1922] Blum: On the Effect of Low Salinity on Teredo Navalis 365 



It appears from table 6 that 10 per cent of the borers survived a 

 stretch of fifty-eight days of salinity below 5 parts per 1000. During 

 May, however, several peaks occurred which reached salinities of 

 4 parts per 1000 or more (see fig. 4). 



Our experiments have indicated 5 parts per 1000 as the average 

 lethal salinity, but it was also observed (fig. 2) that a small propor- 

 tion of the organisms could extend their siphons and carry on their 

 vital functions for some time in a salinity of 4 parts per 1000. It 

 may be possible that this small proportion of the organisms are able 

 to renew the water in the burrow when the salinity rises to 4 parts 

 per 1000, and that this may account for the nearly 100 per cent 

 survival of the organisms during the month of May (see table 6). The 

 slight mortality during this time (May 13 to May 18. table 6) may 

 have been due to the death of some of the weaker individuals. Since 

 samples were not always taken at the major tide of the day, however, 

 it is probable that the salinity may have reached 5 parts per 1000 on 

 some of the days when our salinity record shows only 4 parts per 

 1000. Thus it may be advisable to assume 4 parts per 1000 as the 

 critical salinity in interpreting the results given in table 6. The 

 real stretch seems to have occurred during the period when the 

 salinity was below this point, since, in the piles examined, 50 per cent 

 of the borers were dead at the end of nineteen days, 40 per cent at 

 the end of twenty-six days, and 90 per cent at the end of thirty-three 

 days. This corresponds more closely with some of the results obtained 

 in the aquaria (see table 3). 



From this interpretation of the results an estimate of one and one- 

 half months of a salinity below 4 parts per 1000 as the period required 

 for the destruction of all the teredos seems reasonable. The borers 

 are extremely resistant, however, and it may be that the greater period 

 of fifty-eight days (recorded of a salinity below 5 parts per 1000) is 

 not enough to exterminate them wholly. Estimates of the lethal 

 stretch must be very conservative at present and only careful observa- 

 tion for several years can give reliable results. 



It may be noted that there was apparently no further mortality 

 between June 26 and July 29, during which period the salinity was 

 very frequently above 5 parts per 1000. 



During the years 1859-1869, Dutch investigators made observations 

 upon Teredo navalis along the coast of the Netherlands and in the 

 Zuider Zee. Frequent records were taken of the salinity at various 



