Life in the Open Ocean. Joseph J. Torres

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Life in the Open Ocean - Joseph J. Torres


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utilize the heat generated by metabolism to maintain a constant body temperature. “Endo” is the Greek root for internal or inside; an endotherm’s body temperature results from heat generated within. Mammals and birds modulate the amount of metabolic heat lost to the environment (or gained from it) by a variety of mechanisms, including insulation (fur, feathers, blubber) and adjustment of blood flow to limit or facilitate heat exchange with the external environment. Endotherms, in layman’s terms, are “warm‐blooded.”

      The terms endothermy and ectothermy were created to precisely define how a species’ internal body temperature comes to be the way it is: by virtue of internally generated heat or by interaction with the external environment.

      Poikilotherm (from the Greek “poikilo” or “varied”) may be considered as the older version of ectotherm. A poikilotherm has a body temperature that changes with the external environment, so it is certainly an ectotherm. However, as we just discussed, ectotherms dwelling at a constant temperature are also homeotherms. So, using the old terminology, a poikilotherm could also be a homeotherm when living at constant temperature, which is confusing at best.

      Temperature Effects on Survival: The Tolerance Polygon

      It is important to note that the change in species composition at oceanic boundaries such as the Antarctic Polar Front is not due to the short‐term lethal effects of temperature change. Instead, a suite of factors is involved, including inefficiencies in reproductive strategies and timing, metabolic inefficiency, absence of preferred prey, and competition from similar species for resources that result in the gradual demise of the replaced species. However, characterizing a species’ tolerance to temperature is highly instructive because it introduces two basic rules of physiological response to temperature and to other environmental challenges like salinity. The first rule is that the short‐term range of temperature tolerance within a species, population, or individual is not rigid or immutable; animals can adjust their range of tolerance over a period of time in response to changes in external temperature. The second rule is that upper and lower limits exist for all species that cannot be exceeded, even after allowing for biological adjustment.

      The internal adjustment process that raises or lowers lethal limits takes time to accomplish and is described by two terms. When the adjustment phenomena take place in the natural habitat (e.g. seasonal temperature change), the process is called acclimatization. When adjustment is induced in the laboratory, the phenomenon is called acclimation.

Schematic illustration of thermal tolerance and lethal limits.

      Sources: (a) Adapted from Fry and Hochachka (1970), figure 2 (p. 81); (b) Brett (1952), figure 7 (p. 282).

      The polygon for Oncorhynchus keta indicates that it is a fairly eurythermal species. Polygons for Antarctic species would encompass only a small fraction of the lower range, whereas highly eurythermal species such as the brown bullhead catfish (Ameriurus nebulosus) would be very much larger.

      Studies of temperature tolerance in a variety of different organisms suggest the following.

      1 Generally, upper and lower lethal limits can be modified considerably by different acclimation temperatures, e.g. the warmer the temperature of acclimation, the higher the upper lethal limit.

      2 There are absolute upper and lower lethal limits beyond which an organism cannot adapt, and these limits can be determined with precision.

      3 It takes longer to acclimate to cold temperatures than to warm ones.

      4 The tolerance polygon of an organism relates well to habitat and geographic area, as shown in the example below.

pg41

      In addition to knowing the zones of tolerance or the limits to survival of a species, it is important to understand the physiological responses of an organism to temperatures within its environmental range.

      Temperature Effects on Rate Processes – The Q10 Approximation

      Animals have varying reactions to temperature changes within their zone of tolerance. Reaction to temperature within an animal’s environmental range is usually assessed using a rate function, heartbeat for example, or a filtration rate for species such as clams that pump water through their feeding apparatus. Most commonly, metabolic rate is used; metabolism is an excellent index of an animal’s rate of energy consumption and is readily measured by monitoring an individual’s rate of oxygen consumption. The rate of increase or decrease in reaction rate over a T°C change is standardized by the Q10 approximation, which is the factor by which a reaction velocity (e.g. rate of oxygen consumption) is increased for a rise of 10 °C.

      (2.1)equation

      where K1 and K2 are velocity constants corresponding to temperatures T1 and T2. Reaction velocity is generally used instead.

      For virtually all rate functions in which we are interested, the biological rate increases by a factor of approximately 2 for each 10 °C rise in temperature: that is, Q10 ≈ 2. However, the Q10 of an animal’s metabolic rate varies slightly over a range of temperatures,


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