14 MADRAS FISHERIES BULLETIN VOL. XI, 



the lighter gravity of flood water causing it to pass over the deeps 

 without clearing out completely the saline water which was there 

 before the onset of the floods. Spawning in the case of these few 

 surviving oysters appears to be deferred till about the end of August 

 when the floods have partly subsided and tidal conditions are re- 

 established. Much variation exists however in this cycle of events, 

 for under normal conditions the common Indian oyster is a most 

 irregular spawner; except when the floods are on, some individuals 

 with ripe gonads can always be found ; hence there is no definite 

 and universal spawning season as the case of the English oyster 

 {0. ediilis). Mortality amongst oysters during the rains is confined 

 to those living well within the backwaters ; those close to the mouth 

 sufl'er less and many survive, except in exceptional years when the 

 floods are very long continued. Much, too, depends on the physical 

 conformation of the backwater. In such a one as that at Cochin, the 

 oysters on the beds 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 

 December to the middle of August, with a partial break about 

 April when a large proportion of oysters spawn. August and 

 September constitute the main spawning maximum, induced in 

 part by the hot weather prevailing then and in part by the lowering 

 of 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 is the effective factor. A second maximal 

 spawning 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 ( Mnr,^aritifcr(i viil,i>'/ii'is) and closely akin to 

 those of the Portuguese oyster f'O.s/nv/ (Uigulata). In all these species 

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

 forth freely into the surrounding water when a sufficient stimulus is 

 experienced. Fertilization takes place in the water outside the 

 parents' bodies, trochophore and veliger forms being in turn as- 

 sumed 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 back- 



