Ecological Isolation in Artemia: Population Differences in Tolerance of Anion Concentrations

Abstract

ABSTRACT Artemia cysts were collected from 15 North American habitats: 6 sea-water salterns where the prevailing anion is chloride and nine inland lakes where the prevailing anion is either chloride, sulfate, or carbonate. On a yeast diet, 14 of the 15 populations have optimum nauplius-to-adult viability in media with sodium concentrations of 1.1-1.3 M (with sodium comprising > 90% of relative molar cationic composition). However, nauplii from Jesse Lake require less concentrated media (0.6 M sodium) for the first week of development. The range of anionic tolerance of each population greatly exceeds the range encountered in the source water. Saltern shrimps live in waters with less than 5% sulfate (expressed as percentage molar anionic composition), yet thrive in media with up to 29% sulfate. Penley Artemia tolerate laboratory media which are 23% carbonate, although Penley Lake is reported to contain < 1% carbonate relative molar anionic composition. Each of the four populations from carbonate lakes tolerates 0.2% to 66% of the anionic molar composition as carbonate. Media with high carbonate concentrations (up to 11 g carbonate/l, pH 9.7, obligate low magnesium and calcium) killed shrimps from high-chloride habitats but permitted nauplius-to-adult development of Artemia from 5 low-chloride lakes (Penley, Fallon, Kiathuthlanna, Jesse, and Mono lakes). Low-carbonate media (<0.2 g combined carbonate and bicarbonate/1) distinguished clusters of populations by the criterion of tolerance of chloride/sulfate (Cl/SO4) ratio. In complex solutions, differences in Cl/SO4 ratio are confounded with changes in osmolarity, ionic strength, pH, and specific gravity. Observation of viability of Artemia in an array of media matched for the other parameters revealed authentic population differences in tolerance of Cl/SO4 ratios. In order of descending relative chloride tolerance, the sequence is: 3 Caribbean populations, 6 other populations from high-chloride habitats, and 3 populations tested from low-chloride habitats (Fallon, Penley, and Jesse). Jesse nauplii show the least tolerance of high absolute chloride concentration. In medium No. 70 (1.1 M sodium, Cl/SO4 molar ratio of 2.5, 80 g total dissolved solids/1) all populations except Jesse Lake Artemia have high fertility and viability, much higher than when cultured in sea water. Interpopulation matings in No. 70 and other permissive media produced fertile F, and F2 progeny. Thus, these 15 populations belong to the A. franciscana Kellogg superspecies. Some populations (or clusters of populations) are ecologically isolated in nature and represent incipient species or sibling species.