CHLORIDE ION REGULATION AND OXYGEN CONSUMPTION IN THE CRAB OCYPODE ALBICANS (BOSQ)

¹ Edward Martin Biological Laboratories, Swarthmore College, Swarthmore, Pennsylvania

1. Ocypode albicans, a brachyuran crab with a terrestrial habitat, is able to regulate the chloride ion content of the blood in the dry habitat and in sea water of salinities ranging from 120 to 600 millimoles of chloride per liter. The mean value of the chloride ion concentration in the blood is 378 millimoles of chloride per liter. This value is lower than that of sea water at the site the crabs were collected (480). It is suggested that the lower concentration of chloride is easier for the terrestrial animal to maintain in an environment where chloride is available only from food or from occasional contact with the sea.

2. In sea water containing less than 120, or more than 600 millimoles of chloride per liter, the internal concentration is not maintained, but tends to fall in dilute solutions, rise in the more concentrated ones. This internal shift is significant in a period of twenty-four hours, and proves fatal if continued for an additional twenty-four hours. The antennal gland functions in the regulation of the internal chloride ion concentration. It re-absorbs water when the crab is in air and in hypertonic solutions. It excretes water when the crab is in hypotonic solutions, and may excrete chloride also. The evidence points to another organ functioning in the reverse way; this is assumed to be the gill membrane.

3. The oxygen consumption of Ocypode in air is of the order of 0.139 cubic centimeters of oxygen per gram fresh weight per hour. In water the crab uses a gill bailer and gill rakers constantly, and the respiration rate is elevated. The increase is least when the crab is in an environment in which the chloride ion concentration equals that of the blood. In hypotonic and hypertonic environments the oxygen consumption is further increased. This is caused by the increased activity of the ion-regulating membranes of the antennal glands and gills.

4. Comparison of individual responses to the different environments indicates a difference in individual ability to meet the stress. The differences are more widely marked in environments most different from the internal medium. It is suggested that this is a measure of the variation of a physiological mechanism which was probably important in the evolution of the crab from the aquatic to the terrestrial habitat.