16 THE BIOLOGY OF MARINE ANIMALS 



from warm and cold waters, which are either due to acclimatization or 

 have a genotypic basis. The widely distributed common solitary sea-squirt 

 Ciona intestinalis has been shown to comprise several distinct physiological 

 races which have different temperature optima for breeding. 



In addition, some species, which are found at the surface in colder seas, 

 extend across the equator, but in deeper and colder waters around 400 

 metres or more (19). 



Temperature is a major environmental factor determining the range of 

 a species, and many examples of this effect may be found among benthic 

 as well as pelagic animals. The reef-building madreporarians are unable to 

 tolerate temperatures below 20°C and are restricted to shallow inter- 

 tropical waters between latitudes 25°N. and 25°S. The restrictive effect of 

 temperature may be operative on somatic metabolism, or on reproductive 

 activity. Thus the native British oyster Ostrea edulis breeds when the 

 temperature rises above 15°C whereas the Portuguese oyster Crassostrea 

 angu/ata requires a sea temperature of 20°C before it will commence 

 spawning, and hence cannot reproduce itself in British waters where the 

 temperature in inshore waters does not exceed 16°C. 



The rate of metabolism is greatly increased in poikilothermic animals 

 by rise of temperature. According to van't Hoff's rule the increase in 

 metabolic rate for each 10°C rise in temperature (Qi ) is two- to threefold. 

 External temperatures, therefore, will profoundly affect all the vital 

 activities of the organism, development, growth, reproduction, digestion, 

 etc. In species inhabiting temperate and boreal waters, vital activities occur 

 at a lower level than in corresponding forms from tropical regions, and will 

 be further depressed during the colder winter months of the year. Similar 

 conditions may be expected in bathypelagic and bathybenthic animals 

 (vide Chapter 4). 



Viscosity and Density. The viscosity and density of sea water are proper- 

 ties of great biological significance in relation to movement and suspension 

 of marine organisms. The viscosity of sea water is slightly greater than that 

 of fresh water, and increases gradually with rise in salinity, and to a much 

 greater extent with fall in temperature. At a salinity of 35% , for example, 

 the increase of viscosity is almost twofold for a temperature drop from 

 25-0°C. 



The viscosity of sea water is high compared with that of air, and offers 

 much frictional resistance to the passage of bodies through it. Neverthe- 

 less, some active nektonic animals are able to swim at surprisingly high 

 speeds, e.g. whales, porpoises, penguins and scombrid fishes. We usually 

 find that active swimmers are streamlined in some manner as an adapta- 

 tion for securing higher locomotory efficiency. Streamlining is rather a 

 loose term, but it refers usually to the possession of a smooth tapering 

 shape with a minimal amount of projecting surface that could offer 

 resistance to progression. Some animals with large rounded heads and 

 tapering trunks are almost ideally streamlined, for example sperm whales. 

 Besides laminar viscosity, which is concerned with the movement of thin 



