Essay on Factors or Variables Affecting Organism Response to Temperature !
The response of an organism to temperature is affected by number of factors such as thermal history, genetic difference, diet size, stage in life cycle, sex, moulting, parasitism, hormones, etc.
1. Generally, animals found in warmer environments can withstand higher temperatures than animals from colder situations. It is also generally true that animal from colder environments tolerate lower temperatures better than those from the warmer climates. These differences between warm-climate and cold-climate populations may reflect basic genetic differences or may be phenotypic expressions resulting from different thermal histories.
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2. The thermal resistance of a species may be closely identified with the genetic composition of the parent stock, as illustrated by hybrid development in two anuran species, Bufo valliceps and B. luetkeni (Ballinger and Mc Kinney, 1966). The lower lethal temperature for development of B. valliceps is 18°C; for B. luetkeni, 22°C. In their hybrids, the lower lethal limit is found to be in between that of the two parents at 195C.
3. In some animals a relationship between the total intake of food and resistance to thermal stress has been demonstrated. Some species are more sensitive to elevated temperatures when they are starved for even short periods of time. Goldfish showed increased resistance to high temperature when placed on a high fat diet (Hoar and Cottle, 1952).
4. The size of animal body is found to have some correlation with thermal lethal limits of animals. In some species, the smaller animals are more resistant to higher temperatures than the larger ones; in some species the reverse is noted, and in other species size is not an apparent variable. Further, some smaller animals die faster at low temperatures than do larger animals of the same species, but at higher temperatures body size is not a factor.
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5. Many species of animals have exceedingly complex life cycles during which the larval stages are not only morphologically dissimilar from the adult but also occupy different ecological niches. In the wharf crab, Sesarma cinereum, thermal requirements of the planktonic zoeal stages are different from those of adults because these larvae are limited to a smallar temperature range than are the adults.
6. The female sex is found to be more tolerant than males to the temperature fluctuations.
7. Among certain invertebrates, such as, in some crabs moulting adversely affected heat resistance, but had no effect on other species.
8. Mud-flat snails, Nassarius obsoleta, which are heavily infected with trematode larvae (i.e., parasites) cannot withstand high temperatures like the non-parasitized snails.
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9. Hormones have been shown to influence cold and warm resistance as well as acclimation (viz., temperature adaptation) to temperature, especially in mammals and a few other invertebrates.
10. Acclimatization (see section 11-4-7) of the fish Tilapia zilli to high temperature of 30°C leads to higher values of haematological parameters (viz., R.B.C. count. W.B.C. count, Hb concentration and Hacmatocrit, etc.), while low temperature causes a depression of these values (Farghaly et al., 1973) Similar changes in blood parameters due to temperature have been observed in many other fishes such as Heteropneustes Fossilis (Pandey, 1977). Salmo gairdneri (De wilde and Houston, 1967), Abramis brama and Lucioperca lucioperca (Molnar et al., 1960).
