BARIMO, JF; WALSH, PJ; Univ of Miami, Miami; Univ of Miami, Miami: Ureotely in the gulf toadfish, Opsanus beta: A means of predator avoidance by chemical crypsis?

Studies of prey detection by olfaction in teleost fishes have focused largely on amino acids as odorants while studies with respect to nitrogenous waste appear few in comparison. Although threshold sensitivities for amino acids are often in the nanomolar range, the gill and renal membranes are thought to be less permeable to amino acids than lower molecular weight compounds such as ammonia or urea. Furthermore, amino acids are more likely to be conserved by fish for protein synthesis and occur as a minor constituent of excreta. In this study, we compare the detection of ammonia, urea or an ammonia/urea mix by the omnivorous predator Lutjanus griseus (gray snapper ). The gulf toadfish, Opsanus beta, is a preferred prey item of L. griseus. O. beta are unique among teleosts in that adults can facultatively shift between excretion of ammonia (ammonotely) and urea (ureotely). The shift to ureotely occurs by regulation of key ornithine-urea cycle enzymes which appears mediated in part by the stress hormone cortisol. Experiments were conducted in 8000 l outdoor mesocosms with flow-through seawater and a sediment/seagrass substrate in an effort to simulate natural seagrass habitat. Odorants were injected into small experimental shelters designed to mimic toadfish burrows which were also equipped with low-light video cameras to remotely monitor snapper behaviors. Preliminary results indicate that grey snapper are more attracted to ammonia than either urea or the ammonia/urea mix with threshold sensitivities well below 5 uM . The discussion is framed in the context of the adaptive significance of facultative ureotely in the toadfish.