622 TT, R. BTJKEAU OF FISHERIES 



Tests on the closing reactions and responses to nutrient fluids have 

 shown that tlie glochidia from mussels livino; in polluted waters are 

 on the whole much less reactive than glochidia from mussels held in 

 pure water. Several specific substances liave been found which can 

 contribute toi this reduction in activity of the glochidia. The result- 

 ing reduction in activity of the glochidia by these jjollution substances 

 amounts practically to the sterilization of the mussel, as the glochidia 

 are often found unable to close, which would, of course, prevent any 

 natural replacement of these mussels as the glochidia would not be 

 able to attach themselves to fishes. 



Condition of fresh-water mussels and of mussel beds. — Detailed 

 physiological studies of the blood and internal organs of over 1,000 

 Iresh-water mussels Avere made to ascertain the normal conditions in 

 healthy mussels. These data are noiw being used to determine the 

 effects of pollution on fresh-water mussels. 



The adult fresh-water mussel has been found to be very sensitive 

 to a variety of substances which may be present in fresh water, in 

 fact contrary to the view commonly held, these studies have shown 

 that many species of fresh-water mussels are fundamentally clean- 

 water animals and that their ability to adjust themselves to conditions 

 of stream pollution is sharply limited. Many substances placed in 

 solution in the water surroiniding the mussels were soon recovered 

 in the blood of the mussels, and the average fresh-water mussel was 

 found to have little ability to prevent the entry of toxic and other 

 undesirable substances into the blood, but that it depends upon tem- 

 porary closure of the shell to keep out noxious substances. Conse- 

 quently these mussels are unable to withstand the conditions in many 

 streams where contaminating substances are constantly poured into 

 the water, as the mussels can not remain closed indefinitely. 



Purification of water hy mussels. — An elaborate series of experi- 

 ments has been in progress since early spring dealing with the ques- 

 tion of fresh- water mussels as water purifiers. The removal of silt, 

 organic debris, and other materials, under various conditions by these 

 animals is being checked. 



Two special pieces of apparatus have been evolved for this work : 

 (1) A device which maintains a constant condition of turbidity in 

 the aquarium under observation so that mussels may be studied for 

 long periods of time under conditions similar to those in rivers and 

 streams just after heavy rains, when the water contains large amounts 

 of silt and clay in suspension; and (2) an apparatus by means of 

 which very minute quantities of suspended matter in the water may 

 be measured, thus making quantitative studies possible. 



The activities of individual mussels have been recorded continu- 

 ously for long periods of time (in many cases in excess of 1,000 

 hours). From these observations it has been fonnd that the efficiency 

 of a fresh-water mussel as a water clearer has been greatly overesti- 

 mated in the popular accounts of these animals. Many mussels were 

 found to remain closed for more than 50 per cent of the time, and 

 they can not purify water during the time they are closed. Again 

 the mussels in an effort to protect themselves close frequently when 

 pollution materials are added to the water so that during the time 

 when the purification of water is most desired, the mussels are most 

 apt to be closed. 



