Although most fish are exclusively ectothermic, there are exceptions. Certain species of fish maintain elevated body temperatures. Endothermic teleosts (bony fish) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus). All sharks in the family Lamnidae Ц shortfin mako, long fin mako, white, porbeagle, and salmon shark Ц are endothermic, 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. See also gigantothermy. Endothermy, though metabolically costly, is thought to provide advantages such as increased muscle strength, higher rates of central nervous system processing, and higher rates of digestion. An ectotherm, from the Greek ? (ektos) "outside" and (thermos) "hot", is an organism in which internal physiological sources of heat are of relatively small or quite negligible importance in controlling body temperature. Such organisms (for example frogs) rely on environmental heat sources, which permits them to operate at very economical metabolic rates. Some such animals live in environments in which temperatures are practically constant, as is typical of regions of the abyssal ocean. In contrast where temperature varies so widely as to limit the physiological activities of other kinds of ectotherms, many species habitually seek out external sources of heat or shelter from heat; for example, many reptiles regulate their body temperature by basking in the sun, or seeking shade when necessary. In contrast to ectotherms, endotherms rely largely, even predominantly, on heat from internal metabolic processes. In ectotherms, fluctuating ambient temperatures affec
the body temperature. Such variation in body temperature is called poikilothermy, though the concept is not widely satisfactory and the use of the term is declining. In small aquatic creatures such as Rotifera, the poikilothermy is practically absolute, but other creatures have wider physiological options at their disposal, and they can avoid ambient temperature changes, or moderate their effects. Various patterns of behavior enable certain ectotherms to regulate body temperature to a useful extent. To warm up, reptiles and amphibians find sunny places and adopt positions that maximise their exposure; at harmfully high temperatures they seek shade or cooler water. In cold weather, honey bees huddle together to retain heat. Butterflies and moths may orient their wings to maximize exposure to solar radiation in order to build up heat before takeoff. Many flying insects, such as honey bees and bumble bees, also raise their internal temperatures endothermally prior to flight, by vibrating their flight muscles without violent movement of the wings. Such endothermal activity is an example of the difficulty of consistent application of terms such as poikilothermy and homoiothermy. In addition to behavioral adaptations, physiological adaptations help ectotherms regulate temperature. Diving reptiles conserve heat by heat exchange mechanisms, whereby cold blood from the skin picks up heat from blood moving outward from the body core, re-using and thereby conserving some of the heat that otherwise would have been wasted. The skin of bullfrogs secretes more mucus when it is hot, allowing more cooling by evaporation. During periods of cold some ectotherms enter a state of torpor, in which their metabolism slows or, in some cases, such as the Wood Frog, effectively stops. The torpor might last overnight or last for a season, or even for years, depending on the species and circumstances.