LARVAL TRANSPORT BY OCEAN CURRENTS 457 



especially the epifauna species, increases enormously from the 

 Arctic toward the Tropics (Thorson, 1952), it is obvious that by 

 far the best chances for long-distance larval transports exist in 

 tropical regions. 



Three main types of pelagic invertebrate larvae are known. One 

 of these, which has a pelagic life of a few days to a few hours, may 

 be disregarded here. The types which are of interest in our problem 

 are the following. 



The lecithotrophic pelagic larvae develop from fairly large, yolky 

 eggs. They do not feed on the plankton, but rather exclusively on 

 yolk within the egg cell from which they originate. This larval 

 type accounts for about 10 per cent of the species with pelagic 

 larvae in temperate as well as tropical sea areas. In most cases 

 their pelagic larval life is fairly short. In a few cases, however, 

 they may have a pelagic life of up to two months, and, if so, they 

 must be considered in our calculations. 



The planktotrophic larvae with a long pelagic life originate from 

 small eggs with little yolk, and are often produced by the mother 

 animal in huge quantities. They are known in the literature under 

 the names veliger, tornaria, pilidium, trochophora, etc. They 

 derive all or nearly all their food from the plankton and thus are 

 fully dependent on the food conditions there. This larval type is 

 found in more than 70 per cent of all marine invertebrates in recent 

 seas, and it is this larval type which predominates in all speculation 

 on larval transports. It also is among the parent animals giving 

 rise to this larval type — and only there — that we find those 

 enormous numbers of eggs per female per season (some 1100 to 

 500,000,000 eggs or more, average about 1,000,000 eggs) which 

 seem to be of fundamental importance as a counterbalance to the 

 great loss of larvae during a long pelagic life (Thorson, 1950, 1952). 



For the present purpose, it seems less important to study the 

 length of pelagic larval life within such groups or species, the adults 

 of which are sedentary (cirripeds, serpulids) or sessile (actinians), 

 since they may easily be transported over the oceans by other 

 means than larval drifts, for instance by driftwood and, in our 

 time, to an increasing extent, by ships or, in certain cases, even 

 on the pontoons of hydroplanes. Sedentary species often have a 



