OSMOTIC BEHAVIOR IN AN INTERTIDAL LIMPET, ACMAEA LIMATULA

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Abstract
Over the range of salinities from 25% to 150% sea water, the aspidobranch gastropod Acmaea limatula shows no ability to osmoregulate. A. limatula is isomotic with the medium, within 24 hours, at all salinities from 50% to 150% sea water; in 25% sea water the animals die. The mean body water, as a percentage of body weight, is 78% for animals in 100% sea water, 82% for animals in 50% sea water, and 73% for animals in 150% sea water. The percentage change of body weight as water is approximately the same in hypo- and hypertonic media; a 28% increase in 50% sea water and a 28% decrease in 150% sea water. But calculations of the mobile (osmotically active) water, as percent original body weight, shows that it differs in hypo- and hypertonic media; 27.5% in 50% sea water and 43.7% in 150% sea water. This suggests that salts play a greater role in effecting an osmotic pressure change in the blood of animals in 50% sea water than of animals in 150% sea water. Field desiccation experiments show that A. limatula responds in a similar way to both air desiccation and salinity dehydration, i.e., the concentration of blood rises. However, when the animals are exposed to air, the presence of water between the soft parts and the shell (extra-visceral water) seems to be of adaptive significance in that it serves both in an osmotic and temperature buffering capacitv. Animals in which the extra-visceral water is removed show a more rapid rise in blood concentration as compared with animals retaining the extra-visceral water. And, since the increase in the concentration of the extra-visceral water and blood is a direct function of the temperature (through evaporation), the difference in the blood concentration between the 2 groups is undoubtedly due to the presence or absence of extra-visceral water.

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