In chemistry, an endothermic reaction is one that requires heat to break the bonds of the reactants. In other words, the reaction absorbs heat, causing its surroundings to get colder. Expressed in a chemical equation:
Reactants + Energy → Products
When using the calorimeter, the change in heat of the calorimeter is equal to the opposite of the change in heat of the system. This means that when the solution in which the reaction is taking place loses heat, the reaction is endothermic.
All sharks in the family Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are known to have the capacity for endothermy, and evidence suggests the trait exists in family Alopiidae (thresher sharks).
The degree of endothermy varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks who maintain body temperatures elevated in excess of 20 °C above ambient water temperatures.
Endothermy, though metabolically costly, is thought to provide advantages such as increased contractile force of muscles, higher rates of central nervous system processing, and higher rates of digestion.
Endothermy, the ability to warm the body through metabolic activity, is absent in most fish, but is present in two families of sharks and three families of bony fish.
The authors compare the distribution of endothermy with a molecular phylogeny based on the DNA sequence of the mitochondrial gene for cytochrome b.
Their comparison indicates that endothermy occurs independently in each of the three groups of fish, and is not inherited from a common ancestor.
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