CHAPTER 12 



Behaviour 



The previous chapter, and indeed the very name plankton, emphasizes 

 that these organisms drift where the currents take them, and suggest that 

 their own swimming powers are neghgible. This is true in the broad sense 

 but, although they cannot swim to stem a current or to migrate in the way 

 fish can, most zooplankton organisms arc capable of quite defmite movements 

 which can be classed as behaviour, and these need a chapter to themselves. 



First they swim to catch their food, vigorously if they are carnivorous 

 predators actively searching out their prey, less actively perhaps if they are 

 herbivores or detritus feeders swimming about to bring new pastures within 

 their range. We are so accustomed to the almost universal use of sight by 

 the familiar land animals in fmding their food, especially the carnivores, that 

 it is difficult to realize how little importance is attached to sight in aquatic 

 invertebrates. Certainly the vertebrate fish have efficient sight and so have 

 the cephalopods (the squids and octopuses, p. 62) whose eyes are very 

 reminiscent of mammalian eyes. Some Crustacea and their larvae have com- 

 pound eyes not unlike those of insects. Though it is certain that many fish 

 and the cephalopods use their eyes to see their food, many of the planktonic 

 Crustacea being filter feeders probably use theirs instead for seeing movement 

 rather than detail and so escaping from their enemies. Most planktonic animals 

 are quite blind or have eye-spots that are sensitive to light but not to details 

 of shape. 



Water, being almost incompressible compared with air, is a much 

 better medium for transmitting small pressure diffi:rences and one of the 

 most important senses of marine creatures is their highly developed sense 

 of 'touch'. It seems as though this enables them to perceive the vibrations 

 in the water made by the movements of other creatures, and perhaps also 

 the echoes they receive back from stones and other objects whose presence 

 they could then sense, and so take any necessary avoiding action. It is un- 

 canny to watch an arrow-worm chasing a copepod and realize that both are 

 sightless, and yet arrow-worms can successfully catch fast swimming young 

 fish (Plate XXXII), and a medusa can stretch out its extensible stomach to 

 grab a passing small fish. Blind fish can swim in aquarium tanks without 

 bumping into the walls. 



Smell, too, is a highly developed sense in marine animals, more efficient 

 than we humans can appreciate — the story of the eels finding the fresh 



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