Sharks - KK

Introduction
Most sharks (the exceptions are the mackerel sharks and thresher which are endotherms) are poikilothermic or, informally, "cold-blooded” meaning that they are unable to control their own body temperature through their metabolism, but that their body temperature is controlled by the surrounding water temperature. Therefore if the temperature around them drops, so does their body temperature.

Metabolism, digestions, strength and movement is improved as the body temperature increases. However, the higher the internal temperature, the more energy is spent maintaining it.

Thermoregulation in Sharks
Sharks are able to conserve energy by rising to the top of the ocean, where the water is warmer, so that their body temperature rises without much extra effort from their side. By doing this, digestion is aided, muscle activity is improved and metabolism is boosted Rising to the top is an example of behavioural thermoregulation, which refers to their choosing specific habitats or behaviours based solely on the water temperature. This may happen during the safer night hours to digest food faster using less energy. The sea’s temperature is not constant or all that predictable. Therefore, the sharks that are able to implement behavioural thermoregulation have an advantage over other poikilotherms that cannot.

Research
4 whale sharks "volunteered" to test some time/depth recorders for Michelle Thums of the University of Western Australia and her colleagues.

Vertical movement like this has not often been studied, despite its importance for aquatic animals. With negative buoyancy, most pelagic fish species swim towards the surface quite often, but elasmobranchs (eg. sharks and rays) spend long periods at the surface following dives. Rhincodon typus, the whale shark is the world's biggest fish and spends an average of 49% of its time at the surface between its diving stints (of over 1000 metres). Results supported the idea that thermoregulation took place by using warm surface waters as a heater for the body. Fish physiology demands these higher body temperatures for optimal performance. In warm waters, above 25°C, no relationship exited and it's perhaps necessary to get rid of excess heat at some temperatures by diving deep to cooler water