ION AND WATER BALANCE OF THE HYPO- AND HYPEROSMOTICALLY STRESSED CHITON MOPALIA MUSCOSA

¹ Moss Landing Marine Laboratories, California State Universities, Moss Landing, California
² Dept. of Biological Sciences, California State University Hayward, Hayward, California 94542

1. The effects of external salinity changes on whole-animal volume, blood ions, and muscle-tissue water content of the chiton Mopalia muscosa were investigated. The data indicated that short-term low-salinity adaptation in the chiton M. muscosa is achieved by volume control mechanisms that are quickly activated.

2. Blood Na⁺, Cl^-, K⁺, and Mg²⁺ were isoionic to the SW concentration in salinities ranging between 60 and 125% SW. Blood Ca²⁺ concentration was hyper-regulated in hyposmotic SW.

3. Regulation of cell volume occurred in salinity-stressed chitons, as water content of foot-muscle tissue was regulated in both hypo- and hyperosmotic media.

4. When exposed to hyposmotic SW the chitons at first (0-4hr) gained weight; this was followed by a period in which the rate of weight gain approached zero (4-6 hr) and finally by a period of weight loss (6-24hr) (volume regulation). Exposure to hyperosmotic SW resulted in weight loss and little volume regulation.

5. Volume control in hyposmotic media is accomplished, in part, by a loss of solute.

6. Low temperature, 7°C, decreased both the water influx and volume regulation.

7. Volume regulatory mechanisms are activated within 1 hr after exposure to 60% SW.