98 Water 



stenohaline and euryhaline forms— these terms are entirely relative 

 The terms also do not bring out the important difference in the effect 

 of time of adaptation. Some species can withstand wide differences 

 in salinity if they become adapted slowly, but are unable to tolerate 

 rapidly changing salinity. Relatively few species can endure both a 

 wide range and a rapid change in salinity. Interestingly enough, 

 some euryhaline organisms may grow best in intermediate salinities, 

 or even require them for a part of their life cycles, although our 

 knowledge is scanty in this area. The division rate of certain species 

 of phytoplankton has been shown in laboratory cultures to be twice 

 as great at a salinity of about 20%o than at salinities of about 10%o 

 or 30%o ( Braarud, 1951 ) . We also know that the American oyster 

 can live in full sea water and can tolerate fresh water for short 

 periods, but observations indicate that the larvae of this species will 

 not settle at salinities above 32%o nor below 5.6'/(c. The optimum 

 salinity for settlement in this estuarine animal is 16 to 18.6/fp. Chang- 

 ing salinities in estuaries may control the growth, reproduction, and 

 distribution of more of the inhabitants than we now realize. 



Limits of distribution are sometimes sharply determined by salinity 

 toleration. The vegetation around the margins of salt lakes and in 

 the spray zone near the ocean shore exhibits a characteristic grada- 

 tion of species from those most tolerant of salt to those least tolerant. 

 In San Francisco Bay the slight difference in tolerance between two 

 species of pile-boring mollusks was of great economic importance. 

 In the upper regions of the Bay wharves and other wooden structures 

 had been built without any special protection against shipworms be- 

 cause the local species of the genus Bonkia was unable to grow in 

 salinities below 10%o which frequently occurred in the area. In 1913 

 another species of wood-boring mollusk, belonging to the genus 

 Teredo, was introduced into San Francisco Bay, and this species 

 could tolerate salinities as low as 6%o. Spreading rapidly, Teredo 

 attacked the wooden structures of the area and within a few years 

 had caused destruction amounting to more than $25,000,000. 



Many interesting problems in relation to salinity tolerance remain 

 unanswered. In most cases we do not know why one species can 

 tolerate a rapid or extensive change in the salt content of its medium 

 and another cannot. Why the euryhaline species have not come to 

 dominate the open ocean as well as the coastal regions is unknown. 

 Another question of interest is how oceanic birds and mammals can 

 balance their water budgets without a supply of fresh water. Many 

 of these forms eat marine invertebrates whose salt content is nearly 



