8 



THE BIOLOGY OF MARINE ANIMALS 



geographical variation, becoming more abundant over the continental 

 shelf and in higher latitudes. In general there is a tendency for a few species 

 of planktonic animals to predominate in catches made in far northern and 

 southern waters, but these occur in immense numbers. In tropical waters, 

 on the other hand, the population density is low but there is great diversity 

 and richness of species. 



In temperate and sub-polar waters there is a rich seasonal growth of 

 nutritive phytoplankton which in turn regulates the abundance of plank- 

 tonic animals. Around the British Isles the seasonal increase in plankton 

 first becomes noticeable between February and March as a rapid bloom- 

 ing of the diatom pasturage. This is followed in April by the hatching of 



8,000 



Jan. Feb. Mar. April Ma.y June July Aug. Sept. Oct. Nov. Dec. 



Fig. 1.3. Graphs Showing the Annual Variation of Phyfoplankton 

 and zooplankton in the english channel (1934) 



Continuous line, phytoplankton; broken line, zooplankton. (From Harvey, Cooper, 

 Lebour, and Russell (28).) 



vast hordes of planktonic animals which batten upon the phytoplankton. 

 Owing to the depletion of nutrient salts and the grazing effect of planktonic 

 animals, the plants decline in abundance. A second minor outburst of 

 phytoplankton may follow in the autumn (Fig. 1.3). During the winter 

 months the plankton content of the surface waters sinks to a low ebb. In 

 arctic and antarctic waters there is only a single annual outburst of plank- 

 tonic life, in the summer. 



The permanent members of the zooplankton are animals such as fora- 

 minifers, copepods, euphausiids, siphonophores, ctenophores, salps, 

 chaetognaths, pteropods and so on, which spend all their life adrift. These 

 are the holopelagic forms. In coastal waters another element comes into 

 prominence, namely the temporary plankton. This embraces the drifting 

 larval stages of numerous littoral and benthic species such as polychaetes, 

 decapod Crustacea, echinoderms and molluscs. With these should be 



