124 



CEYLON PEARL OYSTER REPORT. 



inconvenience rather than real injury. There are only two classes of cases where the 

 matter may become more serious, (l) where rapidly growing corals and sponges of 

 large size settle on the shell and practically envelop it (figs. 13 and 14) or 



overweight it .to such an extent as 

 to interfere with the movements and 

 nutrition of the oyster ; and (2) where 

 the acorn -barnacles (Balanus amphi- 

 trite, and other species) spread, as 

 they sometimes do, all over the shells, 

 and are so large and active as to 

 compete successfully for the micro- 

 scopic food in the water and so lead 

 to enfeeblement and, it may be, 

 diseased conditions in the pearl 

 oysters. Young barnacles in the 

 Ceylon seas appear to settle down in 



Fig. 14. Pearl oyster enveloped in a Sponge 

 (t'arhychalina spinilamella, DENDY). 



April or May, and then grow with 

 astounding rapidity, so that in a few 



weeks, rocks, shells, boats, stakes, ropes and any other objects in the water become 



covered with an almost continuous layer. The living corals on the surface of the 



shells may also act by depriving their host of food, and we certainly find that the 



Ceylon pearl oyster cannot exist on the living coral reefs or where there is much 



live coral scattered over the bottom. 



The action of the little " smothering " mussel (fig. 15) called " Suran" by the divers 



is also, probably, partly mechanical and partly of the nature of competition for food. 



It weaves its tough byssal threads round neigh- 

 bouring stones and dead and living oyster 



shells, entangling all in a matted mass in which 



it alone appears to flourish. It is, however, 



small and can have no effect upon adult oysters. 



It can only, then, do harm when it gets in large 



quantities amongst a bed of young oysters of its 



own size, and forms a blanket over and around 



them, interfering with respiration and nutrition. 



But it is very rarely sufficiently abundant, in 



Ceylon waters, to cause serious injury. 



Octopod cuttle fishes (such as Polypus herd- 



mani) are abundant on some parts of the banks 



and are well known to subsist on oysters and mussels. Crabs are also numerous 

 and no doubt cause some destruction ; and there may be other members of the 

 associated fauna that play their part in decimating the oyster beds, 



Fig. 15. "Suran," the small mussel 

 (Modiola barbata) that entangles stones 

 and small pearl oysters in its byssus; 

 natural size. 



