They have no eyes, so how do sea urchins see?

SEA urchins have no obvious eyelike structure, but they react to the presence both of predators hoping to feast on them – such as fish and crabs – or, indeed, something that they may want to eat themselves. And they are experts on knowing what’s what.

I carried out research on the ecology of sea urchins living in deep-sea coral reefs off the coast of Ireland and it showed a clear relationship between the way they position themselves on the reefs and the presence of predators. The complexity of this behaviour was demonstrated by the urchin’s ability to sense and respond differently to various predators.

In the presence of fish, sea urchins hid at the base of the coral infrastructure but when crabs were close by, they had a tendency to perch as high as possible on coral branches – presumably to escape the crab’s deadly claws. Even more peculiar is the fact that sea urchins are able to distinguish between active and inactive predators.

It is baffling that such a small, globular, and simple bodied animal with no head, arms or legs could be capable of providing a highly sophisticated response to environmental cues. Without eyes, how do sea urchins perceive fish and crab predators? How do they distinguish one predator from another (e.g. fish vs. crabs)? And how can they tell whether a predator is on the hunt or not?

Tiny claw-shaped structures known as pedicellariae, as well as the tube feet of the adult sea urchin, act as the sea urchin’s feet and hands. These microscopic structures are hidden amongst the urchin’s thick web of spines and help it to move around and manipulate food. They also have the unsuspecting role of sight, or at least light sensitivity; in other words, these structures can detect the presence or absence of light.

This sensory activity is linked to receptors known as G-protein-coupled receptors (GPCR), which are concentrated on the tip and base of the pedicellariae and tube feet.

Pedicellariae and tube feet cover the entire surface of an urchin’s body and so they collectively act as a giant eye that permits urchins to “see” predators and also to seek shade when trapped in rock pools during warm summer months. Most interestingly, the genes responsible for light sensitivity in the sea urchin govern the development of animal eyes, including that of humans.

Light sensitivity is not the only tool used by sea urchins to recognise predators. GPCRs also give sea urchins a powerful sense of “smell” (known as chemosensory) allowing them to detect the chemical odour that nearby predators release in the water. They can smell injured urchins, which further help them determine whether predators are approaching. By this same mechanism, sea urchins can smell their food from a distance and, thanks to chemosensory, they can do this in a highly selective manner.

Angela Stevenson conducted her PhD at TCD where she was awarded an Irish Research Council Postgraduate Scholarship to study sea urchin ecology in deep-sea coral reefs off the coast of Ireland. She recently returned to Ireland from Washington, D.C., where she was working in ocean policy. She is now hoping to pursue research on deep and shallow water coral reefs around the globe.