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.
References
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
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