There are two extremes of temperature regulation in organisms. Homeotherms are organisms that regulate body temperature to a constant level, usually above that of the ambient (surrounding) environment. A constant and relatively high body temperature enables biochemical reactions to occur in a relatively constant internal environment and at a relatively high rate. Most birds have a body temperature of about 40℃, whereas the temperature of most marine mammals is about 38℃. Because such temperatures are much higher than that of most seawater, marine homeotherms lose heat rapidly to the surrounding environment.

There is another completely different style of living. Poikilotherms are organisms whose body temperature conforms to that of the ambient environment. All subtidal marine invertebrates and most fishes fit into this category. There is an interesting intermediate status in which body temperature is usually somewhat higher than ambient temperature. Strong-swimming fishes, such as skipjack tuna and yellowfin tuna, have this intermediate status. Their rise in temperature above ambient conditions stems from metabolic heat generated by muscular activity (swimming) combined with a heat retention mechanism. The temperature rise is probably necessary to generate the increased biochemical reaction rates that are needed for sustained activity. In contrast, some intertidal animals are not true Poikilotherms, they maintain themselves at lower-than-ambient body temperature, using both evaporation and circulation of body fluids to avoid being heated at low tide by the Sun. Their body temperatures, therefore, differ from that of an inanimate object of the same size and shape that might be placed on the shore. Intertidal organisms absorb and lose heat directly to the air. Darker-colored forms can absorb more heat than can light-colored forms, therefore, variation in color can reflect differences in adaptation to the capture of solar energy at different latitudes.

Ocean temperatures are usually less than 27℃ and may be less than 0℃ in some locations and during some seasons. Therefore, most homeothermic mammals and birds must lose heat continuously to the environment. Their skin is the main pathway of heat loss, especially by direct conduct of heat from the skin to the contacting colder water. Because animals have a circulatory system, heat loss from the body surface also occurs as warm interior blood is transferred and moves into contact with the periphery of the body. Their bodies also radiate heat, usually in the infrared part of the spectrum. Finally, as animals exhale, the resulting evaporation of water involves a considerable loss of heat.

The first line of defense against heat loss is a well-insulated body surface. Marine birds deal with this problem by means of specially adapted feathers. A series of interlocking contour feathers encloses a thick layer of down feathers that traps stationary air, which in turn acts as an insulating layer. Whales, porpoises, and seals are insulated against the lower sea temperatures by a thick layer of subcutaneous fat. Sea otters lack such a layer, but they constantly preen and fluff up a relatively thick layer of fur. Such mechanisms are only partly successful, however, and to generate more body heat to maintain a constant temperature, marine mammals usually must have a higher metabolic rate than similarly sized terrestrial (land) animals.

In marine mammals that have limbs, the limbs are the principal sources of heat loss because they expose a relatively greater amount of body surface area per unit volume to cold water. However, warm arterial blood must be supplied to limbs, such as the flipper of a porpoise. Heat loss in porpoises is minimized by a countercurrent heat exchanger. The arteries are surrounded by veins, within which blood is returning to the core of the animal. At any contact point, the artery, which is on the inside, is warmer than a surrounding vein, so heat is lost to the returning venous blood flow. Heat is thus reabsorbed and returned to the porpoise’s body core. This spatial relationship of circulatory vessels minimizes heat loss to the flipper and thence to the water. Although the anatomical details are quite different, fishes such as skipjack tuna have a circulatory anatomy based on the same overall design. Arteries and veins in the near-surface musculature are in contact, and in arteries and veins, respectively, blood flows in opposite directions.

生物体的温度调节存在着两个极端。温血动物是可以将体温调节至恒定水平的生物体，其温度通常高于周围环境的温度。恒定且相对高的体温使得生化反应能够在气温相对恒定的内部环境中发生，且发生的速率相对较高。大多数鸟类体温约为40℃，而大多数海洋哺乳动物的体温约为38℃。由于这些动物的体温比大部分海水的温度高得多，因此温血动物会迅速地向周围环境散热。 还有另一种截然不同的生活方式。 变温动物是体温与周围环境一致的生物体。所有亚潮带的海洋无脊椎动物和大部分鱼类都属于变温动物。有一个有趣的中间状态，即体温通常略高于周围环境的温度。长时间游泳的鱼类，如鲣鱼和黄鳍金枪鱼都具有这种中间状态。由肌肉活动（游泳）所产生的代谢热量以及保温机理，使得这些鱼类会将体温保持在高于周围环境温度的水平上。温度升高可能是产生持续活动所需的提高生化反应速率所必需的。相比之下，一些潮间带的动物并不是真正的变温动物，它们将自己的体温维持在低于周围环境的温度下，同时运用体液的蒸发和循环，以避免太阳落山时自己的体温升高。因此，它们的体温不同于可能放置在岸上的相同尺寸和形状的无生命物体的体温。潮间带生物直接从空气中吸收和散发热量。深色形态可以比浅色形态吸收的热量多，因此，颜色的变化可以反映出在不同纬度的地区吸收太阳热量的差异。 海洋温度通常低于27℃，并且在某些地点和某些季节可能会低于0℃。因此，大多数温血哺乳动物和鸟类必须不断向环境中散发热量。他们的皮肤是散发热量的主要途径，特别是通过让皮肤直接接触较冷的水。由于动物具有循环系统，随着温暖的内部血液转移并移动到身体的外围，热量将会从体表散发。通常，在光谱的红外部分他们的身体也可以散发热量。最后，在动物呼气时，由此产生的水蒸发会造成大量的热损耗。 阻止热损耗的第一道防线是隔热的体表。海鸟通过改变羽毛的状态来达到隔热的效果。一层厚厚的羽毛包裹着那些互相交错的廓羽，用以吸收静气，而静气反过来又起到了隔热的作用。鲸鱼，海豚和海豹有一层厚厚的皮下脂肪，使得它们可以抵御较低的海水温度。海獭缺乏这样的一层皮下脂肪，但它们不断地整理、抖动它们较厚的软毛。但是，这种方法只能一定的作用，海洋哺乳动物为了产生让身体产生更多的热量，将体温保持在一个恒定的温度，通常会比同等大小的陆地动物的代谢率高。 在拥有四肢的海洋哺乳动物中，四肢是造成热损耗的主要原因，因为四肢使得动物的每单位体积的体表面积相对较多地暴露于冷水中。然而，必须将温暖的动脉血供应给四肢，例如供应给海豚的鳍肢。通过逆流换热器，可以让海豚的热损耗降到最低。动脉周围是静脉，静脉中的血液可以回流到动物的核心部位。动脉内侧的任何接触点比周围的静脉温度高，因此丧失的热量会流回到静脉血流中。因此，可以重新吸收热量，并将热量返回到海豚的身体核心。循环血管的这种空间关系可以最大限度地减少鳍肢和周围水域的热损耗。虽然像鲣鱼这样的鱼类在结构上有很大差别，但是基于相同整体构造，它们拥有一个循环解剖结构。近表面肌肉组织中的动脉和静脉是相互联系的，动脉和静脉中的血液分别向相反的方向流动。