The adaptive significance of ureotelism in the gulf toadfish, Opsanus beta

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Marine Biology and Fisheries

First Committee Member

Patrick J. Walsh - Committee Chair


The gulf toadfish Opsanus beta (Batrachoididae) is one of the few teleosts that maintains a functional ornithine-urea cycle during adult life and possesses the capability of facultatively shifting from ammonotely to ureotely within 24 h under laboratory conditions. Three hypotheses were examined with respect to the adaptive significance of ureotely in O. beta: environmental loading of ammonia, nest maintenance and chemical crypsis.Ureotely was first studied in response to micro-habitat variations in environmental ammonia and other environmental variables using glutamine synthetase (GSase) activity as a proxy for ureogenesis in order to examine the ammonia loading hypothesis. Extremely weak, but significant correlations emerged between environmental ammonia (NH3) concentrations and hepatic GSase activity from both burrowing toadfish (p = 0.005, r2 value = 0.073) and nesting toadfish (p < 0.001, r2 value = 0.076). Mean urea and TAmm concentrations found in shelters occupied by toadfish (n = 281) were 9.8 +/- 0.83 and 13.0 +/- 0.7 mumol-N·l -1, respectively.The mechanism of ureogenesis and patterns of urea excretion across early-life history stages of O. beta were investigated to examine the nest-fouling hypothesis. Juveniles were the most sensitive developmental stage with an TAmm 96-h LC50 value was 875 mumol-N·l -1 which was over an order of magnitude above values found within nests. Furthermore, 40 d exposures at mean and maximum NH3 concentrations occurring within nests revealed no observable detrimental effects and, in fact, growth in terms of wet or dry weight was greatest in the 300 mumol-N·l -1 TAmm treatment.Chemoreception experiments with the toadfish predator Lutjanus griseus were performed to determine behavioral responses toward ammonia and urea in an examination of the chemical crypsis hypothesis. Our results indicate that L. griseus was more responsive to ammonia than either urea or an ammonia/urea mix with threshold sensitivities <5 mumol-N·l -1. Additionally, L. griseus was more responsive to an amino acid-ammonia mix than either an amino acid-urea mix or amino acids without waste-N. These results suggest urea masks or camouflages ammonia odors, but not those of amino acids. Thus, chemical crypsis may confer a selective advantage to ureotelic toadfish because they are less likely to be chemically detected by teleost predators.


Biology, Ecology; Biology, Animal Physiology; Agriculture, Fisheries and Aquaculture

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