Coexistence of sexual and parthenogenetic Artemia populations in Lake Urmia and neighbouring lagoons

We studied the Artemia populations existing in Lake Urmia (north-westem Iran), one of the largest habitats of Artemia in the world, in order to settle the long-standing controversy over the sexual status of the endemic Artemia populations. Experiments were carried out in the laboratory and in the field. Cysts, collected from different sites of the lake and peripheral lagoons, were hatched and cultured to adults in the laboratory. Adult sexual and parthenogenetic animals were isolated and newly hatched nauplii from them were cultured to maturity in different salinities, ranging from 15-80 ppt. Survival levels and percentage of animals attaining adulthood were measured over a period of 30 days. In the field experiment, cysts taken from Lake Urmia were hatched and the resulting nauplii were inoculated into six earthen ponds (80-140 ppt) constructed in the vicinity of the lake. Population composition in each pond was determined over a period of two years. Results indicated that both sexual and parthenogenetic Artemia coexist in Lake Urmia. While the lake itself is dominated by sexual Artemia, the asexual populations were found to be restricted to particular areas in or near the lake. Artemia appearing seasonally in the lagoons adjacent to the lake were exclusively parthenogenetic. Parthenogens could grow, mature and reproduce at very low salinities (15-33 ppt), whereas higher salinities (above 50 ppt) were required for A. urmiana to attain sexual maturity. We consider salinity to be a major abiotic factor determining the distribution of these sexually different populations within and outside the lake.