LARVAL TRANSPORT BY OCEAN CURRENTS 463 



might be sufficient for a larval swarm to conquer new coastal 

 zones, to gain a foothold there, and to found a permanent popu- 

 lation. Actually, the recent pattern of distribution of marine 

 coastal faunas represents the accumulation of all "transport 

 miracles" during thousands of years, and our larval transport 

 problems have to be considered with this in mind. 



Let us for a moment return to the larvae, first to the polychaetes, 

 which are known to be very cosmopolitan. When comparing the 

 number of polychaete species from western Europe with those of 

 the Indo-Western Pacific area, you will find that some 30 per cent 

 of the species are common to both, whereas the distribution of 

 common species in such groups as echinoderms, prosobranchs, and 

 lamellibranchs is less than 2 per cent. This seems mainly to be due 

 to the fact that the individual polychaete species (in contrast to 

 species of the other groups mentioned) may vary the mode of 

 reproduction from pelagic to nonpelagic, etc., according to the 

 season and locality. Thus they are always able to respond in the 

 most favorable way to ecological conditions (Thorson, 1950, p. 32). 

 In some cases, long-distance larvae also may have contributed to 

 the cosmopolitan success of this group. At least it is a fact that 

 such species of polychaetes which actually have long-distance 

 larvae : for instance Polydora ciliata and Chaetopteriis variopedatus 

 also have a worldwide distribution. Polychaete larvae seem to have 

 better conditions for long-distance transport than most other 

 groups. Day (1937) and Wilson (see his review of the whole prob- 

 lem, 1952) have clearly shown the ability of polychaete larvae to 

 postpone their metamorphosis for days and even up to several 

 weeks until they reach a substratum suitable for metamorphosis 

 and settling. Even when the metamorphosis cannot be postponed 

 any longer, some of these larvae may metamorphose in "mid- 

 water" to continue the planktonic life as "pelagic bottom stages." 

 The "Benham stage" of Arenicola is probably such a mid-water 

 metamorphosis. All in all, polychaete larvae thus seem to have 

 still better chances for long-distance transport than is immediately 

 seen from Fig. 1. 



The asteroids on the average have a fairly short pelagic life, but 

 species of the genera Astropecten and Asterias may have a larval 



