 900) might offer a refuge for species from shallow reefs beleaguered by warming, pollution, and overfishing. Or as a recent report by the United Nations Environment Programme asked: “Mesophotic Coral Ecosystems: A lifeboat for coral reefs?”

Recent findings have buoyed hopes for refuges in the depths: Scientists have found the corals that build shallow reefs—scleractinian or stony corals—growing far deeper than was known, and some species of fish and invertebrates live on both shallow and deep reefs. Kosaki recalls seeing deep corals still thriving downslope from “shallow corals horrifically bleached as far as the eye could see.”

Yet studies are just getting started to rigorously test the “deep reef refuge hypothesis,” first proposed in 1996. The 15 February issue of Science Advances includes one of the first, testing whether deep populations of corals might reseed shallow ones. Pim Bongaerts, a research scientist at the University of Queensland in Brisbane, Australia, and his colleagues zeroed in on two coral species that live on both deep and shallow reefs in Bermuda. To find out whether larvae from the deep corals can repopulate shallow reefs, the scientists sequenced part of the genome of shallow and deep populations of each species to see how much they had mixed in the past. In one species, the fragile saucer coral (Agaricia fragilis), the genomes of deep and shallow individuals showed significant divergence, suggesting limited genetic exchange. Another species, the blushing star coral (Stephanocoenia intersepta), showed no such divergence, suggesting recent mixing, the authors report.

Bongaerts and colleagues conclude that reseeding may happen in only a few coral species. “The deep reef refuge hypothesis … can be very relevant to individual species, but should not be assumed as a broader ecosystemwide phenomenon,” Bongaerts says. “Deep reefs are unlikely to represent a lifeline for shallow coral reef biodiversity.”

Kosaki notes that even if deep reefs can't be counted on as a refuge for many shallow corals, they may be more effective havens for fish, like the kumu, that traverse the two types of habitat. “The results show that it's a case-by-case, species-by-species sort of thing,” Kosaki says.

  • * in Honolulu

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