St. Andrews Marine Lahoralory. 61 



5. Note on Injuries to Oysters by Boring Forms. 



It is well known that oysters, amongst other attacks to 

 which they ai'e subject, suffer extensively from the burrowiag 

 habits of an annelid {Polydora), as well as from the borings 

 of a sponge [Cliona). Such have been referred to in the 

 * Annals ' by Dr. Hancock and others, as well as the writer. 

 Both forms chiefly attack the upper (convex) valve, which 

 they by-and-by penetrate, rendering its interior unsightly by 

 dark blotches or excrescences. In the case of the Whitstable 

 " native " this is a serious blemish in the interior of the other- 

 wise pearly white valves, the latter feature having been gene- 

 rally looked upon as the distinguishing characteristic of this 

 favoured oyster. It has sometimes been supposed that sucli 

 inroads, both of annelid and sponge, are due to the impurity of 

 the water caused, for instance, by the dredgers of the Thames 

 depositing their debris in the proximity of the beds. While 

 the latter is objectionable for various reasons, a glance at the 

 habits of the chief depredators makes it probable that the 

 cause just mentioned has little to do with the attacks. Both 

 annelid and sponge perform the functions of universal 

 destroyers of dead shells, calcareous and other rocks, even in 

 very pure water, inshore and offshore. At Whitstable * the 

 chief enemy is Polydora, which propagates in the usual 

 manner by eggs and free-swimming larvaj, the latter occurring 

 in great numbers throughout a considerable part of the year. 

 The purer tlie water the greater the probability, therefore, 

 that these larvae will settle on fresh sites and extend the 

 colonies of borers. At the same time Polydora is partial to 

 inshore localities in which a considerable amount of mud is 

 present in the currents. 



It is not easy to get rid of the annelids after they have 

 tunnelled the shells. Even though every oyster were to be 

 treated in a solution, it is clear that what would kill the 

 annelid would also endanger the oyster. It is satisfactory, 

 however, to know that while the marks on the interior of the 

 valves are unsightly, they do not interfere with the qualities 

 of the oyster as food, nor should they be an objection to the 

 serving of the mollusk in the convex valve, as now obtains in 

 London and elsewhere. 



• I have to thank Mr. Sibert Saunders for specimens of these. 



