Straits and in Suisun Bay the water evidently becomes sufficiently fresh 

 during the rainy season to prove lethal to the borers, as it does to most 

 other marine life. 



In the lower bay Teredo may be found at any season of the year. 

 Investigations in the spring and early summer, however, showed many 

 of the burrows to be empty, thus supporting the conclusion that the 

 annual autumnal or winter death rate is extremely high, only a limited 

 number surviving to propagate at the opening of the next breeding 

 season. 



The- environmental conditions destructive to Teredo are therefore 

 to be sought above Crockett, or in the surface waters in the Straits 

 near Crockett itself. The following tables and the salinity graphs 

 (Plates 15 and 16) present the available data from the records of 

 salinities at Crockett, Port Costa (one mile above), Martinez (five 

 miles above) and at several other points which are of significance in 

 the destruction of Teredo. It is well known that Teredo survives in 

 its burrows for some days both in fresh water and out of water. In 

 the Committee's laboratory normal Teredo in their burrows placed in 

 running fresh water in November and December were all killed off 

 within nine days, most of the deaths occurring on the eighth and ninth 

 days, at 50-65 F. Controls in standing and running salt water sur- 

 vived. In Carquinez Strait, Teredo in piling are subject for varying 

 periods to water of varying degrees of salinity. This is due mainly to 

 the interaction between tides entering from San Pablo Bay, and run- 

 off from the rivers entering Suisun Bay above Martinez. At each re- 

 current flood tide more highly saline water enters from San Pablo Bay 

 along the bottom and at the ebb the fresher water pushes down the 

 straits from Suisun Bay on the surface. The average difference between 

 the salinities at high tide at surface and bottom at Port Costa for Janu- 

 ary, February, March and April, 1921, was 2.9, 2.7, 3.7 and 4.0 parts per 

 1000, respectively, while the minimum and maximum differences be- 1 

 tween surface and bottom on any one daily tide were 0.2 and 6.0, 0.3 

 and 6-4, 0.0 and 9.0, and 0.6 and 9.1, respectively. Teredo near the 

 mud line have a better chance of survival than those nearer the sur- 

 face, since the salinities are higher there than at the surface. 



The borers are able to protect themselves for a certain period of 

 time against the invasion of much fresh water by closing the orifice of 

 the burrow with their pallets and thus retaining the salt water in their 

 burrows. In time their respiratory needs compel a resumption of the 

 circulation through their siphons. The repetition of this process in time 

 dilutes the salt water and the animal becomes relaxed, no longer with- 

 drawing the siphons when irritated/ and soon dies. 



The sense organs at the tips of the siphons apprize the animal of 

 the relative salinity of the water at the orifice. With each returning 

 flood tide and increased salinity the circulation of water through the 



