Transport hy Water 49 



rivers and estuaries although the planktonic larvae might be expected 

 to be carried away since the net water movement is always toward 

 the sea (Ketchum, 1951). In some situations the denser, more saline 

 water near the bottom tends to move predominantly into the estuary 

 whereas the greater flow in the surface strata is seaward. Observa- 

 tions in certain estuaries indicate that the older oyster larvae tend to 

 drop to the bottom on the ebb tide and to rise into the water on the 



Single winter egg in ephippium 



Redrawn from Needham and Lloyd, 1937 



Fig. 2.11. Ceriodaphnia reticulata showing eggs developing in the brood pouch. 



flood tide (Carriker, 1951). In this manner a sufficient number of 

 young oysters work their way upstream to repopulate the upper 

 regions of the estuary. In barnacles the problem is solved by the fact 

 that, although the younger larvae are found in the upper water layers, 

 the larvae approaching the setting stage tend to concentrate at deeper 

 levels where the net drift carries them up the estuary (Bousfield, 

 1954). 



