166 POPULATIONS OF THE SEA 



Since the volume and weight of an object varies as the cube of its 

 median diameter and the surface area only as the square, it is 

 obvious that the likelihood of movement by wind increases with 

 decrease in size, assuming similar specific gravity and, on the part 

 of aquatic organisms, proximity to air-water boundary. As early 

 as 1830, Ehrenberg described a variety of microorganisms from 

 atmospheric dust, but Charles Lindbergh made the first in situ 

 collections of high-altitude aerial plankton on his flight over the 

 Greenland ice cap in 1933 (Meier, 1935). Gislen (1948, pp. 124-125) 

 reports that it is common for animals up to a few millimeters long 

 (especially fish) to be transported by winds, and he records aerial 

 transport of animals as much as 10 cm long. If inferred prevailing 

 or seasonal wind tracks fall in the right places and directions for 

 the times involved, they could provide sweepstakes routes (Simp- 

 son, 1940a, pp. 152-157; 1953, pp. 23-25) whereby very rare but 

 significant successful passages might account for the transport of 

 exotic marine biotal elements that otherwise appear to demand 

 the movement of established land barriers or the emplacement of 

 temporary shoals across historic deeps. 



With regard to the more orthodox forms of transport by water 

 currents, various limiting factors are in\olved. Most important 

 are the characteristic sites of adult and larval existence and the 

 presence of tropisms with regard to light, current, or bottom; the 

 direction, orientation, temperature, and depth of the current or 

 passage along which movement may take place; and the nature 

 and orientation of natural barriers and filters. 



Planktonic eggs and larvae are more easily dispersed, but those 

 that stay on or near the bottom are less likely to be eaten or to 

 drift into unfavorable areas. The advantage varies according to 

 the conditions and degree of continuity of tolerable adult and 

 breeding habitat. The dispersal and persistence of species that 

 inhabit warm and shallow waters is favored by larvae that drift 

 near the surface in the mild equatorial currents, provided larval 

 existence is not so long as to carry them beyond contiguous 

 favorable bottom, or is long enough to ensure dispersal across 

 surrounding deeps and near-continuous occupancy of more remote 

 shoals. A similar degree of drifting for the stenothermal benthos 

 of cold and deep waters presents greater hazard of introduction to 



