PHYSIOLOGICAL CORRELATES OF WATER CHEMISTRY REQUIREMENTS IN FAIRY SHRIMPS (ANOSTRACA) FROM SOUTHERN CALIFORNIA

Abstract

ABSTRACT In an effort to explain distributional differences among 4 species of fairy shrimps (Anostraca) in southern California, we correlated their ionoregulatory abilities with ion levels in their native pools. Branchinecta sandiegonensis Fugate and Streptocephalus woottoni Eng, restricted to dilute coastal vernal pools in San Diego county, were strong hyperregulators that maintained constant hemolymph [Na'] (average = 82 mmol 1-1, and 69 mmol 1 1, respectively) when external Na' levels were below 60 mmol 1 1. In higher concentrations, however, hemolymph levels rose significantly and mortality increased; no S. woottoni survived in 100 mmol 1-1 Na+. Similarly, both species maintained constant hemolymph levels in alkalinities up to 800–1,000 mg 1-1, but did not survive higher concentrations. In contrast, Branchinecta mackini Dexter, which inhabits saline and alkaline pools, regulated hemolymph ion levels more loosely, but were tolerant of higher ion concentrations. Hemolymph Na+ concentrations rose gradually from 50.0 ± 3.0 mmol 1-1 in 1 mmol 1-1 water to 105.1 ± 1.8 mmol 1 1 in 100 mmol 1 1 water, but there was no mortality. They could tolerate alkalinities up to 2,000 mg 1-1 (double that of the other 2 species), although hemolymph [Na+] fell from 88.8 ± 1.5 mmol 1-1 to 78.0 ± 3.8 mmol 1 '. The fourth species, Branchinecta lindahli Packard, which is found in both types of pools, exhibited intermediate ionoregulatory abilities. In high external Na' concentrations they allowed internal levels to rise, but in low external concentrations (<20 mmol 1-1) they were able to maintain elevated hemolymph levels. These results indicate that the distributions in southern California of the 4 species of fairy shrimps examined here were greatly influenced by the water chemistry of the pools due to different ionoregulatory capabilities.