OSMOREGULATORY CAPACITIES OF TWO CARIDEAN SHRIMPS, SYNCARIS PACIFICA (ATYIDAE) AND PALAEMON MACRODACTYLUS (PALAEMONIDAE)

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Abstract
Osmoregulation of 2 caridean shrimps adapted to various salinities was studied by measuring Cl concentration of blood and osmotic pressure of blood and urine by freezing point depression. The atyid Syncaris pacifica appears confined to fresh water, as are practically all species of this relict family, and has been collected only in Marin and Sonoma Counties, Calif. In contrast, Palaemon macrodacytlus has been recently introduced to the San Francisco Bay estuarine system and now occuples numerous brakish localities there. Distinct adjustment responses are shown by the 2 species when they are exposed to environments of different salinities. P. macrodactylus is a strong regulator; over the ranee of 2-150% sea water (SW), body fluid concentrations change only slightly, like many decapods, P. macrodactylus excretes urine which is isosmotic to blood throughout the tested salinity range. On the other hand, urine of S. pacifica is hypo-osmotic to blood in the natural habitat (urine about 25% of total blood concentration). In higher salinities blood, urine and medium are practically isosomotic. There have been few physiological reports on atyids or on non-European palaemonids. Urine hypo-osmotic to blood has previously been reported in only one decapod group (Astacura). Both atyids and crayfishes have long histories in fresh waters. All palaemonids studied are osmoregulators, but none are yet known to produce a markedly dilute urine as an adaptation for life in environments of low salinity.

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