GENKKAL SUMMARY AND RECOMMENDATIONS. 



121 



disappearance, from the Muttuvaratu Paar, of 72 million oysters, one year old in 

 L897, before 1899 was probably due to this cause; and also the sweeping away, by an 

 exceptionally strong current, of the oysters on the north end of the East Cheval Paar 

 between November, 1887, and February, 1888. 



(II.) Next in importance come, we consider, the ravages of natural enemies, the 

 most noteworthy of which are: 



(a) Voracious fishes, chiefly rays (Rhinoptera javanica and other allied species) 

 and file-fishes {Balistes mitis, B. stellatus, &c). 



(b) Boring Gastropod Mollusca, chiefly Sistrwm spectrum and Pinaxia coronata, 

 along with species of Xassa, Mitre.?, Purpura, and Turbinella. 



(e) Boring Sponges (Cliona margaritiferoe). 



(d) Boring worms (Polydora horneUi). 



(e) Starfishes, chiefly Pentaceros linchi, P. nodosus, and Luidea maculata. 



(f) Smothering Lamellibranch Mollusca, such as Modiola barbata, the " Suran," 

 which weaves nests and other entanglements around masses of young oysters, and 

 may, when present in quantity, cause serious mortality. 



(<j) Crabs and cuttle fishes, and possibly other animals also, which can tear off the 

 byssus and crush the shell. 



(It) Associated animals, such as Corals, Barnacles, and Sponges, adhering to the 

 shell, which, mechanically or by competition for food, cause injury and even death. 



A few of these natural enemies call for some further remarks. 



The file-fishes (several species of Balistes) and also the " Vellamin " (Lethrinus, spp.) 

 feed upon immature oysters. We have found the broken shells in the stomach ; but 

 although these fish frequently snip pieces 

 out of the margins of quite large shells, 

 they probably do not destroy adult 

 thick-shelled oysters. Shells which are 

 rendered rotten by the borings of the 

 sponge Cliona fall an easier prey to all 

 oyster-eating fish. 



The larger Elasmobranchs, such as 

 the Eagle-Rays, and allied forms, may 

 cause very serious reduction in a bed of 

 mature oysters. In 1903 Mr. HoRNELL 

 found large rays feeding on the Periya 

 Paar Karai, and, on exploring the bottom in a diver's dress, obtained abundance of 

 the crushed shells left by the rays. Shells broken up by these fish have a peculiarly 

 cracked and splintered appearance, which is characteristic (see fig. 9), the fragments 

 of the brittle nacre being held together by the tougher prismatic margin. 



In regard to these various fish enemies of the oyster it is necessary to bear in 



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Fie. 9. 



Fragment of pearl-oyster shell crushed 

 by the teeth of a large Kay. 



