SEA MUSSEL MYTILUS EDULIS. 215 



ENEMIES AND PARASITES. 



The sea mussel, as are all the smaller marine species of animals, is preyed upon by 

 a host of enemies. The destructive forces with which it has to compete are so numerous 

 it seems almost incredible that the species can maintain itself so successfully. From the 

 moment the egg is laid to the end of its life, dangers of various sorts threaten it constantly 

 from every side. Inanimate as well as animate forces unite in working toward the destruc- 

 tion of this mollusk. The animate forces which act against the life and welfare of the 

 mussel may be divided into the active enemies, which include the predacious animals, 

 and the passive, which comprise a number of sedentary organisms that intercept the 

 food supply or cause depositions of silt which interfere with the digestive processes or 

 smother the mollusk. 



The following account does not by any means include all the enemies of the sea 

 mussel. It serves merely to show some of the tremendous forces with which the 

 species has to contend in order to maintain itself. 



INANIMATE DESTRUCTIVE FORCES. 



A slight change of current may cause a deposition of sand over the beds which 

 may be acres in extent and smother the mussels out of existence. Some years ago such 

 a wholesale extinction by this agency took place in Menemsha Pond, Marthas Vineyard, 

 Mass. The bed, a photograph of which was published in a previous paper of the author 

 (Field, 1911), was in perfect condition in August, 1911, but when visited in July, 1912, 

 nothing but a barren flat of white sand was visible at low tide. Investigation revealed 

 the presence of the decaying shellfish about 4 inches below the surface. Exposure at 

 low tide to the frost of winter proves fatal to enormous numbers. The young larval 

 mussels succumb to sudden falls of temperature and are often swept up on the shore 

 by winds, waves, and tidal currents to perish by the millions. 



ACTIVE ENEMIES. 

 STARFISH. 



The starfish Asterias forbesii and A. vulgaris, are the arch enemies of the sea mussel. 

 The former species ranges from Massachusetts Bay southward to the Gulf of Mexico 

 and is abundant south of Cape Cod. A . vulgaris ranges from North Carolina to Labrador, 

 but is abundant only north of Cape Cod. The starfish feed upon almost any kind of 

 mollusk, but the sea mussel constitutes their favorite food. The method of feeding 

 is to seize the shellfish in such a position that the mouth of the starfish comes to lie 

 opposite the opening of the shell (fig. 203, opp. p. 216). Then by attaching its numerous 

 tube feet to the opposite valves it sets up a constant pull, which in the case of a large 

 starfish has been shown by Sehiemenz (1896) to equal more than 2 ) i pounds. The 

 starfish can rest by shifting its work from one set of muscles to another, while the 

 mussel, relying only on its single set of adductors, becomes exhausted and succumbs 

 to the weaker but tireless pull of the enemy. When the valves open, the starfish turns 

 its stomach inside out and envelops it about the soft parts of the prey and digests 

 them outside its own body. This accomplished the starfish withdraws its stomach 

 and moves on in search of another victim. Young starfish, especially, have voracious 



