SECT. 4] BIOLOGICAL SPECIES, WATER-MASSES AND CURRENTS 383 



to admixture of other waters, or to changes in temperature, and so forth, 

 experienced during dispersal from the hatching or nursery area. But it should 

 be pointed out here that there is some reason to believe that the larval forms 

 may succumb more readily to changes than do the expatriates of the per- 

 manent plankton, although some larvae apparently do survive as drifting 

 organisms for periods longer than considered normal for the species, when 

 conditions favorable to metamorphosis to the benthic stage are not encountered. 

 However, larval stages generally cannot be expected to be found, except as 

 mere stragglers, at very great distances from the distributional range of the 

 adult. The limits of the adult range may be controlled by the tolerance of 

 the planktonic larvae, rather than by that of the benthic stage. However, the 

 floating larvae of some near-shore shallow-water or intertidal adults can be 

 traced to distances ranging up to 150 or more miles offshore over great depths, 

 giving evidence of offshore flow of water. 



To illustrate how these biological constituents may be useful as indicators in 

 oceanographic, zoogeographic and ecological studies, the remainder of our 

 discussion will be concerned chiefly with a review of a number of recent surveys, 

 mainly in the North and South Pacific and adjacent seas, that have aided 

 our concepts and understanding of the dynamics of the oceans and of their 

 inhabitants. 



The literature contains many reports, largely from the Atlantic, showing 

 the application of this kind of plankton study. Some of these have been re- 

 viewed by Hardy (1956) and extended most recently by Tebble (1960). For 

 recent work by Japanese and Russian investigators in the West Pacific see 

 especially Bogorov (1955), Brodskii (1957) and Tokioka (1959). 



It is not always possible to distinguish sharply between the application of 

 biological indicators toward identification of water-masses per se as opposed 

 to application as indicators of water currents. Often these are one and the 

 same. However, in studies of the terminal extensions of' water-masses or of 

 the more local movements of water intruding from or into separate areas, the 

 application becomes quite distinct. For purpose of discussion, we shall treat 

 the surveys under distinct headings although much overlapping is of necessity 

 implied. 



These discussions will not be deeply concerned with the precise physiological 

 or ecological actions of temperature, salinity, etc. that function to confine an 

 organism to a particular water-mass. This is a field that also needs much 

 research, but we shall mainly be occupied in presenting some aspects that 

 contribute to the descriptive phase of oceanography ; these aspects are currently 

 experiencing an upsurge in the less explored Pacific and adjacent waters. 



2. Water- Masses and Biological Species 



The studies of distribution and systematics are interdependent in the 

 development of biogeographical concepts. Oceanic populations found to be 

 small in range sometimes prove to be ecotypically or genotypically distinct 



