1917] Barroivs: Shipworm in San Francisco Bay 41 



at Oleum. From the salinity table just quoted, the bottom salinity is 

 seen to be consistently greater than the surface density, in one instance 

 by as much as 6.48 parts per 1000. While Teredo diegensis will prob- 

 ably live in water of a salinity of 10 or 12 parts per 1000, a slightly 

 greater density of a few points per 1000 seems directly or indirectly 

 to attract greater numbers of the borers and to stimulate their greater 

 activity. It is noteworthy that an average annual difference in salinity 

 of from 2.89 to 4.28 parts per 1000 within but a few fathoms of depth 

 should cause so distinct a difference in the abundance and activity of 

 these borers as has been reported. 



It is probable that there may be a direct physiologic adaptation 

 between a certain degree of salinity and the wellbeing of these wood- 

 borers, such as is noted in the case of most marine animals, and which 

 effectually prevents their existence in water of either unusually low 

 or high salinity. It is possible also that the relation of shipworms to 

 salinity may be only an indirect one, and that the distribution of 

 shipworms may be at least partly dependent upon some other cause, 

 such as the presence of certain classes of plankton organisms, upon 

 which the shipworm may feed to the best advantage, which are in 

 their turn directly dependent upon water of a certain density for 

 flotation or of a certain salinity for their own physiologic wellbeing. 



That Teredo diegensis is extremely sensitive to the amount of 

 water in circulation around the place where it may have settled ap- 

 pears from the fact that the damage to the Mare Island dikes was 

 much greater in the brace piling, consisting of isolated timbers set 

 at an angle away from the dike wall, than in the sheet piling which 

 formed the face of the wall, although both the brace piling and the 

 sheet piling were more or less infected for their full length. The 

 water washing more freely around the isolated brace timbers than 

 over the closely set sheet piles of the wall of the dike probably brought 

 to the borers in the brace piling a much more abundant supply of food 

 than to the borers located in the sheet piling, thus suggesting the close 

 ultimate relation between boring activity and food supply. 



Conclusions 



1. It, therefore, appears that shipworms of the species, Teredo 

 diegensis Bartsch, may be intermittent residents of the vicinity of 

 Mare Island near the mouth of Carquinez Strait; that the excessive 

 damage caused by these borers in 1913 came about through the 



