Effects of Salinity on the Reproductive and Lifespan Traits of Artemia Parthenogenetic Lineages with Different Ploidy Levels

Although previous studies have investigated the reproductive (performance and mode) and lifespan traits of parthenogenetic Artemia, ploidy level has not been considered. Four parthenogenetic Artemia lineages, i.e., diploid, triploid, tetraploid, and pentaploid, were examined to determine the role of ploidy level under osmotic stress conditions (50, 100, and 150 ppt). Although the reproductive mode of the pentaploid lineage is unaffected by changes in salinity, it is entirely switched to oviparity in the diploid lineage at 50 ppt and in the tetraploid lineage at 100 ppt. Moreover, tetraploid reproduction is completely inhibited at 50 ppt. Although oviparity has been proposed as an adaptive strategy enhancing Artemia fitness at high salinities, the exclusive oviparous reproduction observed in the diploid parthenogenetic lineage at 50 ppt suggests that low salinity may also act as an environmental stressor, driving oviparity to ensure the next generation. On the basis of lifespan data, the tetraploid lineage presents greater euryhalinity than other ploidy levels do, whereas the pentaploid lineage is more stenohaline. Additionally, discriminant function analysis revealed that diploid and tetraploid lineages display heterogeneous reproductive/lifespan patterns across salinities, whereas triploid and pentaploid lineages exhibit homogeneous patterns within their respective groups. Our findings challenge the prevailing view that the ploidy level of parthenogenetic Artemia is correlated with tolerance to critical hypersaline conditions. In conclusion, we propose that the life history of parthenogenetic Artemia is influenced not only by ploidy level but also by the multifactorial integration of environmental conditions (particularly salinity and temperature) and local intra-variation/adaptation within isolated habitats.