NATURE ADRIFT 



the living plankton maintain its level or change it to suit a changing en- 

 vironment? 



The plants, as discussed in Chapter 3, are small, often with long spines, 

 so that their ratio of surface area to weight is very high. Some contain oil 

 globules which increase the buoyancy, whilst the very tiny nanoplankton 

 organisms are often motile and can actively swim towards the light. 



The animals are mostly much more able to swim and many do keep 

 their level by swimming upwards, and gradually falling back in a resting 

 phase before repeating the process, but many must be incessantly active. 

 Fish with swim-bladders adjust the amount of gas in them and so are able 

 to achieve a true balance; most tish without swim-bladders are deep-sea 

 fish with reduced skeletal parts and they are often fatty which reduces their 

 weight. Their body fluids are less salty than sea water and this, too, helps 

 to give them a positive buoyancy. Their muscles (which are chiefly of the 

 relatively heavy protein) are reduced and weak because the fish attract their 

 prey by lures instead of chasing. 



Cuttlefish possess a 'bone' (it is frequently given to canaries and other 

 caged birds to exercise their beaks on) which is very light and contains gas 

 spaces which make it buoyant thus reducing the weight of the 

 cuttlefish in water to almost nothing. One deep-sea squid achieves this 

 neutral buoyancy by having a very large cavity inside it equivalent to 

 two or three times the rest of the animal (Plate XV) and this cavity is 

 filled with a liquid lighter than the sea water. This is achieved by replacing 

 the sodium part of the salt of sea water by the ammonium ion, which is 

 naturally produced as a product of excretion. 



Some siphonophores produce a gas-tiUed float (Fig. 17; 2, 3 and 4). 

 Many copepods and other Crustacea store oil or fat, particularly in the 

 colder waters where the food is richer, but must rely on their own muscular 

 powers when food is less abundant. It is significant that warm water is 

 less dense than cold water and animals have therefore more work to do in 

 warm water to keep from sinking. One way of easing the difliculty is to 

 follow the lead of the diatoms and grow long feathery hairs which increase 

 the surface area and hence very considerably the resistance to sinking. 

 Copepods of the same species, for example, are often very much more feathery 

 in the tropics than in cold water, the hairs becoming litdc plumes (Fig. 38). 



By and large, however, the planktonic organisms keep to, or change, 

 their level by their own unceasing efforts. These may be momentarily 

 relaxed, but only to have the lapse made up by increased effort. There is 

 no real rest for them, and for so very many of them the bottom is two 

 miles or so below and in everlasting darkness, a most inhospitable environ- 

 ment for all except those specially adapted to hve there. 



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