Coping with temperature fluctuation
Most fishes are ectothermic, so their body temperature reflects that of the surrounding environment. Fishes that experience changing environmental temperatures, such as those characteristic of diel or seasonal changes, have several cellular and subcellular mechanisms for adapting to the newest of conditions. Many physiological adjustments are the result of switching on or off genes that are responsible for the manufacture of particular proteins. For example, acute heat stress initiates the synthesis of stress proteins, also known as heat shock proteins or HSPs, which maintain the structural integrity of proteins that otherwise would become denatured at higher temperatures, thereby allowing them to function biochemically.
To compensate for the decreased rate of biochemical reactions at low temperatures, fishes may increase the concentration of intracellular enzymes by altering the rate of enzyme synthesis, degradation, or both. Increased cytochromec concentration in Green Sunfish (Centrarchidae)that were moved from 25 to 5°C is due to a greater reduction in the degradation rate than in the rate of synthesis(Sidell 1977).
In some fishes alternative enzymes (termed isozymes)may be produced to catalyze the same reaction more efficiently at different temperatures. Isozymes are regulated by switching on or off the different genes that control their production. Rainbow Trout (Salmonidae) acclimated to 2 versus 18°C exhibit different forms of acetylcholinesterase,an enzyme important to proper nerve function because it breaks down the neurotransmitter acetylcholine(Hochachka & Somero 1984). The ability of Long jaw Mud suckers (Gobiidae) to tolerate rather wide ranges of temperature is probably due to the fish’s ability to regulate the ratio of isozymes of cytosolic malate dehydrogenase, an important enzyme in the Kreb’s cycle (Lin & Somero1995).
Polyploid species have extra sets of chromosomes (see Polyploidization and evolution), and may havea better capacity to cope with a wide range of temperatures; perhaps the multiple copies of genes provide more opportunitiesfor evolution to bring about changes in alleles that may prove to be beneficial. For example, among cyprinids, Goldfish and Common Carp are both polyploid and can tolerate a wide range of temperatures, and the polyploidy Barbel can acclimate better to different temperatures than can the diploid Tinfoil Barb (O’Steen & Bennett 2003).
Laboratory acclimation studies, in which a single variable such as temperature is altered while other factors are controlled and remain constant, can be helpful in understanding how fishes respond to a change in a single variable. However, in their natural habitats, fishes usually acclimatize to simultaneous changes in several variables, such as temperature, photoperiod, and perhaps reproductive condition as seasons change. The absence of natural seasonal cues, such as changing photoperiod, may cause an artificiallyacclimated fish to respond somewhat differently than one that has been naturally acclimatized. For example, laboratory acclimated fishes typically have higher metabolic rates at higher temperatures (see Metabolic rate), yet seasonal reproductive cycles cause naturally acclimatized sunfish (Centrarchidae) to have higher metabolic rates in spring than in summer (Roberts 1964; Burns1975). Other studies also have shown seasonal changes in metabolic rate that were independent of temperature in trout (Salmonidae; Dickson & Kramer 1971), two minnows (Cyprinidae; Facey & Grossman 1990), sunfish(Evans 1984), and sculpin (Cottidae; Facey & Grossman1990).
Some fishes exhibit allozymes, alternative forms of the same enzyme that are controlled by different alleles ofthe same gene. Different populations of the species may exhibit higher or lower frequencies of the appropriate alleles depending on their geographic location. Livers ofMummichog (Cyprinodontidae) along the east coast of the United States exhibit two allozymes of lactate dehydrogenase, an important enzyme in carbohydrate metabolism. In Maine, the frequency of the allele for the form more effective at colder temperatures is nearly 100%, and the frequency decreases progressively in populations further to the south (Place & Powers 1979). In Florida, the alternative allele, which codes for the form more effective at higher temperatures, has a frequency approaching 100%.
Acclimation to cold temperatures includes modification sat the cellular and tissue level as well. Fishes, as well asother organisms, can alter the ratio of saturated and unsaturated fatty acids in their cell membranes to maintain uniformity in membrane consistency (Crockett & Londraville2006). The proportion of unsaturated fatty acids, which are more fluid at colder temperatures (e.g., compare vegetable oil and butter at low temperature), increases in those species that are active during winter. Some fishes also decrease cholesterol levels in cell membranes to increase fluidity at lower temperatures. Fishes that live in very cold habitats, such as polar seas (see Polar regions), often show cellular-level metabolic adaptations such as enzymes that function well at low temperatures and more mitochondria in their swimming muscles (Crockett & Londraville2006). Therefore, they can function better at lower temperatures than would a nonpolar fish acclimated to very low temperature.
Decreased muscle performance at low temperatures can be compensated for at several levels of muscle function.
Acclimation of Striped Bass (Moronidae) to low temperatures results in a substantial increase in the percent of red muscle cell volume occupied by mitochondria (Egging ton& Sidell 1989), and an overall increase in the proportion of the trunk musculature occupied by red fibers (Jones &Sidell 1982); both of these adaptations would increase the aerobic capability of the fish. Muscle fibers of Goldfish(Cyprinidae) show an increased area of sarcoplasmic reticulum at lower temperatures (Penney &Golds pink 1980),which would make available more calcium ions to help activate more proteins needed for contraction.
At colder temperatures fishes may utilize more muscle fibers to swim at a particular speed than they use at warmertemperatures (Sidell &Moreland 1989). Because lower temperatures require the recruitment of more muscle fi bersto sustain a given speed than is necessary at higher temperatures, maximum sustainable swimming velocities are lower at low temperatures (Rome 1990).
Temperature changes may affect ion exchange at the gills in a few different ways (Crockett & Londraville 2006).
Higher temperatures typically increase molecular activity, causing increases in diffusion rates. Changes in membrane fluidity due to changes in the saturation of fatty acids or concentration of cholesterol, as discussed earlier, can also affect membrane permeability – less fluid membranes tend to be more permeable. Freshwater fishes often show increased activity of Na-K adenosinetriphosphatase(ATPase) at lower temperatures, whereas marine fishes show increased Na-K ATPase activity at higher temperatures. Both trends suggest increased metabolic activity to maintain osmotic balance as temperature changes.
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