4 2 4 ANIMALS INTRODUCED UNAWARES 



which have clung through thick and thin to their hosts 

 during the vissicitudes of transportation. An interesting 

 recent example is furnished by the appearance of the 

 American Slipper Limpet (Crepidula fornicata] in the 

 Thames estuary. The first sign of its presence there was a 

 dead shell found on the shore at St Osyth in 1891, although 

 a fisherman had recollections of the "Crow Oyster" ex- 

 tending back some fifteen or twenty years. In 1893 a living 

 example was found amongst Oysters from the River Crouch, 

 and thereafter records came with ever-increasing frequency, 

 until it was discovered that the Slipper Limpet, from being 

 a rarity, had become a pest. Its numbers on the oyster 

 beds became so troublesome that endeavours were made to 

 eradicate it, a special crushing apparatus being arranged 

 for converting into manure the " Limpets" dredged from the 

 bottom. About 191 1, the Blackwater Fisheries alone yielded 

 35 tons of Slipper Limpets in four weeks ; and since then 

 the multiplication of the alien has been even more rapid, for 

 in twelve months in 1914-15 upwards of 1000 tons, 

 dredged chiefly from the estuaries of the Blackwater and 

 the Coin, were crushed and used for manure by the farmers 

 of the district. The precise relationship between the Slipper 

 Limpet and the Oyster is unknown, but whether the former 

 be a semi-parasite or only a constant messmate, there seems 

 to be little room for doubt that it was introduced with foreign 

 and probably American Oysters, brought for relaying in the 

 oyster beds of the Thames estuary. 



