458 BOUNDARIES OF THE SEA 



more or less developed brood protection of the early larval stages, 

 and in such cases larvae may be ready to hatch in a fairly advanced 

 stage, to metamorphose and settle when the ship arrives in a new- 

 harbor. This significantly improves the chances for dispersal of 

 the species concerned. 



When, later on, we compare the figures for pelagic larval life 

 and the chances for current transport in the individual species, we 

 must realize that the animals may have obtained their recent 

 pattern of distribution by several other means than larval trans- 

 ports: crabs may be dispersed on fouling sides of ships (Bertelsen 

 and Ussing, 1936) or in ballast tanks (Peters and Panning, 1933), 

 egg capsules of prosobranchs on sides of ships or in cargoes with 

 living oysters (Chapman and Banner, 1949), etc. It will take too 

 long to consider all these means of transport, but they must not 

 be forgotten in our calculations, and it must also be kept in mind 

 that most modern ships crossing the oceans will in one or two 

 hours travel the same distance as does the average surface current 

 in about twenty-four hours. Therefore, invasions of new species 

 by ships may in future severely obscure the picture of natural 

 dispersal of marine invertebrates. 



The figures, which I have brought together so far for the duration 

 of pelagic larval life in 195 species of bottom invertebrates are 

 mostly based on rearings in aquariums, though a fair percentage of 

 them originate from observations in nature. It has, however, been 

 stressed by several biologists that in most cases development 

 under laboratory conditions will advance more slowly than in the 

 sea, the larval life thus being longer than the average one. Most 

 laboratory experiments are, however, carried out at room tempera- 

 tures, which are often some 2 to 5 degrees centigrade higher than 

 the temperature of local sea areas at the same season. Thus, the 

 higher temperatures accelerating the development, and the 

 artificial aquarium conditions postponing development may 

 counterbalance each other to give a final figure, which roughly and 

 for all the larvae will hardly deviate much from the average figures 

 in nature. In such species, where control observations on larval 

 development made under natural conditions are compared with 

 aquarium experiments, the results do not in most cases differ 



