Abstract
Gulf toadfish (Opsanus beta) are well studied for their ability to produce and excrete urea as an alternate form of nitrogenous waste. Urea is only produced by a few adult teleost species in substantial quantities because its production is energetically costly compared to ammonia. Toadfish uniquely excrete urea in distinct pulses and the process is controlled physiologically by the stress hormone cortisol and the neurotransmitter serotonin (5-HT). Decades of research have ruled out several explanations for the adaptive advantage of this pulsatile urea excretion and the remaining hypothesis is that signals are release via urea pulses for chemical communication. However, the use of pheromones for communication is not well understood in marine teleosts as most studies focus on chemical signals released in freshwater fish and crustacean urine. Therefore, the main objectives of this dissertation are to further characterize the neuroendocrine control of pulsatile urea excretion and determine whether toadfish utilize urea pulses across the gill to releases signals that communicate reproductive and social status. Using physiological and behavioral experimental approaches, the present studies determined that the 5-HT that triggers urea pulses is likely coming from circulation rather from local release within the gill because plasma 5-HT decreases over the course of a urea pulse and then increases after the pulse. Additionally, we confirmed that pulsatile urea excretion is not under nervous control through the branchial branches of the glossopharyngeal and vagus cranial nerves. These nerves are also not responsible for local release of 5-HT from gill storage sites as nerve-sectioning did not affect abundance of gill 5-HT and its metabolite. Ours was the first study to determine that plasma 5-HT fluctuates seasonally in teleosts. We suggest that higher circulating 5-HT measured during breeding season, that was strongly correlated with toadfish gonadosomatic index (GSI), may be responsible for consistently elevated pulsatile urea excretion during breeding season. Toadfish respond with a reduced pulse latency, a greater pulse frequency, and an increase in swimming activity when presented with chemical cues of a conspecific of the opposite sex during breeding season, suggesting that they may be releasing chemical signals via urea pulses to communicate sex and reproductive status. However, perhaps more convincingly, when olfaction was experimentally blocked in pairs of male toadfish, pulsatile urea excretion was inhibited in anosmic males compared to controls and behavior in subordinate anosmic males was altered. These changes in behavior and urea excretion in response to anosmia suggest that the transfer of chemical signals plays a role in establishing social hierarchies. Additionally, peaks in agonistic behavior occurred around the time of urea pulses in the majority of toadfish pairs, suggesting coordination of urea pulses and behavior. This dissertation significantly advanced our understanding of pulsatile urea excretion by providing physiological and behavioral evidence that it is involved in the communication of reproductive and social status. Furthermore, because it has already been found that circulating 5-HT plays a role in teleost reproduction and aggressive behavior, the newly determined importance of circulating 5-HT in the control of pulsatile urea excretion and its seasonality provides an essential link between communication of reproductive and social status and pulsatile urea excretion.