Abstract
The brine shrimp Artemia is a micro-crustacean, well adapted to the harsh conditions that severely hypersaline environments impose on survival and reproduction. Adaptation to these conditions has taken place at different functional levels or domains, from the individual (molecular-cellular-physiological) to the population level. Such conditions are experienced by very few equivalent macro-planktonic organisms; thus, Artemia can be considered a model animal extremophile offering a unique suite of adaptations that are the focus of this review. The most obvious is a highly efficient osmoregulation system to withstand up to 10 times the salt concentration of ordinary seawater. Under extremely critical environmental conditions, for example when seasonal lakes dry-out, Artemia takes refuge by producing a highly resistant encysted gastrula embryo (cyst) capable of severe dehydration enabling an escape from population extinction. Cysts can be viewed as gene banks that store a genetic memory of historical population conditions. Their occurrence is due to the evolved ability of females to “perceive” forthcoming unstable environmental conditions expressed by their ability to switch reproductive mode, producing either cysts (oviparity) when environmental conditions become deleterious or free-swimming nauplii (ovoviviparity) that are able to maintain the population under suitable conditions. At the population level the trend is for conspecific populations to be fragmented into locally adapted populations, whereas species are restricted to salty lakes in particular regions (regional endemism). The Artemia model depicts adaptation as a complex response to critical life conditions, integrating and refining past and present experiences at all levels of organization. Although we consider an invertebrate restricted to a unique environment, the processes to be discussed are of general biological interest. Finally, we highlight the benefits of understanding the stress response of Artemia for the well-being of human populations.
Highlights
Because the processes of life are complicated there is no simple solution to the problem of surviving, as individuals and species, the unpredictable challenges that environments pose to populations
We review the suite of adaptations expressed at different domains to understand how they account for the complex adaptive phenotype that is generated and maintained by natural selection, though sometimes Artemia populations experience cycles of expansion and contraction favoring drift
A look at the existing Artemia species and their distribution shows three striking facts in which the species and the environment are tightly linked: (1) there are six sexual species, a relatively low number, most of them geographically restricted to salty lakes in specific regions in Eurasia at, or close to, the Mediterranean area where Artemia species diverged from the ancestral species some 80 million years ago (Baxevanis et al, 2006)
Summary
Because the processes of life are complicated there is no simple solution to the problem of surviving, as individuals and species, the unpredictable challenges that environments pose to populations. The optimum is estimated under laboratory conditions at 60 g L−1 in experiments where several fitness parameters were compared (Lenz and Browne, 1991), the maximum being close to NaCl saturations in solar ponds (340 g L−1), whilst the lower limit depends on the upper salinity tolerance of fish predating on Artemia, in extreme cases being as high as 100– 130 g L−1 as reported by Van Stappen (2002).
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