456 BOUNDARIES OF THE SEA 



data for some 25 more species may be found in literature, so 

 far not available to the present author. Also knowledge of the 

 direction and velocity of the oceanic surface currents, which may 

 transport the pelagic larvae, has increased considerably since 

 Gardiner's time. It therefore seems reasonable to reexamine the 

 whole question, the more so since larval transport by currents is 

 regarded as an important means of distribution in nearly all papers 

 and books discussing marine zoogeography (see, for instance, 

 Ekman, 1953). 



It must, however, be stressed, that this does not mean that the 

 whole question is solved now. Many problems are still unsolved, 

 but so many new facts have been brought together that it seems 

 worth while to re\iew present knowledge to find out what actually 

 is known and what points must be tackled in the years to come in 

 order to make the best progress. 



The chief problem, which we shall consider, is to what extent 

 surface currents may transport the larvae of coastal forms (i.e., of 

 forms not able to reproduce beyond the limits of the continental 

 shelf) across the ocean from one continent to another, from one 

 continent to a group of islands, from one group of islands to 

 another group, etc., thus introducing new faunal elements in 

 already well-established communities beyond their normal bound- 

 aries. To examine these questions it seems reasonable first to get 

 some idea about the length of pelagic larval life in the different 

 groups of bottom invertebrates, then to collect similar information 

 on the velocity of the most important surface currents crossing 

 the oceans, and finally, to compare and discuss the two groups of 

 information obtained. 



As is well known, nearly all invertebrate species living in the 

 Arctic and Antarctic coastal zones as well as all inhabitants of the 

 deep sea seem to have a nonpelagic larval development. In cold 

 temperate seas, for instance the waters round Denmark and 

 Scotland, about 65 per cent of all species of bottom invertebrates 

 have pelagic larvae, and in the tropical coastal areas pelagic larval 

 development will be the rule in about 85 per cent of all species of 

 bottom invertebrates (Thorson, 1950). Since, furthermore, the 

 total number of species of marine coastal-dwelling invertebrates, 



