Distributions of diploid and pentaploid brine shrimp Artemia parthenogenetica in an illuminated thermal gradient

We explored the behavioral response to horizontal gradients of light and heat of sympatric diploid and pentaploid Artemia parthenogenetica. When exposed to a uniformly illuminated thermal gradient, pentaploids had a mean distribution that was oriented toward temperatures 2.9 °C higher than that of diploid animals. In a light gradient, diploids were distributed at higher light intensities than pentaploids. When the distributions in an evenly illuminated thermal gradient were compared with those occurring when thermal and light gradients ran in opposite directions, it was found that diploids tended to occur at lower temperatures, whereas pentaploid distributions were not significantly different for the two conditions. Our findings suggest that pentaploids are not as strongly attracted to light as diploids, in either the presence or the absence of a thermal gradient. We predict that in the field, warmer areas of a pond may be exploited by pentaploids during the day and by diploids during the night.