Deep sea
Deep sea species exhibit lower changes of entropy and enthalpy compared to surface level organisms, since a high pressure and low temperature environment favors negative enthalpy changes and reduced dependence on entropy-driven reactions. From a structural standpoint, globular proteins of deep sea fish due to the tertiary structure of G-actin are relatively rigid compared to those of surface level fish. The fact that proteins in deep sea fish are structurally different from surface fish is apparent from the observation that actin from the muscle fibers of deep sea fish are extremely heat resistant; an observation similar to what is found in lizards. These proteins are structurally strengthened by modification of the bonds in the tertiary structure of the protein which also happens to induce high levels of thermal stability. Proteins are structurally strengthened to resist pressure by modification of bonds in the tertiary structure. Therefore, high levels of hydrostatic pressure, similar to high body temperatures of thermophilic desert reptiles, favor rigid protein structures.