Metabolism, homeostasis and growth

Metabolism consists of the sum of anabolism (construction) and catabolism (destruction) with the release of energy, and achieving a fairly constant internal environment (homeostasis). The aquatic external environment favours differences from mammalian pathways of excretion and requires osmoregulatory adjustments for fresh water and seawater though some taxa, notably marine elasmobranchs, avoid osmoregulatory problems by retaining osmotically active substances such as urea, and molecules protecting tissues from urea damage. Ion regulation may occur through chloride cells of the gills. Most fish are not temperature regulators but a few are regional heterotherms, conserving heat internally. The liver has many roles in metabolism, including in some fish the synthesis of antifreeze seasonally. Maturing females synthesize yolk proteins in the liver. Energy storage may include the liver and, surprisingly, white muscle. Fish growth can be indeterminate and highly variable, with very short (annual) life cycles or extremely long cycles with late and/or intermittent reproduction.