142 CEYLON PEARL OYSTER REPORT. 



(1.) The local abundance of microscopic food. This was shown by the richness 



of the plankton in our tow-net gatherings. 

 (2.) The absence of competing organisms, such us those that are present in 



quantity on coral ground. 

 (3.) The impossibility of overcrowding, as there are no extensive sheets ot rock 



emerging from the sand, and therefore the pearl oysters cannot, at these 



localities, form a continuous layer almost hiding the underlying rock, as they 



do in some places. 



These are all important points, especially perhaps the last. The oysters on these 

 more favoured grounds, although abundant enough, are not overcrowded, but lie in 

 separate bunches or little isolated groups. Sometimes, as on the Periya Paar Kerrai, 

 the foothold consists of little patches of flat rock (see fig. 36, p. 109), 3 or 4 feet in 

 extent and separated by sandy tracts 2 or 3 yards in width. At other places, as on 

 the South Cheval, little or no rock appears on the bottom, and the oysters are 

 attached in bunches to the scattered natural " culch," fragments of dead coral and of 

 calcretes, broken shells and Lithothamnion masses, lying on the sand. A certain 

 amount of sandy bottom, interspersed with harder tracts, and with plenty of " culch'' 

 natural or artificial scattered over the sand, is probably the most favourable 

 ground for the Ceylon pearl oyster from the point of view of those interested in the 

 fisheries. The presence of more culch in many places would, however, be an 

 improvement. It would give more foothold for new broods of oysters, and at the 

 same time woidd probably help to stiffen the shifting sand and prevent, in some 

 degree, the wash-out and turn-over of the surface which is caused by a heavy swell. 



The reliable paars which produce less rapid growth and are characterised by their 

 stunted oysters (the " Koddaipakku " variety of the divers and the Inspectors' 

 reports) are the Muttuvaratu, much of the West Cheval and the Dutch Modragam. 

 These are all localities with great stretches of continuous rocky bottom which may be 

 covered, during favourable seasons, by myriads of pearl oysters closely crowded 

 together. Thus Captain Donnan estimated, in 1902, that on the sandy East Cheval 

 (11,804,676 acres) there were 74,41 3, 000 pearl oysters, while on the rocky West 

 Cheval (10,500,000 acres) there were 123,357,600, showing a much denser population 

 per acre in the latter case, but not equalling the density on the Muttuvaratu Paar, 

 where an area of rather less extent (10,206,725 acres) supported in March, 1902, the 

 enormous total of 277,000,000 pearl oysters which have since suffered great losses, 

 due no doubt to their overcrowded condition. 



2. Shell- Repair. 



Pearl oysters have considerable recuperative power after injury, and can usually 

 repair damage done to the shell. This is of importance from two points of view, first 

 in connection with the ravages of some of their natural enemies, and secondly, because 



