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Brine shrimp

Parthenogenesis and shrimp genetics

Brine shrimp inhabit areas across the globe, both in the western hemisphere and eastern hemisphere. Based on morphological and life history characteristics, there are 3 main groups of Artemia present: Old World zygogenetic, New World zygogenetic, and Old/New World parthenogenetic shrimp. Parthenogenetic shrimp are grouped into a single species, Artemia parthenogenetica, but are genetically variable within the species. Zygogenetic shrimp are divided into multiple species and varieties. All Western Hemisphere (New World) shrimp are diploid.

Parthenogenesis is a method of asexual reproduction where fertilization by males is not required. This phenomenon has been discovered in the eastern hemisphere (e.g., Mediterranean basin, Spain, Asia), and is found in areas where habitat conditions are stable and uninterrupted. Parthenogenesis has yet to be confirmed in the western hemisphere. parthenogenetic females produce a higher proportion of females than males, skewing the sex ratio. Researchers attempt to document parthenogenetic brine shrimp populations by finding a skewed sex ratio.

Because parthenogenetic shrimp have not been identified in the Western Hemisphere, comparisons between the two species are made in Europe and Asia, and then researchers speculate on what may happen if the co-habitation occurred in the Western Hemisphere. In the Eastern hemisphere, sexual populations have a higher probability of cyst production over parthenogenetic populations. Parthenogenetic populations tended to give live birth. The two reproductive methods were typically are not found in the same locations, due to competitive advantages in different habitats. Sexual populations have a higher fitness in harsh, unpredictable habitats due to high cyst production and the maintenance of diversity through sexual reproduction; parthenogenetic shrimp have a higher fitness in more stable, predictable habitats due to their ability to produce large quantities of offspring without diverting energy to cyst production. Laboratory experiments have shown that one species is able to out-compete the other species in as little as 2-3 generations, due to a large niche overlap.

A recent publication by Campos-Ramos et al (2003) stated that parthenogenetic cysts were found in commercial cyst products. This could be due to an adaptation by GSL shrimp, an introduction event into the lake, or if commercial processors mixed cysts from other areas into their processing procedure. An introduction or development of parthenogenic shrimp in the GSL could alter cyst production, the algal growth cycle, the overall shrimp population, and species that rely on brine shrimp in the lake. Because A. parthenogenica has lower cyst production compared to A. franciscana, harvest levels (see Brine shrimp harvest) would have to be modified to ensure a viable population remains in the spring to re-seed the lake. If algae are grazed at a rate different from current rates, it is possible that the balance and timing of brine shrimp reproduction could be upset, resulting in population crashes. A population crash would create food shortages for birds and impact the commercial harvest of shrimp eggs.

But there are ecological reasons this has not occurred. The Great Salt Lake is not an easy place to live, and conditions are always changing. The ability of cysts to remain in diapause and wait out unfavorable conditions, and the fact that genetic diversity is maintained through sexual reproduction, gives GSL shrimp the best chance to survive. In a more stable environment, the ability to produce a great deal of offspring could allow populations to recover from infrequent disturbance without wasting resources in cyst production. Apparently, brine shrimp populations in the eastern hemisphere are distributed according to environmental conditions, with sexual varieties living in areas with poor habitat conditions and asexual varieties living in areas with stable, high quality habitat.