| Popis: |
Daphnia pulex de Geer and Daphnia magna Straus populations cannot be maintained in defined (sensu stricto) media containing less than 0.1 part per billion (ppb) of selenium. A concentration of 1 ppb is sufficient to satisfy minimal needs in otherwise sufficient media. In the first generation with no selenium added to the medium or detected in it, media deficiency is shown by a premature cuticle deterioration visually similar to senescence, by progressive loss of distal segments of second antennae (primary swimming appendages), and by a shortened lifespan. No progeny attain reproductive maturity in the second generation. Although experimental animals in prime condition exhibit a shortened lifespan in the first generation maintained at 0.5 ppb selenium, culture lines can be maintained at 0.5 ppb for indefinite numbers of generations if established as young orthoclones . Tests in organic-rich media indicate a significant sparing effect of organic additions. This selenium requirement is reminiscent of that for the stability of feathers in domestic fowl. |
