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The extremely euryhaline mollusc, Elysia chiorotica, does not utilize intracellular free amino acids for cell volume regulation during osmotic stress. Instead, Elysia utilizes an osmolyte previously unknown from animals, proline betaine. Although proline betaine occurs in some plants and Elysia forms a symbiosis with an algae, the proline betaine in Elysia seems to be a product of the animal.
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Pierce et al. (1984) studied this question.
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