LARVAL TRANSPORT BY OCEAN CURRENTS 461 



5.5 per cent of all larval species examined thus far remain in the 

 plankton for a period longer than 3 months. Only the last category 

 may be regarded as true "long-distance larvae," which have to be 

 considered as the chief objects for transoceanic transport. The 

 diagram also shows that some systematic groups, as far as the 

 species examined are concerned, have no long-distance larvae at 

 all. This is the case for the ophiuroids, the holothurians, the 

 prosobranchs, and the lamellibranchs, whereas polychaetes, aster- 

 oids, echinoids, and especially decapod crustaceans have at least 

 some species which can be transported over wide areas. 



If we return now to the sources of error of this diagram, some 

 will want, no doubt, to stress the enormous variability in the 

 length of the pelagic larval life as related, for instance, to tempera- 

 ture. A few examples will demonstrate this. In the sea urchin, 

 Evechinus chloroticus, the length of the pelagic lar\'al life may vary 

 from 6 to 17 weeks (Mortensen, 1921; Maxwell, 1957). In the 

 oyster, Crassostrea virginica, the larval life may \'ary from about 

 7 days at 24-27° C to 21 days at some 18° C (J. Nelson, 1908; 

 T. C. Nelson, 1928), and Dr. Loosanoff (verbal information) has 

 found that Venus niercenaria may vary the length of its larval life 

 from 7 to 20 days. In the lobster, Homarus americanus, the larval 

 life may vary from about 9 to 11 days at 22-23° C to 15 or 16 days 

 at 18-19° C (Hadley, 1906; here taken from Ehrcnbaum, 1907), 

 and according to Templeman (1940) even from 14 days at 21° C 

 to 49 days at 10° C. 



Dr. Costlow and Dr. Bookhout (verbal information) have shown 

 that the pelagic lar\al life of several species of crab larvae in 

 rearing experiments in North Carolina may vary from 15 up to 

 40 days in the individual species according to the ecological 

 conditions. 



If we now use this information for Fig. 1, the following may be 

 deduced. There is no reason to believe that the durations of pelagic 

 larval life given in this diagram are minimal values. On the con- 

 trary, they seem to give average or perhaps a bit higher than 

 average values. So, even if it is assumed that extraordinarily low 

 temperatures, poor food conditions, etc., on certain occasions may 

 prolong significantly the larval life, there is certainly no reason to 



