SEA MUSSEL MYTILUS EDULIS. 



20I 



Williamson (1907) made observations on the growth of some mussels which he 

 kept in the laboratory for one to two years. He divided them into three sets, accord- 

 ing to size, and designated them as A, B, and C. Lot A consisted of individuals 0.3 

 inch long, lot B of specimens 1 inch in length, and those of lot C measured about 1.5 

 inches in extent. Individuals of the A group, on the average, doubled their length 

 during the first year, but owing to the artificial conditions they assumed a peculiar form, 

 which was described roughly as barrel-shaped. At the end of the second year the 

 increase in size was very slight. The greatest growth observed during the entire period 

 was three-eighths inch; the least, one-eighth inch. Lot B gained from one-sixteenth to 

 five-sixteenths inch the first year ; at the end of the second year the total gain in length 

 was from one-eighth to three-eighths inch. Lot C did least well of the three. During the 

 two years five out of the seven specimens died. Of the remaining two, one showed an 

 increase of one-eighth inch in length, while the other exhibited no growth at all. 



The above experiments were performed under such artificial conditions that the 

 results appear to be of little value. In order to find out the actual rate of growth under 

 normal conditions, the author selected five groups of mussels which were located in 

 different environments and situated where they were least likely to be disturbed. Five 

 specimens were selected from each group and measured for three successive summers. 

 During this period two of the experimental groups disappeared. The three remaining 

 ones, however, represent a variety of situations and show some interesting results. 



Table 1 shows the record of specimens kept on Pine Island, Woods Hole, Mass. 

 They were firmly attached by byssal threads to a rocky bottom which was kept per- 

 fectly free from mud by a very swift tidal current that swept over it. The shellfish 

 were so situated that they were exposed at low tide for a period of from two to three 

 hours daily. 



Table i. — Rate of Growth of Mussels on Pine Island, Woods Hole, Mass., in Millimeters. 



Table 2 represents the growth of mussels which were attached to one of the piles 

 of the Government wharf at Woods Hole, Mass. They occupied a position slightly 

 below the level of low-tide mark and were exposed only at very low tides. A moderately 

 strong current of water which was rich in food materials flowed over them. Slack 

 water prevailed for about three hours each day. In this situation it will be observed 

 the mussels grew more than twice as fast as those on Pine Island. 



