Except for sea birds and mammals, marine organisms are poikilothermic, i.e. their body temperature is always close to that of the surrounding water and varies accordingly. In the coldest parts of the sea, where temperatures are close to —2.0oC, the blood of fishes would be below freezing point but for the antifreeze action of relatively high concentrations of glycoprotein in the plasma. However, some large and active forms have body temperatures higher than water temperature due to the release of heat by metabolism. In fast-swimming sharks and tuna the temperature in the swimming muscles is sometimes 10oC above water temperature.
The physiological effects of temperature change are complex but, in simple terms, rates of metabolic processes increase with rising temperature, usually about 10 per cent per IOC rise over a range of temperatures up to a maximum (Figure 4.5) beyond which they fall off rapidly. Death occurs above and below certain limiting temperatures, probably because of disturbances of enzyme activity, water balance and other aspects of cellular chemistry. Marine creatures usually succumb more rapidly to overheating than to overcooling. The limits of distribution of a species in the sea do not coincide closely with the normal occurrences of rapidly lethal temperatures but are much more restricted. In freak climatic conditions, extremes of heat or cold may have rapid and devastating effects on marine populations, especially those of the shore, but in normal circumstances temperature probably controls distribution in subtle and gradual ways through its influence on several major processes including feeding, respiration, osmoregulation, growth and reproduction, especially the latter.
Temperature regulates reproduction in several ways. It controls the maturation of gonads and the release of sperms and ova, and in many cases the temperature
tolerance of embryonic and larval stages is less than that of the adults. Temperature has therefore a major influence on the breeding range and period, and on mortality rates during early stages of development and larval life. Along the fringes of distribution there are usually non-breeding zones where the adults can survive but cannot reproduce, the population being maintained by spread from the main area of distribution within which breeding is possible. Several Lusitanian species which are quite common along south and west coasts of the British Isles, e.g. the crawfish Palinurus vulgaris, probably seldom if ever breed in these waters. They are carried into the area from the south as larvae which successfully metamorphose and complete their development here.
In temperate seas many species virtually cease feeding during the winter. In some cases reduced feeding is simply the result of shortage of food, but many creatures definitely stop eating below a certain temperature. Food requirements are reduced during cold periods because the respiration rate is low and growth ceases. These interruptions of growth may produce periodic markings in growing structures; for instance, the annual winter rings on fish scales (see page 363).
Despite the depressing effects of cold on growth, it is nevertheless generally observed that where the distribution of a marine species covers a wide range of temperature the individuals living in colder areas attain larger adult sizes than those in the warmer parts of the distribution. This trend is associated with a longer growing period, later sexual maturation and a longer life in cold water. There are exceptions to this trend, and some species reach larger sizes in warmer water e.g. the gastropod Urosalpinx cinerea and the sea urchin Echinus esculentus.
Apart from direct physiological effects, changes of temperature have certain indirect effects by altering some of the physical properties of the water, notably density, viscosity and the solubility of gases, which in turn influence buoyancy, locomotion and respiration. There are instances e.g. the summer and winter forms of the diatom Rhizosolenia hebetata (Figure 4.6), where the morphology of a species appears to vary with changes of temperature, possibly because of alterations in viscosity and buoyancy. The viscosity of water falls considerably with increasing temperature, which may partly account for the increased setation of the appendages of many warm-water planktonts as compared with cold-water forms. The greater surface area of finely divided appendages increases their floating ability.
Figure 4.6 Rhizosolenia hebetata in (a) summer and (b) winter forms.
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