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Physiology
The Ninespine Stickleback is a euryhaline and eurythermal species of teleost fish, occupying both freshwater and marine habitats in higher latitudes of the world. Recently, this species has been under great examination due to pond populations' adaptations of morphology, life history, and behavior which separates them from their marine conspecifics. Many recently isolated populations (<11,000 years) have been studied to identify ranges of phenotypic plasticity and possible genetic drift between populations. It has been found that marine populations exhibit metabolic rate reduction when in colder conditions, in addition to having an elevated hepatosomatic index (HSI) at all temperature conditions as compared to freshwater populations in those same conditions. Hepatosomatic index refers to the proportional size of the liver of the fish to its body mass, as in indication of energy reserves. In addition, certain freshwater pond populations have shown to be able to metabolically compensate to survive a wide range of temperatures, six degrees Celsius to nineteen degrees Celsius. Alterations to metabolic rate, such as decrease in standard metabolic rate, allowed pond population individuals to survive less than favorable conditions. Such physiological differences between populations has not been determined to be exclusively due to plasticity or generic changes; however, there has been found to be significant genetic diversity between marine and freshwater populations.

Due to their climatic versatility, Ninespine Stickleback face much variation in dissolved oxygen in their environment. Ninespines are physoclists, so they lack a connection between their swim bladder and their digestive tract. Responses to hypoxia may include increased ventilation rate or slower gill perfusion. This means they cannot gulp air from the surface of the water, as physostome fish do, in order to compensate for an environment of low dissolved oxygen. Ninespines must rely solely on their gas secretion and absorption abilities, using aquatic surface respiration (ASR) when facing a hypoxic situation. ASR is when the fish rise almost to the surface to ventilate with the top millimeter of water which is high in dissolved oxygen. Ninespine Stickleback do not regularly experience nocturnal hypoxia in their natural environments, and therefore, have slower and decreased control of their swim-bladder lift, beginning ASR at a much higher dissolves oxygen concentration, in hypoxic environments as compared to other stickleback species, such as the Brook Stickleback.