RESISTANCE OF THE PURPLE SEA URCHIN TO OSMOTIC STRESS

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Abstract
The sea urchin, Strongylocentrotus gurpuratus, resists 70% to 120% sea water briefly and 80% to 110% sea water for many days. Damage at extremes is indicated by loss of activity, pigment and appendages, and by failure to respond to food, probing and light The sea urchins lose weight in hypertonic solutions and gain weight in hypotonic solutions, but recover on replacement into sea water. Respiration is little altered by tonicity of sea water. Addition of sucrose to diluted sea water protected sea urchins from osmotic damage but excess sucrose in sea water created a stress like excess salt. Sea urchin eggs develop normally over the range 90% to 110% sea water, abnormally in more dilute and more concentrated sea water. Diluted sea water with tonicity restored by adding sucrose was almost as favorable for development as sea water, while sea water with added sucrose was deleterious. Since the maximal osmotic variations in the natural environment of the sea urchin are generally small, the sea urchin is capable of withstanding such changes with an ample safety factor.