174 MADRAS FISHERIES BULLETIN VOL. XlV, 



are on, some individuals with ripe gonads can always be found ; 

 hence there is no definite and universal spawning season as in the 

 case of English oyster (O. cditlis). The mortality of the oysters 

 tkiring the rains is confined to those living well within the back- 

 waters ; those close to the mouth suffer less and many survive, 

 except in exceptional years when the floods are very long continued. 

 Much, too, depends on the physical conformation of the backwaters. 

 In such an one as that at Cochin, the oysters on the betls in the 

 harbour usually survive, the considerable (relative) depth of water 

 over the beds constituting a protection. 



On the Coromandel Coast, as the floods take place generally 

 from the end of August until November, the season runs from 

 January to the middle of August, with a partial break about April 

 when a large proportion of oysters spawn. August and September 

 constitute a spawning maximum, induced in part by the hot weather 

 prevailing then and in part by the lowering of the salinity due to 

 flood water: in a wet season the latter is the main factor; in 

 exceptionally dry ones or when the rains are long deferred, the 

 former in the effective factor. A second maximal spawning some- 

 times takes place in March and April, and between this time 

 and August, spawning individuals can always be found. 



The reproductive habits of this oyster are similar to those of 

 the Indian pearl oyster {Margaritifera vulgaris) and closely akin 

 to those of the Portuguese oyster {Ostrea angiilata). In all these 

 species the sexes are separate ; the ripe ova and spermatozoa are 

 poured forth free into the surrounding water when a sufficient 

 stimulus is experienced. Fertilization takes place in the water 

 outside the parents' bodies, trochosphere and veliger forms being 

 in turn assumed during the growth of the fertilized ova. The 

 free-swimming stages last for several days and in this time they 

 may be carried by currents for considerable distances, thus ensuring 

 wide dispersion throughout the neighbouring coasts. The free- 

 swimming spat when ready to attach is easily obtained in any 

 suitable estuary or backwater by the employment of spat collectors ; 

 experience shows that ordinary country roofing tiles, arranged in 

 low piles of several rows, are the most suitable form to use. During 

 our first experiments at Pulicat, we arranged these collectors with 

 a view to catch the spat produced during the September spawning 

 season; experience shows that it is preferable to aim at obtaining 

 the necessary spat during spring (April-May) as the parent 



