340 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
III.— ABSENCE OF OYSTER SPAT IN DEEP WATER. 
The shells of exposed Oyster ledges are usually well adapted for collecting spat, 
especially those clusters that immediately fringe the stream. A few feet below the 
level of low tide the appearance of spat becomes unusual, and still more unusual as 
the water gradually deepens. The question of why oyster spat is absent in deep water 
is of practical importance to oyster-culture in this region. To determine the cause 
would settle definitely the method to be pursued in the collecting of young oysters 
and thus prevent a possible waste of labor and capital. Some of the more important 
reasons assigned to this absence of the young in the deeper water are as follows : 
(1) The extreme density of the water preventing the swimming embryos from 
sinking at the fixative stage; a suggestion of Lieut. Robert Platt, U. S. A., command- 
ing the steamer Fish Hawk. 
(2) The extreme softness and film-covered character of the bottom preventing the 
oyster from fixing. 
(3) The suspension of slowly depositing silt in the deeper water, whose clogging 
action is fatal to the delicate respiration of the microscopic young. 
(4) Changes in the composition of the oyster-bed water, either in its salts or food 
constituents, at different levels. 
If the view of Lieut. Platt be established, the process of spat-collecting will be a 
simple one for the culturist. If the silt and mud theories be correct, the importance of 
establishing the relation of the size of the seed to be planted successfully to the char- 
acter of the bottom is a vital one. If the water variations at different depths are at 
all marked, showing that oyster food is greatest in amount near the shore in shallow 
water, and that there, too, occur the best conditions of saltness and liminess of the 
water, a valuable suggestion is gained. It will likewise be of value to determine 
whether all or some of these possible causes act in concert. 
1. Density of water preventing oyster fry from settling . — It can be shown most clearly 
that South Carolina oysters, living in a water density of 1.017 to 1.024 specific gravity, 
occur only on raccoon reefs between the extremes of high and low water marks. At 
low water it is interesting to note how clearly the stakes and piling define this oyster 
belt, as we have seen in Plate lxv. The region of mid-tide mark shows the greatest 
luxuriance of growth ; the oysters decrease gradually in size and number above and 
below, becoming straggling and finally disappearing. Occasionally, as the pile is 
eaten by teredos, the oysters drop to the bottom and become of the “dropped-off” 
raccoon type. If the proposition be proven that the embryo at the fixative stage can 
not sink in water of high density, but must form raccoon ledges, some corollaries must 
naturally follow : 
(1) Spat should in no instance be found in deep water of high specific gravity. 
(2) As the water becomes fresher, the oysters, with spat of undeniably recent 
attachment, as in the North, should be found in favorable localities covering the bot- 
tom; and, accordingly, 
(3) As the water becomes less salt the raccoon ledges should gradually and 
entirely disappear. 
Of course, the only indisputable proof would be to fertilize eggs, carry the embryos 
as far as possible into the swimming stage, and by actual experiment show that the 
