The ontogeny of light-dark response in Triops longicaudatus as a response to changing selective pressures

The branchiopod shrilnp. Triops longicaudatus, inhabits ephemeral pools, an d individuals must develop rapidly if breeding is to occur. Age-related dif ferences in physiology, foraging behavior acid predation pressures should m ake it more adaptive fur young individuals to dwell near the surface, and t he adults on the bottom. Consequently, young T. longicaudatus should be pho topositive whereas older individuals should be photonegative. We tested lig ht-dark preferences in the first five instars of lab-reared T. longicaudatu s. Photonegativity increased with age, with instar 2 preferring lighted env ironments significantly more than instars 4 and 5. The first two instars we re more active under dark conditions than under light conditions, suggestin g an orthokinetic mechanism for habitat segregation. These changes may be c orrelated with the development of the compound eyes.