AN ANALYSIS OF RESPONSE TO OSMOTIC STRESS IN SELECTED DECAPOD CRUSTACEA

¹ Department of Zoology, University of California, Los Angeles, California

1. The decapod Crustacea, Emerita, Callianassa, Upogebia, Cancer antennarius. C. gracilis and Pugettia cannot regulate osmotically and from lack of tolerance are generally stenohaline.

2. Pachygrapsus, Birgus, Hemigrapsus and Uca can regulate osmotically in concentrated and dilute sea water. Hemigrapsus is the weakest hypo-osmotic regulator of the four species.

3. Among species where osmotic regulation occurs, it is established immediately and may be long lasting or may grow weaker with time. Although blood concentrations may fluctuate in a given stress, the phenomenon is not common.

4. Equilibrium of the blood concentration, where perfect regulation does not occur, is usually established within 24 hours following immersion; changes occasionally occur later when extreme stresses are imposed.

5. Estimates on the solute space volumes were calculated as 40% for Emerita, 54% for Pachygrapsus and about 50% for Birgus.

6. Concentration changes occurring in the blood of Pachygrapsus and Emerita are caused mostly by salt rather than water exchanges.

7. There is a dynamic flux of salt and water in the gill chamber of Pachygrapsus, thus furnishing further evidence that the gills are osmo-regulatory organs.

8. The osmotic regulating crustaceans Cambarus, Pachygrapsus, Hemigrapsus nudus and H. oregonensis have less permeable exoskeletons than the non-regulators, Cancer gracilis, C. antennarius and Pugettia by a factor of at least three.

9. Pachygrapsus, Uca, H. nudus and H. oregonensis sustain greater osmotic gradients when greater osmotic stresses are imposed. The two species of Hemigrapsus are weak regulators in small stress, but the osmo-regulatory activity accelerates as stresses increase. A sensitivity to absolute salinities is suggested.

10. Uca averages greater respiratory rates in greater osmotic stresses, but this is not necessarily so for individual specimens and the average differences are small. A discussion of the energetics of osmotic regulation reaches the conclusion that such increases in metabolism are not direct reflections of increased osmotic work but of muscular or other activity.