LARVAL TRANSPORT BY OCEAN CURRENTS 469 



the oceans (for instance currents running from West Africa toward 

 Brazil and the Antilles) will carry the larvae a shorter distance 

 than currents which are gradually cooled (i.e., the Gulf Stream) — 

 perhaps a fact of much significance. 



Figure 3, in which an attempt has been made to combine the 

 average velocity of ocean-crossing surface currents (here indicated 

 through the number of weeks used to pass a known distance) with 

 the time spent by the larvae in the plankton, is based upon all 

 available information including the most recent British and 

 American pilot charts. The current velocity has been calculated 

 for the seasons of the year when each current is most direct and 

 most rapid. In some cases the currents for the northern summer (S) 

 as well as the northern winter (W) are indicated. The numbers 

 referring to the currents are given to the left, first for the Atlantic 

 Ocean, next for the Indian Ocean, and finally for the Pacific Ocean. 

 Within each of these areas the currents moving from east to west 

 are given first. At the top are figures to indicate the number of 

 weeks used for the current to flow the indicated distance. The 

 deadlines for pelagic life in 70 per cent (dotted line) and 80 per cent 

 (solid line) of all larval species examined so far have been indicated. 

 The hatched area shows the number of weeks during which even 

 long-distance larvae tend to metamorphose and disappear from 

 the plankton. 



It will be seen that under average conditions even most long- 

 distance larvae have a much too short pelagic life to survive the 

 critical distances across the eastern Pacific (i.e., from Galapagos 

 to Christmas Island or from IXIellish Bank to California), and even 

 the most rapid currents from West Africa to Brazil and the 

 Antilles will in most cases be too slow even for long-distance larvae. 



These current calculations are based on average velocities, and 

 what we are interested in is the maximum velocity within a space 

 of some hundred years. It is well known from old measurements 

 that the Gulf Stream, for instance, may vary its velocity signifi- 

 cantly not only during the course of a year, but also over long 

 periods of time. On the assumption that such optimal conditions 

 might now and then occur within some hundred years, there is 

 hardly any doubt that the true long-distance larvae may sometimes 

 have opportunities to cross even the eastern Pacific, and, under 



