261 



tions having a greater difference of concentration. Tlius tlie dilution of tlie Wiiter 

 in the burrow would take place more slowly when tlie hlocks were surrounded by 

 water of 2 parts per 1000 salinity than when siu'rounded iiy water of zero salinity and 

 the teredos would sur\ive for a longer period. 



Teredos in the piles at Crockett have survived greater jjeriods of low salinity 

 during the past three years than have the teredos under any conditions of low salinity 

 in the aquaria (see tables 38 and 41). The short period of sur\i\al in the latter case 

 may be due, however, to the greater relative surface exposed for diffusion in the speci- 

 men lilocks than in piles. Besides having their cut surfaces exposed in most cases, 

 these blocks were scrublted free of l)arnacles and other marine growths which cover 

 the exterior of all jiiles. Thus there was a re1ati\'ely larger surface exposed for diffusion, 

 including the face of the block, the split surfaces, the open ducts of the wood at the 

 cut ends, and the very large surface exposed by the opening up of many burrows in 

 cutting the lilock. As is shown in table 40, more teredos survi\ed low salinities wiien 

 the cut surfaces of the blocks were ccnered with paraffin, although this covering diil 

 not adhere closely in some places and was thus an imperfect seal. There must also 

 be a slight leakage through the pallets although they fit tightly and form a \'ery 

 effecti\e plug. 



At times during the ]:)eriod ol low salinit\' .it Crockett, when several ])iles were 

 pulled at the same time from the same locality it was foimd that there was a marked 

 difference in sur\ival in teredos from different piles. With conditions apparently the 

 same for all the piles, it seems reasonable to assume that the difference of survival 

 was due to differences in the porositx' of the wood in the piles, which would allow 

 diffusion to take place more rapidly in some piles than in others. A markedly greater 

 percentage of teredos sur\ives in a hard, close-grained ]iile than in one of a more soft 

 and spongy texture. The most plausible explanation of this is that in the former case 

 the salinity of water in the burrow of the animals is not so soon reduced to the lethal 

 point by gradual filtering in of fresh water during a period of low salinity. 



Although the above evidence is not conclusive, the explanation gi\en seems to be 

 the only one which accounts for some of the phenomena obser^•ed. Hence it 

 seems reasonable to assume that when plugged up in their burrows teredos are killed 

 only by a gradual reduction in .salinity of the water in the burrow, and that this process 

 takes place in part by diffusion through the wood. 



SuRviv.vL Period or Lethal Stretch 



An explanation of the above process becomes important if we wish to predict 

 the survival of teredos by observation of the salinity of the water from day to day. 

 In a daily salinity record such as is represented by figure 99, periods of survival or 

 lethal stretch must be measured as periods during which the salinity remains below 

 5 parts per 1000, as when the salinity rises above this point the teredos are able to 

 take in a fresh supply of salt water. Fluctuations of salinity during such periods must 

 influence the rate of dilution of the water in the burrow, but are of little importance 

 for predicting the survival of the organisms. 



During the winter of 1921-1922 a daily record was kept of the salinity of the water 

 in Carquinez Strait. Samples were taken during the day at high and low tide, from 

 the surface and from a depth of 14 feet. Samples were also taken at two points about 

 one-eighth mile apart, as it was found that differences in the salinity at these two 

 points occurred frequently because of currents and eddies. In preparing the salinity 

 graph, figure 99, the maximum and minimum records from all samples for each day 

 have been plotted. 



