474 
BULLETIN OF THE BUREAU OF FISHERIES 
The table shows that the average number of spat caught on 1,000 square centimeters 
of the egg crate filler is 427, while on the special collector the average is 1,378. This 
value refers to all surfaces, which in the egg crate filler are vertical, and in the special 
collector both vertical and horizontal. It was conclusively demonstrated that the 
special design is more than three times as effective as the standard filler. 
It is to be expected that the difference between the numbers of spat caught on 
horizontal and vertical surfaces would not be as great for concrete-coated paper as 
for plane glass, as described above, since the roughness of the concrete provides a 
large amount of surface which may be at all angles. The vertical concrete wall has 
a large horizontal component in the projecting grains of sand which are large in pro- 
portion to the oyster larvae. Therefore, the values in table 19 do not exactly fit the 
curve obtained with plane glass (fig. 21), but the points fall along a more gently 
sloping curve. The significance of the observation cited above, that flow of water 
along surfaces is necessary for most efficient setting, is well demonstrated by the 
difference between the numbers of spat caught per 1,000 square centimeters of vertical 
surface on the two types of collectors. The special collector is about two and one- 
half times as effective, considering only the vertical walls. 
On most grounds egg crate fillers collect large quantities of silt which fills the 
cells and kills the spat. Frequently only those caught on the upper edges survive. 
In the case of the special collector, however, the water is able to flow through the cells 
and prevent deposition of silt. Even on soft ground only the bottom layer suffers 
a loss as the collector settles into the surface. Such an example is shown in the upper 
photograph of figure 22, while the lower surface of a collector placed on firm ground 
shows little mortality. 
It would appear that the habit of attaching primarily to an under surface has the 
function of protecting the delicate young spat from various unfavorable conditions, 
such as hot sunshine, and deposition of silt. Immediately suggested by the results 
is the possibility that the larvae are photosensitive and react negatively, causing them 
to collect in the shadows where they set. Such a view would imply that setting takes 
place almost entirely during the day, and that at night the larvae would not concen- 
trate on under surfaces. 
To determine whether light is a factor in the setting behavior of larvae, two sets 
of wire frames, each containing 15 glass plates, were placed on an oyster ground so 
that the plates were horizontal and allowed to remain for about 24 hours. The plates 
of one set were painted black on the upper surfaces, the others left clear. Both 
surfaces of each pane of glass were carefully examined with a binocular miscroscope 
and all spat counted. On the lower surfaces of the black glass 435 spat had caught, 
while on similar surfaces of the clear glass 616 were counted. Not one was found on 
the upper surfaces of either group. It is not considered significant that the clear glass 
caught more spat than the black, but it is important that the shadow under the latter 
did not result in any increase in the catch. 
In other experiments, which are described in a later section, it was demonstrated 
that larvae set as well at night as during the day, and that in all cases the lower sur- 
faces of horizontal panes receive almost all of the spat. It is therefore obvious that 
light is not an orienting factor in the setting behavior of larvae of tins species. The 
pigment spots of mature larvae have often been looked upon as possibly sensitive to 
light, but Prytherch (1934) concluded that larvae are not photosensitive, and that the 
pigment spots have an entirely different function. The present results confirm his 
conclusion. 
