l46 Musael Beds; their Productivity and Maintenance 



During my examination of the stomach content of Mi/tilus, I found 

 that Nematodes were present in a large number of individuals. The 

 matter is being investigated, and these thread worms may prove to be 

 true parasites. 



The above will suffice to show that, as regards animal foes, the amount 

 of damage wrought in the Cardigan Bay mussel areas is inconsiderable. 

 Indeed, their activities may be regarded as positively beneficial in their 

 effects, inasmuch as they serve to thin the crowded population of the 

 scars to some small extent. 



C. L. Walton^ has found that mussels adherent to vertical rock faces, 

 piles, etc., break away and fall to the ground when they attain a length 

 of 2-i" to 3|", and a weight in correspondence. It is suggested that ; 

 this happens only where the mussels are subjected to frequent changes 

 of medium, and not when alwa3^s bathed in water. 



Seaweed, when present in quantity near mussel beds, is often the 

 cause of a high rsrte of mortality among the shellfish, which are apt to 

 become smothered beneath masses of it. " Eel Grass" (Zostera sp.) and 

 the commoner Ulvae may be cited in this connection. This source of 

 danger, again, is rarely or never serious in Cardigan Bay. 



Industrial pollution of rivers and estuaries may sometimes affect 

 mussel beds in an adverse manner. At the present time, I am investiga- 

 ting the probable effects of plumbism on mussels from Aberystwyth 

 Harbour, 99 % of which show a singularly uniform malformation in 

 shell growth. 



The greatest calamity to which mussel beds are subjected, how- 

 ever, is that of becoming silted over by shifting sand. Whole beds are 

 often destroyed in this manner, as referred to in the foregoing. 



Sand is also important in this connection in quite' a different way. 

 In August 1917, I received a sample of fairly large mussels from the 

 entrance to the Harbour at Pwllheli. The shellfish were fairly well 

 fleshed, but extraordinarily light in weight. This was owing to the fact 

 that the attrition of water borne sand and shingle had worn away a 

 large part of the two outer shell layers. Practically, only the inner- 

 most (pearly) layer remained to protect the animal at the anterior end, 

 and, although this was relatively thick, it is improbable that, in such 

 conditions, many individuals would survive for any great length of 

 time. 



' 7'lie Food Vnhie of iScii Mussels, by 1. A. Field, GoveiJinu-nl i'liiiling OUice, W'asli- 

 iuiiton. U.S.A. 



