257 



The Lethal Salinity 



The foregoing observations indicate a general interruption of function in salinities 

 below 6 parts per 1000, while below 4 parts per 1000 there is little or no activity. We 

 may expect that above this critical range (4 to 6 parts per 1000) the teredos are 

 able to continue to live and function, as regards salinity, while below this range the 

 organisms will die if exposed long enough. The salinity below which the average 

 individuals are unable to live may be called the lethal salmily. This salinity has been 

 more accurately placed at 5 parts per 1000 by the following experiments. 



Teredos were exposed by splitting away the outer surface of the specimen blocks 

 and breaking away the thin shell around the pallets, so that the animals were unable 

 to plug themselves up in their burrows. Individuals exposed in this manner lived for 

 1 1 days in slowly running water of 5 parts per 1000 salinity, after which time the results 

 were obscured by fouling of the aquaria. The activity of these teredos as manifested 

 by the sensitivity of the siphons at the end of this time was equal to that of specimens 

 in 6 and 15 parts per 1000 salinity, as is shown in table 36. 



TABLE No. 36 



ACTIVITY OF TEREDO NAVALIS, EXPERIMENTALLY EXPOSED IN VARIOUS 



SALINITIES. 



Individuals exposed to salinities below the lethal (5 parts per 1000) were killed 

 off very rapidly, as is shown in table 37. The criterion of death used in these experi- 

 ments was the first appearance of degeneration of the tips of the siphons. As soon as 

 this degeneration appeared, the specimen blocks were changed to water of a salinity of 

 15 parts per 1000, but the animals never revived after this degeneration had begun. 



Harington (1922) reports experiments on the effect of low salinities upon the 

 larvae of a teredo, probably T. norvegica. He shows that these larvae were able to 

 survive for at least a short time in salinities as low as 10 parts per 1000, although 

 swimming was inhibited below 17.5 parts per 1000. No observations of this kind have 

 been made on the larvae of T. navalis. Since T. norvegica is usually found in open sea 

 water (i.e., water of a salinity of 35 parts per 1000 or thereabouts), however, it is prob- 

 able that the larvae are less adapted to low salinity than are the larvae of T. navalis, 

 which inhabits brackish waters. It may be expected that the larvae of the latter form 

 might be able to live in approximately the same salinities as the adult. 



