SURVIVAL OF 



263 



TABLE No. 41 



TEREDO NAVALIS IN PILES AT CROCKETT DURING I'ERIOD OF LOW 

 SALINITY, SEASON OF 1921-1922 



*Salinity remained below 5 parts per 1000 from April 26 to June 24. 



**Salinit\' remained below 4 parts per 1000 from Ma>' 17 to June 19. 



***Salinit\- remained below 3 parts per 1000 from Ma\' 27 to June 19. 



Reference to figure 99 shows that duruig that season (1921-1922) the sahnity 

 remained below 5 parts per 1000 from April 26 to June 24, below 4 parts per 1000 

 from May 17 to June 19, and below 3 parts per 1000 from May 27 to Jime 19. 



Table 41 shows the sur\ival of teredos in piles pulled from tiie fender lines at 

 Crockett, at intervals during the period of low salinity. 



It appears from table 41 that 10 per cent of the borers sur\ i\ed a stretch of 

 58 days of salinit>- below 5 parts per 1000. During May, however, several peaks 

 occurred which reached salinities of 4 parts per 1000 or more (see fig. 99). 



It has since been found also that bottom salinities at Crockett (cf., figure 10(1, 

 showing surface and bottom salinities at Martinez, where conditions are cjuite similar 

 to those at Crockett) may be from two or three to six parts per thousand greater than 

 salinities at the surface. As the depth along the wharf at Crockett is from 25 to 40 

 feet, samples taken at a depth o1 14 feet, on which tal)le 41 is based, do not adequately 

 represent the bottom conditions. 



It has been a matter of repeated obser\ation at Crockett that teredos in the 

 lower end of a pile, in the region of the denser water, will survive long after all those 

 at higher levels in the same pile lia\'e been killed off. Of a considerai)le number of piles 

 at this locality examined in the spring and summer of 1922, the highest level at which 

 any survivors occurred at the end of the fresh water [period was 17 feet above the mud, 

 in water 33, feet deep at low tide. It would appear therefore that the conditions 

 represented in figure 99 and table 41, based on samples taken at a depth of 14 feet, 

 represent the conditions which are just lethal to the organisms. At this level all died. 

 A few feet lower in the pile a few survived. 



Our experiments ha\e indicated 5 parts per 1000 as the average lethal salinity, 

 but it was also observed (fig. 98) that a small proportion of the organisms — those 

 individuals most highly resistant to conditions of low salinity — could extend their 

 siphons and carry on their functions, at least in part, for some time in a salinity of 

 4 parts per 1000. This fact may explain the sur\-i\al of a few individuals at a le\"el 

 just below that represented In' the salinity figures gi\en. It is further probable that, 

 since samples were not always taken at the major tide of the day, the salinity may have 

 reached 5 parts per 1000 on some of the days when our salinit\- record shows onl\- 



