By Sarah Ward
Living Seas Officer
Feet aren’t really a body part you’d associate with creatures living in an aquatic environment; most animals swim effortlessly, drift in the current, or crawl across the sea bed. However, one group of marine animals do indeed have feet, just not in the same sense that us land-lubbers have them.
Echinoderms are an exclusively marine group of invertebrates; starfish, sea cucumbers and urchins are all part of this group. Animals in this group all have an outer skeleton of calcified plates - the Greek, ‘echinodermata’ literally translates to ‘spiny skin’ – and their tube feet protrude from pores which are organised in rows radiating from a central point (all animals in this group exhibit radial symmetry).
Tube feet are small projects that come out of a water-vascular system, a series of canals that run around the body. It is essentially a water hydraulic system that allows movement of the tube feet by moving water in and out of them using valves. Most echinoderms have tiny suckers at the end of their tube feet, but some starfish and brittle starts have little paddles.
© Paul Naylor
As you might expect, tube feet are generally used for movement, allowing the animal to seemingly slide across the sea floor, it’s tube feet suckering and un-suckering to the surface below (think about a swan effortlessly gliding across a lake, but in reality its legs are paddling ten to the dozen). However, tube feet are often used for more than just locomotion; they can also play an important role in respiration and gathering food. Some starfish will even prise open a bivalve mollusc with their tube feet.
Echinoderms can be spotted in rock pools on a low tide – take a look for them. Common starfish and brittle stars are frequently seen, if you’re lucky you might even find a sea cucumber or an urchin.