F. S. Wright 145 



In the branchial chamber of large mussels there is frequently present 

 a small crab {Pinnotheres pisum). An examination of the stomach 

 content of several of these crustacean "messmates" of Mytilus revealed 

 the presence therein of the diatoms and other materials^ utilisable by 

 the mussel itself as food, but diverted by the crabs to their own use. 

 I have remarked that it is never (or very rarely), found within the 

 valves of the seed muscles, which, as we have seen, are poorly nourished. 

 This is significant, because, as they are frequent in mussels from the 

 fishing grounds, it would appear that the partnership is a profitable one 

 from the crab's point of view. I have noticed that, in certain-cases, 

 where the female Pinnotheres had attained a relatively large size, it 

 would seem to have exerted considerable pressure on portions of the 

 mantle lobes, and upon the shell beneath. In some instances, this 

 pressure had caused the nacreous or pearly layer to be dissolved away, 

 probably by the action of pathogenic matters secreted b}^ the cells of 

 the mantle at the points aft'ected. Thus, the presence of Pinnotheres 

 (although it may not be parasitic in the strict meaning of the term), is 

 probably more or less harmful to its host. By fishermen and dealers 

 alike, 5a?a/m5-encrusted mussels are regarded" with great disfavour, 

 "because, by rendering the appearance of the shellfish unsightly, their 

 market value is considerabl}^ reduced. My own observations tend to 

 show, however, that, so far from being an unmitigated nuisance near 

 mussel-beds, Balanus may actually prove of some benefit to young 

 mussels. The spat tends to settle (or, rather, to remain) on stones, etc.,- 

 which are encrusted by numbers of Balanus, while other stones in the 

 vicinity, although to all appearance equally suitable, are frequently 

 barren. A Balanus-QOvexeA surface certainly offers a good attachment, 

 and I have frequently remarked young Mytili growing up and thriving 

 actually within the 'parapet" of Balanus, the original tenant of which 

 had died. I have no doubt that large numbers of larval Mytili receive 

 their start in life through the agency of the Balanus colonies. The 

 nauplius and cypris stages of Balanus larvae are not uncommonly 

 present in the stomach content of Mytilus, and they probably form a 

 not inconsiderable part of its food in spring and early summer. The 

 Common Starfish {Asterias ruhens, L.) is a notorious enemy of the mussel. 

 It is occasionally found on the seed beds at Aberdovey, and elsewhere 

 in Cardigan Bay, but, being anything but numerous in these situations, 

 it cannot be classed as a serious pest. 



» The Food Value of Sea Mussels, by I. A. Field, Government Printing Office, Wash- 

 ington, U.S.A. 



