42 University of California Publications in Zoology [Vol. 18 



marked increase in the average salinity of the water of this part of 

 San Pablo Bay, because of the comparatively small amount of fresh 

 water which entered the bay during the two consecutive dry seasons 

 of 1910-11 and 1911-12 ; that these borers may be killed off entirely 

 by a season of unusually heavy rainfall, though a re-injection may 

 occur during the next summer and fall ; and that these borers may 

 occur in this region in small numbers even in years of average rainfall. 



2. A salinity of at least 10 parts per 1000 (approximately) seems 

 to be required for the existence of Teredo diegensis at temperatures 

 ranging from 6° to 19° C. 



3. Damage may be expected at these temperatures from Teredo 

 diegensis in regions where the average annual salinity is as much as 

 13 or 14 parts per 1000, provided that the salinity does not fall for 

 any considerable period below 10 parts per 1000. 



4. An increase in salinity of even 3 or 4 parts per 1000 above the 

 minimum salinity tolerated by Teredo diegensis seems to be effective 

 in considerably increasing the abundance of the borers and in stimulat- 

 ing their activity ; hence their greater activity near the bottom than 

 near the surface in such localities as those under observation here. 



5. The activity of Teredo diegensis is thus, directly or indirectly, 

 related to the salinity. It is possible, however, that the salinity itself 

 may not be the only factor determining the vertical or horizontal 

 distribution of these borers, but that some other condition may to a 

 certain extent be the factor immediately controlling this distribution, 

 such, perhaps, as the plankton food supply, which may be more deli- 

 cately related to the salinity than the shipworm is itself. 



6. Of the several major factors of the marine environment, such 

 as temperature, salinity, depth, pressure, food supply, etc., salinity 

 may usually be regarded as the variable factor most likely to deter- 

 mine, either directly or indirectly, the distribution of Teredo diegensis 

 in bays where all these factors undergo greater variation than is 

 common on the open sea-coast. 



7. Other things being equal. Teredo diegensis seems to be most 

 active in woodwork exposed to a good circulation of water ; hence the 

 greater damage to the brace piling than to the sheet piling in the 

 Mare Island dikes. Thus, being apparently held in check by the 

 minimum salinity which can be endured, the activity of Teredo 

 when once located in timber under tolerable conditions of salinity may 

 depend largely upon the amount of food material which the constantly 

 moving current may bring to it. 



