472 
BULLETIN OF THE BUREAU OF FISHERIES 
EFFECT OF ANGLE OF SURFACE 
Practical oyster growers have often noticed that most of the spat are to be found 
on the under surfaces of shells or other objects in the water. The question therefore 
arose as to whether this was due to some specific reaction of the mature larvae or 
merely to the fact that sedimentation and growth of algae and other organisms on 
the upper surfaces ordinarily prevent attachment of larvae. The opinion is frequently 
encountered that larvae seek the shadows and migrate to the relatively dark under 
surfaces. In order to determine, in the first place, whether larvae actually attach 
more abundantly to lower surfaces even when upper surfaces are equally clean an 
experiment was performed which provided various angles for comparison. Some of 
these results have already been published (Hopkins, 1935). 
Wire frames were made of galvanized hardware cloth of %-inch mesh, each frame 
holding three 8- by 10-inch panes of clear glass 1 inch apart and parallel. Some of 
the frames were designed to hold the panes in either a vertical or a horizontal position, 
others were so constructed that the panes were held at an angle of 45°. Thirty plates 
were used in each of the three posi- 
tions, horizontal, vertical, and at 
the 45° angle. These were placed 
in the water of a dike at low tide on 
one day and removed the following 
day when the dike was exposed. 
They were in the water only 24 % 
hours and may be considered as all 
equally clean since the time was 
too short for any considerable 
amount of fouling. Half of the 
vertical and 45° plates were placed 
perpendicular to the general direc- 
tion of tidal flow, the other half 
parallel, in order to indicate the 
effect of current. 
After removal from the water the plates were allowed to dry, removed from the 
frames, and the number of spat caught on all surfaces carefully counted with a binocu- 
lar microscope. In the analysis of the results the different angles are referred to as 
follows: 0°, under horizontal; 180°, upper horizontal; 45°, under, and 135°, upper, 
surfaces of the 45° panes; and 90°, vertical. In table 18 the results are presented in 
detail. The effect of the angle of the surface on its efficiency as a collector of spat is 
further illustrated in figure 21, in which the average values in table 18 are plotted 
graphically. The correlation between angle and number of spat caught is remarkably 
close, when it is considered that in each case, except for the vertical plates, the area 
of surface was only 2,400 square inches. 
It may be noted in the table that the panes which were parallel to the direction 
of flow of tide caught definitely more spat than those perpendicular to the current. 
This would appear to indicate that setting may be proportional to the rate of current. 
Prytherch (1929) noted that larvae of Ostrea virginica set most abundantly on the 
leeward side of objects in the water, where the current is reduced to a minimum, and 
concluded that the heaviest setting takes place when the current is least. The above 
results suggest that the larvae of the native oyster react differently. 
The values represented in the graph (fig. 21) are the totals, including plates which 
were both perpendicular and parallel to the current. It might possibly be a more 
1200 
1000 
600 
2 
cr> 
EC 
£ coo 
£ 
Z 
400 
200 
0 
Figure 21.— Number of spat caught per 2,400 square inches of glass sur- 
face at different angles: 0°, under horizontal; 180°, upper horizontal; 90°, 
vertical. 
ANGLE OF SURFACE 
