Sea urchins are voracious eaters, destroying kelp beds as they graze. However, devastated kelp beds can re-generate on wave-swept surfaces where sea urchins have more difficulty clinging on. Shigeru Kawamata from the National Research Institute of Fisheries, Japan, explains that sea urchins grip bare rocks with flexible foot suckers that mould to most surfaces. But how would sea urchins fare if the shore were coated in fine mats of slippery microscopic algae? Suspecting that microalgal surfaces could deter sea urchins from moving over them, Kawamata decided to test how well Strongylocentrotus nudus sea urchins move across and graze on thin mats of microalgae (p. 1464).

Encouraging algal mats to grow on sections of simulated seabed, Kawamata replicated the seashore in a flow tube where he could reproduce the waves’ surging motion. Attaching young kelp to algae-encrusted seabed sections and algae-free sections, Kawamata released sea urchins into the flow tube. He then filmed the sea urchins’ activity as they travelled around the flow tube while he adjusted the wave velocity from 10 to 40 cm s–1.

At the lowest wave velocity (10 cm s–1), the sea urchins had no problems gripping onto the slippery algal mats. However, as the water velocity increased, the sea urchins moved more slowly on the slippery surface. They also had increasing trouble hanging onto the algal surfaces at the moderate wave velocities and often became dislodged. And when Kawamata checked how well the young kelp plants had survived, he found that the algal mats had kept the sea urchins at bay, protecting the young kelp that were out of the sea urchins’ reach, while the kelp that were attached to algae-free surfaces were decimated. However, at the highest wave speed, 40 cm s–1, even the sea urchins in the alga-free environments had difficulties feeding as the waves buffeted the kelp.

So, microalgal mats could play a crucial role in helping to re-establish sea urchin-devastated kelp beds as the animals are unable to graze on kelp when they cannot get a grip on the slippery algal surface. Kawamata is keen to find out whether microalgal mats also protect established kelp from sea urchin damage.

Experimental evaluation of the anti-attachment effect of microalgal mats on grazing activity of the sea urchin Strongylocentrotus nudus in oscillating flows
J. Exp. Biol.