INTRODUCTORY 17 



uniform layers of fluid gliding smoothly over one another, progression 

 nearly always involves some degree of eddy viscosity and turbulence as 

 well. A streamlined form produces less turbulence and drag, and the animal 

 accordingly encounters less resistance to its progress through the water. 



The density of sea water is correlated with salinity and temperature. At 

 atmospheric pressure and 0°C the specific gravity of sea water of salinity 

 35% is about 1-028. The specific gravity decreases with rise in temperature, 

 and is increased slightly by high pressures (Fig. 1.5). 



The cells and tissues of marine animals have nearly the same specific 

 gravity as sea water, which accordingly forms a circumambient medium 

 supporting their bodies. The density and viscosity of sea water are also 

 important statically in the flotation of planktonic organisms. To maintain 

 themselves in the surface waters, or at particular levels, the organisms 

 concerned must either be no heavier than the water, if they are quiescent, 

 or expend energy actively in order to counteract the pull of gravity and 

 maintain their position. There are relatively few animals that have an 

 overall specific gravity less than or equal to sea water. This is achieved 

 by siphonophores which have gas-filled floats (pneumatophores), such as 

 the Portuguese man-o'-war Physalia and the by-the-wind sailor Velella. 

 Those teleosts with swim-bladders are able to achieve the same result by 

 controlling the volume of gases in the air-bladder, which acts as a buoyancy 

 organ. The pelagic cephalopods Nautilus and Spirula have a chambered 

 shell containing air, and the planktonic snail Glaucus is said to contain 

 intestinal gases which fulfil a similar role. 



Other adaptations serve to lower the specific gravity of marine organ- 

 isms relative to sea water, and thus reduce the sinking factor. Pelagic 

 animals frequently have the skeleton reduced compared with benthic 

 forms, or have lost it altogether, e.g. the pelagic holothurian Pelagothuria, 

 heteropods, pteropods, the pelagic lamellibranch Planktomya, pelagic 

 crustaceans, cephalopods and fishes. A relative decrease in weight is also 

 achieved by incorporating large amounts of water in the body tissues. 

 This phenomenon is widespread in pelagic animals, many of which have 

 soft transparent tissues of a jelly-like consistency (coelenterates, pelagic 

 annelids, chaetognaths, pelagic cephalopods, salps and fish). 



Light. Light is rapidly absorbed when passing through the surface 

 waters of the sea and the intensity falls off with depth. Transmission of 

 light through sea water is of great biological importance from several 

 aspects. Since daylight provides the energy for photosynthetic activity it 

 is one factor regulating the growth of plants, upon which animals ulti- 

 mately depend for foodstuffs. The majority of animals are sensitive to 

 light which acts as an environmental stimulus. Some of the more complex 

 biological phenomena that are governed by changes in light intensity are 

 phototactic responses, the diurnal migrations of planktonic animals, the 

 incidence of reproductive activity, colour-responses and alterations in 

 pigment-density. The absence of daylight in the deeper waters of the ocean 

 has resulted in peculiar morphological and ethological specializations, and 



