214 



POPULATIONS OF THE SEA 



Clions Umacina 



Fig. 5. The average distribution and abundance of Clione Umacina in 1952 and 

 1956, based on the Continuous Plankton Recorder Surve>'. Crosses indicate the 

 centers of sampled squares in which Clione was absent. The three sizes of circles 

 are drawn at abundance intervals of X3 (F. R. Vane, unpublished). 



lustrated in Fig. 6, which shows the seasonal timing and abundance 

 of a diatom {Thalassiothrix longissima) and a copepod {Corycaeiis 

 angliciis) in the North Sea during the last ten years. Thalassiothrix 

 was relatively rare until 1952 but subsequently it became a com- 

 mon and most consistent part of the plankton in the winter and 

 spring. Corycaeus showed the reverse trend ; appearing regularly 

 during the late autumn between 1949 and 1953, it was almost 

 absent Irom Recorder collections during the last four yeais. 



These two pictures help to emphasize one of the limitations of 

 plots of static distribution ranges which I mentioned above. Both 

 species were found in the same area, in this example, the North 

 Sea. But, because of their different biological seasons and because 

 of different between-year trends they would hardl}^ ever meet 

 each other in the sea. 



Inspection of the temporal shapes of distribution such as those 

 in Fig. 6 showed that there was one kind of seasonal cycle in the 



