258 



TABLE No. 37 



SURVIVAL OF TEREDO NAVALIS, EXPERIMENTALLY EXPOSED IN VARIOUS 



SALINITIES 



Natural Protection Against Unfavorable Salinity 



Although teredos exposed by splitting away the block around the pallets were 

 soon killed when placed in salinities less than 5 parts per 1000 (see table 37), individuals 

 in specimen blocks which had not been exposed in this manner lived for much longer 

 periods in the same salinities. Table 38 gives the per cent of organisms surviving 

 various periods of exposure to salinities of and 2 parts per 1000. 



Table 39 gives comparative results from tables 37 and 38, showing that, in the 

 same salinities, unexposed teredos lived inuch longer than exposed individuals. The 

 organisms therefore received some protection from their burrows, through their ability 

 to prevent the entrance of water from the outside by plugging the burrow entrance 

 with the pallets when the salinity of the water fell below the danger point (5 parts 

 per 1000), and at the same time to retain a quantity of salt water within their burrows. 



Factors Limiting Protection 



We may expect that if the water within the burrow were diluted to a point much 

 below the lethal salinity (5 parts per 1000), the teredos would be killed, since the 

 animals die in a short time when directly exposed to water of less salinity. Such a 

 process of dilution is indicated by the following experiment. 



A specimen block containing teredos was removed from water of 15 parts per 

 1000 salinity and placed above water in a closed aquarium functioning as a moist 

 chamber. In this way, evaporation of the water in the wood was restricted, but no 

 dilution of the water in the burrows was possible. At the end of twenty-two days, the 

 specimen block was placed in salt water and approximately 50 per cent of the original 

 number of teredos extended their siphons. This was a greater survival than that in 

 water of and 2 parts per 1000 salinity for the same period of time. Table 39 shows 

 the survival periods under different experimental conditions. 



Since at the end of twenty-two days a greater percentage of teredos survived 

 in the block not directly surrounded by water than in the blocks exposed to water of 

 either or 2 parts per 1000 salinity, we may assume that in the latter case the water 

 within the burrows was diluted by the fresh water from the outside. 



