SEA MUSSEL MYTILUS EDULIS. 219 



string of the mussels and fly off with them, but they never succeeded in carrying their 

 burden more than a hundred yards without dropping it. The gulls would then return 

 and make other attempts to carry them off. Laughing gulls, Larus atricilla, which were 

 also present on the beds, were never seen to pick up mussels, but it may be that they 

 were feeding on the very young ones, which at the time of the observations were from 

 5 to 8 mm. in length. The author was not able to kill any of the birds to examine their 

 crops for shellfish, but judging from their behavior it is reasonable to assume that they 

 were feeding upon mussels. Black ducks, Anas obscara, were several times seen on the 

 Menemsha mussel beds feeding over places where the young shellfish were particularly 

 abundant. L. L. Dyche {in Field, 1910a, p. 165), at the New York meeting of the 

 American Fisheries Society in 1910, said: 



The eider ducks eat the small ones, about an inch in length, and you will find the ducks oftentimes 

 full of these mussels clear to the throat; I do not believe I would be exaggerating if I should say there 

 was a pint in each. 



MAMMALS. 



Mammals of various sorts depend upon the mussels as a source of food. The common 

 gray rat, Mus decumanus, and the muskrat, Fiber zibelhicus, often eat them. Ingersoll 

 (1887) states that seals, especially young ones, feed largely upon the Arctic mussel, but 

 that the mammal which preys most extensively upon them is the walrus. According 

 to Mr. Dyche: 



They constitute the sole food of the walrus. The walrus crushes and spits out the shell and swallows 

 the mussel. I have killed from 24 to 30 walruses and have found in the stomach on occasions a ball 

 containing two qu arts of pieces of shell and other material from sea mussels. Seals eat squid and small 

 fish, but the only thing that the walrus feeds on in the north is the sea mussel. 



PASSIVE ENEMIES. 



The passive enemies include a large number of plant and animal forms which do not 

 attack the mussel directly but by their habits intercept the currents, causing deposition 

 of silt, which interferes with the nutrition of the mollusk ; or they may so envelop the 

 shellfish as to cut off their food supply and even suffocate them; or they may come 

 into direct competition for the food substances in the water. 



EELGRASS. 



Eelgrass, Zostera marina, is one of the most destructive weeds which grows in 

 profusion on the sheltered beds. It not only intercepts the currents which bear the food 

 supply of the mollusk but causes very often such a heavy deposition of silt that the 

 mussels are smothered or even completely buried beneath it. Their decomposing 

 bodies then form the richest kind of fertilizer on which the eelgrass thrives. 



ALGjE. 



Alga? of various species are oftentimes present in great abundance on the beds or on 

 the mussels which encrust wharf piles, buoys, etc. The most common species found 

 associated with the mussels are Fucus vesiculosus, Laminaria saccharina, Chorda filum, 

 Champia parula, Enter omorpha crccta, Rhabdomia tcnera, and Ulva lactuca. 



