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BIO 3302 - Lectures 18-21.docx

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Lecture 18Slide 118There are a few species of aquatic animals that use urea as their nitrogen endpoint and its not because of environmental water availabilityThe example that we looked at in the last lecture was the Lake Maggoty Telacnea which is a small fish that lives in warm very high pH waterBecause of the high pH the acidtrapping mechanism for ammoniaexcretion doesnt work and they are there for obligate ureoteleThere are a couple more examples of these unusual ureotelic fishThe second group of fish that are ureotelic are fish that weve already encountered and these are the cartilaginous fish the elasmobranchs sharks skates and rays the chimaeras ratfish and the coelacanthThese are the fish that use urea as an osmolite These fish produce high levels of urea and retain urea as a strategy for maintaining water balance in ionic and osmotic strategiesThey are exceptions to the rule that aquatic animals are ammonotelicThey are obligate ureoteles they produce urea in the liver and have body tissue urea levels that would be toxic to most other animalsIts for the specific purpose of allowing them to be osmoconformersThe third example is the very unusual case of the Gulf ToadfishThey are marine fish that are found in the subtidal zoneThey are not exposed to air under normal condition They can be facultative ureoteles and they can excrete ammonia or ureaWhen they produce urea they do it through the ornithineurea cycle and they switch to producing urea under a couple of lab conditions 1 Confinementput a bunch in a small amount of water they will switch to urea or 2 Exposureif you expose them to water with high levels of ammonia they will switch to ureaIf you sample the fish directly from the environment it seems to be about 5050 urea excretion vs ammonia excretionAnd theyre fully aquatic so its not a case of environmental water availabilityNot only are they facultative ureoteles whats most interesting about them is the way they excrete ureaIf you put a toadfish in a box of water and monitor ammonia and urea levels what you find is that there is a little bit of ammonia leaking out over timeAmmonia levels are relatively lowWaterurea levels stay relatively low and then all of a sudden they shoot up not because the fish has emptied its bladderInstead the fish inserts a urea transporter into its gills and dumps urea into the surrounding water roughly once every 24 hoursThis is called pulsatile urea excretion and is an energetic costA researcher and student at uOttawa hypothesized that if the toadfish was sitting in its burrow producing ammonia a predator could follow the trail and feast on the toadfishSo they produce urea as a mechanism to hide from predatorsoIn the experiment they placed a tube burrow into a tank with a camera at one end to watch what happens and a tube where they can release different odorants to attract the predatorThey placed a fake toadfish in the burrow to fool the grey snapper predatorWatch the behaviour of the snapper not the toadfish In the control they injected seawater into the tubeIn the experimental trials they injected ammonia urea ammoniaurea mix and groundup toadfishWhen seawater was injected the snapper didnt reactWhen they injected groundup toadfish the snapper went wildWhen they injected ammonia the snapper was still attracted to the tubeWhen they injected urea the snapper was significantly less attracted than with ammonia When they injected a 5050 mix of AU the result was the same as with urea suggesting that it minimally attracts the predator The toadfish leave their tubes at dawn and dusk which may serve to be a time for them to excrete as much waste products as possibleUricAcid Excreting AnimalsUricacid is the most energetically expensive to produce and the most difficult to move across membranesIts produced in the liver and the synthesis pathways are complex amino acids feed into uric acid synthesis by being converted to glycine aspartate and glutamate and nucleic acids which have nitrogen do the same thing and through many steps produce uric acidOnce uric acid is synthesized in the liver it has to be excreted through the kidneys and specialised uric acid transporters move the uric acid from the liver to the blood and to the tubulesUric acid is a molecule in reptiles and birds that shows significant secretion so it can be filtered but can also be secreted into the nephron to eliminate it as a waste productThis seems to be driven largely by environmental water availabilityIf you look at an aquatic reptile you will often see ammonia production to cut down on energetic costsWhy do some terrestrial animals use uric acid and others use ureaNo clear answer but a lot of people point to the fact that what distinguishes reptiles and birds from amphibians and mammals is the embryonic environmentReptiles and birds lay eggs which have a shellAmphibian eggs lack a shellThe cleidoic egg shelled allows gases and water to move across the shell but whatever else is in the shell stays within the shell ie wasteSo the toxicity of the waste should be as low as possible to avoid damage to the developing embryoBy using uric acid it can accumulate in the egg without endangering the embryoAmphibians dont have shelled eggs and mammals develop in utero where there is waste removal through the placentaThermoregulationTemperature is a major determinant of the internal environmentReaction rate depends on temperature and so has a major influence on all aspects of life temp goes up reaction rate goes upThis is true in the individual reactions that take place in the complex series of reactions that occur in the chemistry of lifeWhen one reaction gets out of sync the rest get messed up and they can no longer support life The Q shows the fold difference in a reaction rate for a 10 C temperature change10Most reactions have a Q of 2 or 3 meaning that the reaction rate doublestriples if 10temperature rises 10 CEvery individual reaction has its own Q value10There is a broad range of temperatures that animals can tolerate 1 C to 50 C Much below animals start to freeze much above proteins denatureAlthough there is a wide range of possible temperatures most animals have a preferred range of temperatures that is quite a bit narrowerIt is also typically matched to the environment they live inIn fish species they have lethal temperaturesFor an Antarctic fish the lethal upper temperature is 6 CFor Rainbow Trout a temperate fish the upward lethal temperature
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