331 



HORIZONTAL DISTRIBUTION, GROWTH AND DYNAMICS OF DISPERSAL 



The shallozv living Fourth, Fifth and Sixth Fiircilias 



It has already been shown (pp. 108-13, Figs. 9-1 1) that once the larvae have accomplished 

 the developmental ascent they show an increasing tendency as they get older to become massed both 

 by night and by day very close to the surface of the sea, Furcilias 4-6 in particular (Fig. 11) 

 evidently being so close that the uppermost (50-0 m.) vertical net, in which these stages are almost 

 exclusively taken, passes it seems only for a second or two through their zone of concentration, 

 capturing in consequence as a rule only small or negligible numbers. In other words, the vertical 

 net can no longer be regarded as an efficient sampler of the larvae at this stage of their development 

 and, therefore, the charts for Furcilias 4-6, while giving a reliable enough picture of their horizontal 

 distribution and its dynamics, provide only an inadequate idea of the actual abundance of these stages 

 during their life in the plankton. A true picture of this abundance can now in fact only be obtained 

 from the stramin nets in which as will be shown presently (p. 366, Figs. 1 15 and 116) the late Furcilias 

 are taken in large, often enormous, numbers. 



SECTOR 60W 



WEDOELL WEST 



LONGITUDE 

 DATE 



O/^IA/ WEDDELL 1,1^1- 

 30W^,^;,,, WOE SECTOR 



59°5S'S2'5l"49°49°4«'.4tf43°4f4d"38'34"3l°3l"30°* I 27'27°27 26' 23 1 8' 

 12 30 9 22 8 9 8 3025 21 26 2OI92I0I89 2 89 124 5 

 647 639 636 637 2298 2300 373 1990 1987 365 369 1142 

 1328 2293 638 W527 375 374 1138 372 368 1994 1144 



LONGITUDE 

 DATE 



* SOUTH GEORGIA 



Fig. 90. Developmental condition of surface larvae in the Weddell drift in March. 



March-April. They first appear, as Figs. 90 and 91 show, in the second half of March, again, 

 like the Calyptopes and early Furcilias in January and February, in Weddell West. Except for one 

 instance, they seem everywhere to be rather scarce and while this might be expected in the first month 

 of their appearance it must in fact be only partly real, our horizontal data showing that at the close 

 of the month the late Furcilias, especially Furcilia 5, may sometimes be present in Weddell West in 

 enormous numbers. In other words this apparent March scarcity must be ascribed in part at least 

 to the low sampling power of the vertical net. In April there is some indication of a spreading from 

 Weddell West into the western part of Weddell Middle, although alternatively the three April 

 occurrences in Weddell Middle might be ascribed to the local growth of a very small percentage of 

 the substantial early Furcilia population (Fig. 90 and p. 325, Fig. 86) recorded there in March. 

 Weddell East is barren except for two very small occurrences, both in late April. Both (p. 321, Fig. 82, 

 Stations 1353 and 2344) can be ascribed to the local growth of the early Furcilias already recorded 

 there. 



It is somewhat surprising, having regard to the position in March, that there are no even minor 

 occurrences of late Furcilias in Weddell West in April, for as Fig. 82 shows the principal larval 

 concentrations then consist in the main of early Furcilias or younger stages. Two alternatives might 

 be put forward to explain this anomaly : (i) our nets simply through chance failed to strike the principal 

 older concentrations or (2), and perhaps the more likely, our stations fell in a year (or years) when for 

 some reason or other the larval development in Weddell West was more retarded than usual. It has 

 already been shown (p. 319, Fig. 81) that there appears to be a direct correlation between the 



38-2 



