39.4 Thursday, Jan. 5 Hunger-mediated phototaxis in adult brine shrimp HARVEY, A.*; WHITFORD, G.; DELORENZO, S.; Georgia Southern Univ., Statesboro; GSU, Statesboro; GSU, Statesboro email@example.com
Nauplii of brine shrimp (Artemia spp.) are known to be strongly attracted to light, but information about the photoresponse of adults is strikingly inconsistent. For example, Aiken and Hailman (1978) found that adults were strongly photopositive, whereas Bradley and Forward (1985) reported equally strong negative phototaxis. The design of their experiments differed in numerous ways, but we found that the contradictory outcomes were not artifacts of these differences: individual adults that were photopositive in the Bradley design were also photopositive in the Aiken design; likewise, photonegative adults were consistently so in both setups. Over 90% of our adults were negatively phototactic, as found by Bradley and Forward. However, the two earlier studies differed not only in their experimental apparatus, but also in the source of their animals: Aiken and Hailman bought theirs from a pet shop (pet shops do not feed adult brine shrimp), whereas Bradley and Forward reared theirs from cysts (which requires regular feeding). Thus, we tested the hypothesis that hungry adult brine shrimp are more attracted to light than are well-fed adults, and found this in fact to be the case: only 14% of individuals from a well-fed stock tank showed a positive photoresponse, but this proportion gradually rose to 60% by the fourth day without food. These results provide a likely resolution for the contradictory results of previous studies: Aiken’s photopositive shrimp were hungry, whereas Bradley’s photonegative shrimp were not. This hunger-mediated phototaxis may be explained by the ecology of brine shrimp in the wild: light levels are highest at the surface of the water, where both phytoplankton (food for hungry brine shrimp) and birds (primary predators of brine shrimp) are in greater abundance.