262 POPULATIONS OF THE SEA 



"animal exclusion." The retarding or lethal effect of water from the 

 English Channel on eggs and larvae of some polychaetes and 

 echinoderms (Wilson, 1948-1954*) should be examined with this 

 hypothesis in mind. It also seems desirable in connection with the 

 antibiotic effect of plankton algae on bacteria (Steemann Nielsen, 

 1955). The hypothesis includes a means for understanding why 

 Calaniis sometimes occurs at the surface in bright sunshine, 

 although it is generally very sensitive to light. Perhaps some bodies 

 of water contain only a few fluorescent substances and therefore 

 they will have very little photodynamic effect. 



One of the basic ideas in this hypothesis is the combined effect 

 of several environmental factors. Rabaud (1918) referred speci- 

 fically to this problem. Moore and Kitching (1939) suggested that 

 tolerance of any one factor by various organisms is greatest when 

 all other factors are optimal. Some observations of marine animals 

 indicate a relationship between light and temperature. Lewis 

 (1959) pointed out that three species of copepods did not reveal 

 any such relationship within their annual ranges of temperature 

 in their natural habitat. However, above or below this range they 

 shun stronger and weaker light intensities respectively. Dehorne 

 (1918) stated that the syllid Myria?iida pinnigera withstands 

 temperatures up to 22°C in the dark, but it dies in light with only 

 a small temperature rise. Moore (1950-1958) has referred on 

 several occasions to the importance of the simultaneous action of 

 light and temperature in vertical migrations of several zooplank- 

 ters. In the speckled trout Salvelinus Jontinalis Sullivan and Fisher 

 (1954) found "that selection of temperature was apparently more 

 precise at low intensities than at high intensities, although the 

 actual temperature selected did not vary with a light intensity." 

 From experiments, Hoar (1956) obtained a correlation in the 

 goldfish between photoperiodicity and sensitivity to temperature. 

 Finally, according to Marshall and Orr (1958) Calaniis living at 

 the same temperatures in August and February needs more oxygen 

 in August. The authors suppose that different stages in develop- 

 ment may provide an explanation. I think it possible that increased 



For reference see Wilson and Armstrong (1958). 



