SEA MUSSEL MYTILl'S EDULIS. 203 



average increase in length made by 2-inch specimens, between March 30 and November 

 15 of the same year, is nine-sixteenths of an inch. 



It appears, therefore, that in both Europe and America the growth rate of mussels 

 under favorable conditions amounts to 1 inch annually for the first two years, after 

 which the rate decreases gradually in the third year and rapidly thereafter. However, 

 as will be described later, the transplanting of old individuals which for a long time have 

 exhibited no growth to a new environment will cause rapid growth to start again (figs. 

 192 and 193). 



FOOD OF THE SEA MUSSEL AND ITS SIGNIFICANCE. 



In a former paper (Field, 1911) the author stated that the food of the mussel con- 

 sisted of microscopic plants and animals and gave a list of 29 species of diatoms and 9 

 species of Protozoa which were found in the stomachs of mussels taken in the vicinity 

 of Woods Hole, Mass., In the light of more recent research, which is discussed later in 

 this section, his attention was turned to the fact that the food organisms mentioned do 

 not form more than one-half of the bulk of material actually ingested and that the re- 

 maining matter, which was considered not worth reporting, may be of prime importance 

 in the nutrition of the mussel. Other observers also have noted the same conditions in 

 the stomachs of different species of shellfish, and likewise also have disregarded the mud- 

 like contents as of no food value. Lotsy (1893), who examined the stomach contents of 

 oysters, reported that he found in addition to the diatoms " a quantity of decaying organic 

 matter at least equal in amount." Quahogs, soft clams, and ribbed mussels living in 

 the same vicinity as the oysters had the same proportion of things in their stomachs. 

 He asserted, however, that the decaying organic matter went through the alimentary 

 tract unchanged. Moore (1913), discussing the food of the oyster, states: 



It appears that finely divided organic debris, or detritus, which constitutes the major part of the 

 material ingested, plays a more important role in the oyster diet than has been conceded. 



In the stomach of the mussel this detritus is present in relatively the same propor- 

 tion as has been reported for the oyster. Figure 194 is a photomicrograph of a sample 

 of plankton tow taken over a mussel bed on Pine Island near Woods Hole, Mass., August, 

 1915. Figure 195 is a photomicrograph of the stomach contents of a mussel on this bed. 

 A comparison of the two pictures shows that the mussel feeds exclusively on fine particles 

 of detritus and the smaller plankton organisms. The larger organisms and those with 

 long spinous processes, as well as the coarser particles of decaying organic matter, are 

 excluded almost entirely from its diet. The feces are discharged in flat ribbonlike seg- 

 ments of varying lengths, which are shown somewhat enlarged in figure 196. After 

 lying on the bottom a few hours they fall apart into the separate fine particles of which 

 they are composed. These are shown in the photomicrograph, figure 197. It reveals 

 the diatom shells empty and, in most cases, finely broken and the detritus ground to a 

 fine powder. 



In an attempt to determine the daily quantity of material ingested by the mussel, 

 measurements were made of the volume of feces cast off each day by a group of 3-inch 

 mussels which were thoroughly washed and placed in a trough having a clean, white 

 bottom. Sea water to a depth of 4 inches was kept flowing over the shellfish, but the 

 current was not permitted to become swift enough to carry away the excrement. Each 



