By Andrea DeBrish
Have you ever visited the beach along the rocky shore? Have you ever investigated the tide pools formed on these rocky shores? This can be a tumultuous environment for any organism to live in. To survive in the intertidal an organism must be able to survive periods out of the water or with limited water, warmer temperatures, wave action and exposure to UV radiation(UVR). These can all be physiological stressors and different organisms will deal with them in different ways. The sea urchin displays a unique behavior called covering or masking which may help it to deal with these stressors. This behavior involves the sea urchin using the suckers at the ends of its tube feet to hold on to bits of shell or algae, keeping them in place over its body.
So what are the sea urchins gaining from covering themselves. There can be a cost associated with the energy needed to continually maintain the covering as well as possible impediments to movement so is it really worth it? Well if they are still doing it then the answer is probably yes.
There are many different studies on many different species of urchins and the results are as varied as the methods. Some attribute the behavior to adults protecting themselves from the damaging effects of UVR, others believe it stems from a need to protect themselves from the sand and other debris violently washed around in the surge.
The green sea urchin, Strongylocentrotus droebachiensis, from the east coast of the US may have multiple reasons for covering. In one study urchins exposed to UVR exhibit significantly more covering behavior than did urchins exposed only to photosynthetically active radiation(PAR), referring to those wavelengths longer than UVR including the visible light we live by. Another study on this species looked at multiple possible stressors leading to the covering behavior and found there may be multiple reasons. In this study urchins responded to UVR by covering but they also did it in response to increased wave agitation. The wave surge actually resulted in greater covering than the UVR. Interestingly the covering decreased with increasing urchin size. This could mean the larger they are the more resistant they are to these stressors. Their study showed no signs of covering in response to the presence of predators.
Another study looked at Paracentrotus lividus and came to the conclusion the urchins cover themselves to prevent being covered by fine grain sand particles and mud. These dirt particles could block openings to their reproductive and water exchange systems. This study discounted the effect of UVR on covering based on the fact that UVR is unable to penetrate very deeply into the water. When they treated urchins with suspended sand particles, they found more shells covering the larger organisms.
A fourth study compared two urchin species, Tripneustes ventricosus and Lytechinus variegates and theirs may be some of the most interesting results. They tested materials used for covering to decide on importance of covering. In T. ventricosus the material used most for covering blocked light indicating it is the main factor involved in covering. For L. variegates either of the plastics, black or clear sufficed to cover with although it consistently chose the natural grass more often. They theorized the grass choice could be simply because it was the natural material or perhaps because it would provide better camouflage with its environment.
It seems there is no strong consensus yet as to the reason sea urchins will cover themselves with debris. Research points to both UVR protection, sediment protection, wave action and even possibly for camouflage. It is very likely there is not simply one reason for the behavior and that its importance changes from species to species. This is a great example of how there is always room for more research.
Adams, N.L. 2001. UV radiation evokes negative phototaxis and covering behavior in the sea urchin Strongylocentrotus droebachiensis. Marine Ecology Progress Series 213:87-95
Dumont, C.P., D. Drolet, I. Deschenes, and J.H. Himmelman. 2007. Multiple factors explain the covering behaviour in the green sea urchin, Strongylocentrotus droebachiensis. Animal Behavior 73:979-986
Fierce, S.E., and H.E. Lapin. 2004. Selectivity of covering material in two sea urchins, Tripneustes ventricosus and Lytechinus variegatus. Dartmouth Studies in Tropical Ecology
Richner, H., and M. Milinski. 2000. On the functional significance of masking behaviour in sea urchins-an experiment with Paracentrotus lividus. Marine Ecology Progress Series 205:307-308