ECOLOGY— DEPTH DISTRIBUTION 173 



ascend is removed and the animals probably sink back to the water layers offering the best combina- 

 tion of other factors such as temperature and perhaps salinity. With the coming of dawn they rise to 

 meet the optimum and follow it downwards.' Thus for many species of cavoliniids a double diurnal 

 rhythm is established ; those liking a low light intensity will rise very near the surface at night and 

 those preferring a higher intensity will lose the stimulus earlier and sink. At dawn the reverse takes 

 place, those liking a low intensity receiving the stimulus earlier and preceding the others to the 

 surface. 



Considering first the two stations WS 996 and WS 997, we find great abundance of Limacina buli- 

 moides by day (4 to 6 p.m.) and by night (1 to 4 a.m.) (Fig. 3). At WS 996 the day-time distribution 

 shows a maximum at 50-100 m. (1542) and relatively few (93) at 0-50 m. From 100-250 m. the 

 numbers fall away markedly to 178, and the fall continues, decreasing fairly evenly down to 750-1000 m. 

 where only six specimens were taken. The species is virtually excluded below 250 m., some of the 

 shells counted here being dead. Of the three smaller day-time samples, no station yielded sufficient 

 material to compare adequately with WS 996. At WS 986 (11 a.m.) (on the line immediately north of 

 the southernmost one — the other four stations being on the southernmost line) there were only eight 

 specimens taken and these were all at 0-50 m. At WS 999 at noon there was a small maximum 

 (twenty-nine) at 100-150 m., and at WS 998 (8 a.m.) Limacina was found only at 100-175 m. where 

 there were eighty. For the night distribution at WS 997 there were 11 16 just below the surface 

 (0-50 m.), many fewer (468) at 50-100 m. and the greatest number anywhere recorded (3142) between 

 100 and 250 m. 



On the basis of much smaller samples than the above, Stubbings finds the vertical distribution of 

 cavoliniids (with the exception of the abnormal Creseis virgula which he regards as not controlled by 

 light intensity) to vary between 250 m. for Creseis acicula down to 650 m. for Clio citspidata and 700 m. 

 for Diacria quadridentata, with Clio pyramidata, Cavolinia globulosa and Hyalocylia striata at inter- 

 mediate optima from 400 to 600 m. All show a well-marked surface migration by night, and well- 

 developed diurnal depth behaviour (see Stubbings, 1937, fig. 1, p. 27). 



When considering the depth distribution and light response of Limacina, it is important to keep its 

 much smaller size in mind. A journey to the surface from 250 m. in the six hours after dusk would 

 entail continuous travel at the rate of 2 ft. per minute, a performance greatly in excess of what I have 

 found L. retroversa capable of maintaining for even a short time (Morton, 1954c). On grounds of size 

 alone we should thus expect species of Limacina to be much less sensitive to changes in illumination than 

 the larger cavoliniids, if only because they are not large or swift enough to swim away from regions of 

 diurnal changes in light intensity. We might expect to find them remaining rather near the surface as 

 grazers on phytoplankton and tolerant of strong light. If they abandon the surface too far by avoiding 

 light during the day, they will be unable to reach it again by night. A similar reasoning may explain 

 the preference for the upper layers of the two smaller and more slender cavoliniids Creseis virgula and 

 C. acicula. These have a relatively tiny wing expanse and no doubt limited diurnal swimming powers. 

 The cavoliniids occurring deepest are the larger and more robust ones. The greatest density of 

 Limacina bulimoides by day at WS 996 is between 50-100 m. This is much nearer the surface than any 

 optimum recorded for cavoliniids (with the exception of C. virgula), and this layer probably represents 

 the maximum light concentration above which Limacina is not happy by day. With the doubtful 

 exception of WS 886, L. bulimoides was never found predominantly in the upper 50 m. by day. The 

 ninety-three individuals present at 0-50 m. at WS 996 were but a small portion of a population in very 

 much greater concentration in the layer immediately below. The most numerous day-time samples 

 came from 50 to 100 m., 100 to 150 m. and 100 to 175 m. Light response by large diurnal depth 

 changes seems to be a feature that arose with the cavoliniids; the more primitive limacinids show it 



