THE PELAGIAL 243 



coastal waters, rich in plankton and small fishes, harbor the hump- 

 backed whales {Megaptera). In open, warm seas the plankton lies at 

 a deeper level and is followed by large crustaceans and cephalopods. 

 It is these which are found in the stomachs of the sperm whale 

 (Physeter macrocephalus) , which is able to reach great depths (up 

 to 750 m.). 



Pelagic biotopes. — Distinct biotopes within the great extent of 

 the pelagial are only vaguely differentiated. Perhaps the most impor- 

 tant differences are those between the oceanic currents and the eddying 

 areas. 



The oceanic currents are obviously of primary importance in the 

 distribution of pelagic animals, and man}' free-swimmisg forms are 

 also influenced by them. The action of the currents differs according 

 to their course, either returning to its origin, forming a closed circuit, 

 or with a distinct ending. At the present time only the surface currents 

 are known, and the currents of the depths are unstudied with reference 

 to their special faunas and effects on distribution. 



A closed current returns its animals to the same position and en- 

 vironment. The distribution of the developmental stages of Calanus 

 fimnarchicus in the Norwegian Sea affords an example. 35 Great numbers 

 of adult animals with eggs appear in spring in the diatom beds near 

 the Shetland and Faeroes islands; carried away by flow of the Gulf 

 Stream, the larvae which emerge from these eggs gradually grow, while 

 the adults die out; these then develop, further north, into "juniors"; 

 the current then returns from Jan Mayen, and the number of juniors 

 lessens as that of adults increases; they apparently winter in deep 

 water, and reenter the circuit at the Faeroes in spring, when the 

 warm water of the Gulf Stream brings them to sexual maturity. These 

 animals thus are carried through an extensive temperature range and 

 must be to a considerable degree eurythermal. This is still more 

 strikingly a requirement in the great closed circuits of the North and 

 South Atlantic. The latter carries water from the neighborhood of the 

 equator to the 48th parallel of south latitude, and organisms in it, 

 especially in its peripheral portion, are exposed to a considerable range 

 of temperature variation. The plankton in such a current requires 

 several months and even years to return to its origin, about l 1 /^ years 

 in the North Atlantic current and 2 1 / 4 in the South Atlantic. Among 

 short-lived plankton animals many generations, even hundreds of 

 generations, are included in this period. Larger animals with longer 

 periods of development are also exposed to considerable differences in 

 successive generations. Lohmann 36 believes that regular differences, 

 "cyclomorphoses," are connected with this cycle, corresponding in coc- 



