Sea cucumbers may help protect coral reefs

They change the chemistry of seawater

Coral reefs are under pressure. The seas they live in are getting ever warmer and more acidic, taking them out of coral’s comfort zone. Pollution is encouraging algal blooms, which stifle their growth. And overfishing is wiping out many species that they depend on for protection.

The over-fished species which have drawn most attention are those that feed on the algae or hunt others which have a penchant for consuming coral. However, Cody Clements and Mark Hay of the Georgia Institute of Technology wish to add what might seem an unlikely candidate to the list: sea cucumbers.

Sea cucumbers, as soft and cylindrical as their name suggests, are unarmoured relatives of sea urchins and starfish. They feed on sediment, digesting the organic matter therein and voiding sand. They are also, in the guise of bêche-de-mer, a delicacy in many parts of the world, particularly east Asia. This demand, along with not enough sea-cucumber farming, has led to a poorly monitored and regulated trade in the animals (one estimate puts it at being worth $1.5bn a year).

That is worrying to conservationists. But Dr Clements and Dr Hay suspected things might be worse, and that stripping reefs of their cucumbers may also have an effect on the coral itself.

Experiments in aquaria show that sea cucumbers influence the chemistry of the seawater surrounding them. In particular, they excrete two substances that might help corals: ammonia and carbonate ions.

Ammonia, a compound of nitrogen and hydrogen, is used to make protein by the symbiotic algae which live in the sea-anemone-like animals, called anthozoans, that build coral reefs. These algae, in turn, provide the anthozoans with much of their nutrition. Carbonate ions, meanwhile, reduce local acidity. Both of these facts suggest corals should benefit from having sea cucumbers around.

To test that idea the two researchers decamped to French Polynesia and set up, off the coast of Mo’orea, an experiment with a species called the black sea cucumber. They put 20 cages around corals planted on reefs fringing the island. They then added two sea cucumbers to ten of the cages and looked, over the following 38 days, for differences in growth rates between corals caged up with cucumbers and those not so accompanied.

There was none—which surprised them, given the expectations raised by the aquarium experiments. But instead of giving up, they doubled down, running the experiment again with 24 cages and four sea cucumbers per cage instead of two.

That worked. Two of the three encaged coral species grew faster in the cucumbers’ presence—in one case by 17% and in the other (an endangered staghorn coral called Acropora pulchra) by a whopping 41%. Only the third displayed no detectable reaction.

Still, two out of three is a significant result, and bears directly on the point of conducting the experiment in the first place. If removing cucumbers also harms coral, which this experiment suggests it will, that adds to the urgency of bringing the bêche-de-mer trade under control. ■