CHARACTERISTICS AND HABITS OF FISHES 27 



and ultimately they increase considerably in size. All 

 edible shellfish need systematic care and attention. 

 Their cultivation by man affords the simplest instance 

 of an attempt at a systematic aquiculture. 



Food. The surface water of the sea abounds in 

 minute forms of vegetable and animal life. This vast 

 floating population of microscopic organisms is called 

 the " plankton." Just as man and all land animals 

 depend ultimately for their food supply upon grass and 

 other green-leaved plants which, under the influence of 

 sunlight, are able to transform the inorganic constituents 

 of the atmosphere and the soil into organic foodstuffs 

 albumen, fat, carbohydrates so the minute unicellular 

 marine plants of the plankton are able, under the 

 influence of sunlight, to convert the inorganic con- 

 stituents of their environment into fat, albumen and 

 carbohydrate. Upon these minute organisms, therefore, 

 directly or indirectly, all marine life depends. 



In addition to these minute plants, the plankton 

 contains nearly all forms of marine life at some stage or 

 other of their life history. Fish are only found in it as 

 eggs, or larvae. Crustacea of all kinds are present, 

 and form one of its most important constituents. Crabs 

 and lobsters spend their larval, free-swimming career 

 among the plankton, until they reach the adult stage 

 and settle down to the bottom. Various minute 

 Crustacea, known as " Copepoda " (lit., oar-footed) 

 spend the whole of their lives drifting about in the 

 surface water. They occur in incredibly large numbers, 

 and are the most abundant of all forms of marine life. 

 These copepoda form the main source of the food of 

 pelagic fish, such as the herring, mackerel and sprat. 



The larvae of the edible molluscs, oyster, mussel, 

 cockle, develop in the warm surface water until they 

 settle to the bottom and begin their adult life. 



