Life can be tough if you're a small creature on a seaside rock; one minute you're on the move, and the next you need to cling on as waves threaten to sweep you away. Patrick Flammang and his Belgian colleagues reveal the trade secret of rock-clambering sea stars and sea urchins: their numerous tiny hydraulic tube feet contain mutable tissues that allow their owners to get a toehold when the sea gets too rough(p. 2277).

Flammang explains that many echinoderms, including sea stars and sea urchins, possess mutable collagenous tissues (MCTs) that can switch from being flexible to rigid in seconds, thanks to special secretory cells. To find out if sea stars and sea urchins have mutable tissues in their feet, Flammang and his team immersed the tube feet of the little sea-dwellers in solutions that they knew would disrupt secretory cells. Then they tested the mechanical properties of the tube feet by stretching each tube foot to breaking point. They knew that other echinoderm mutable tissues become stiffer after bathing in these cell-disrupting solutions, and noticed exactly the same effect in the tube feet of sea stars and sea urchins. They also found that both species'tube feet became more flexible in calcium-free seawater, just as other mutable tissues do. And when they examined the tube feet under a microscope, they saw cells that are remarkably similar to the secretory cells found in other mutable tissues. The team concludes that sea stars and sea urchins possess mutable tissues in their feet, which act as shock-absorbers to help the creatures resist pounding waves. But they also noticed that sea urchin tube feet are stronger, stiffer and tougher than the larger tube feet of sea stars,which they suspect reflects how the creatures walk; sea urchins use their tube feet to drag themselves forwards, while sea stars' tube feet act as flexible levers.

Santos, R., Haesaerts, D., Jangoux, M. and Flammang, P.(
). The tube feet of sea urchins and sea stars contain functionally different mutable collagenous tissues.
J. Exp. Biol.