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

Abstract

Artemia cysts were collected from 15 North American habitats: 6 sea-water salterns where the prevailing anion is Cl- and 9 inland lakes where the prevailing anion is either Cl-, SO42- or CO3-. On a yeast diet, 14 of the 15 populations have optimum nauplius-to-adult viability in media with Na concentrations of 1.1-1.3 M (with Na comprising > 90% of relative molar cationic composition). However, nauplii from Jesse Lake require less concentrated media (0.6 M Na) for the 1st 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% SO42- (expressed as percentage molar anionic composition), yet thrive in media with up to 29% SO42-. Penley Artemia tolerate laboratory media which are 23% CO3-, although Penley Lake is reported to contain < 1% CO3- relative molar anionic composition. Each of the 4 populations from CO3- lakes tolerates 0.2 to 66% of the anionic molar composition as CO3-. Media with high CO3- concentrations (up to 11 g CO3-/l, pH 9.7, obligate low Mg and Ca) killed shrimps from high-Cl- habitats, but permitted nauplius-to-adult development of Artemia from 5 low-Cl lakes (Penley, Fallon, Kiathuthlanna, Jesse and Mono lakes). Low-CO3- media (< 0.2 g combined CO3- and HCO3-/l) distinguished clusters of populations by the criterion of tolerance of Cl/SO42- ratio. In complex solutions, differences in Cl-/SO42- 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 authenic population differences in tolerance of Cl-/SO42- ratios. In order of descending relative Cl- tolerance, the sequence is: 3 Caribbean populations, 6 other populations from high-Cl habitats, and 3 populations tested from low-Cl habitats (Fallon, Penley and Jesse). Jesse nauplii show the least tolerance of high absolute Cl concentration. In medium No. 70 (1.1 M Na, Cl-/SO42- molar ratio of 2.5, 80 g total dissolved solids/l) 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 F1 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.