186 
shown, it seems that at least the final phase of 
the approach was visually directed. 
When the shark selected the wood, the ob- 
ject was either simply bumped or briefly taken 
into the mouth and then rejected. No teeth 
marks were found on the wood following the 
test. In all cases the fish-baits were carried away. 
Only grey sharks were involved. 
If we accept as fact that these sharks orient 
visually during the final phase of their approach 
to a motionless prey, we must then question the 
nature of the stimulating visual image, bringing 
us to the subject of visual acuity. 
Most elasmobranchs reportedly possess an all- 
rod retina with a high ratio of visual cells to 
ganglion cells, which provides for low visual 
acuity. While this would presumably result in 
an inability to see objects in detail, the sharks 
would be able to utilize their visual sense under 
conditions of relatively little light (Gilbert, 
1961). Kato (1962) found no evidence of cones 
in the retina of blacktips or whitetips. Perhaps 
even more significant, he found no specialized 
area, corresponding to the human fovea, which 
is generally assumed to be associated with visual 
acuity. 
At Engebi, we often noted that sharks at- 
tracted by a dead fish suspended on a line often 
struck objects in the immediate vicinity of the 
bait before taking the bait itself. A rock about 
IV 2 ft square, which was situated in the middle 
of a sandy area below the baits, as well as float- 
ing seaweed and a lead weight on the line a 
foot above the bait were often tested with a 
nudge or by actually being taken momentarily 
into the mouth. These observations suggest one 
of two alternatives: (1) these sharks lack the 
visual acuity necessary to distinguish the visual 
appearance, i.e., form, markings, etc. of fish from 
inedible objects in the immediate vicinity; or 
( 2 ) although these sharks do possess the visual 
acuity necessary for such a distinction, it was not 
utilized by them in this situation. 
Tester and Kato (ms) showed that small 
blacktips and greys have the ability to discrimi- 
nate between a number of objects of different 
shape, e.g., squares, triangles, and rectangles, 
and that they make this discrimination from a 
distance of at least 6 to 12 ft. These species then 
possess at least the degree of visual acuity re- 
quired to discriminate between these forms. 
PACIFIC SCIENCE, Vol. XVII, April 1963 
The following experiment was an attempt to 
detect evidence of the utilization of a high de- 
gree of form discrimination ability in the feed- 
ing behavior of these sharks. 
Experiment VIII 
This experiment was designed to determine 
whether or not the visual cues offered by a fish 
through its characteristic morphological features 
are significant attractive visual stimuli to these 
sharks. 
Two fresh dead fish were selected for each 
trial, both being of the same species, but one 
a little larger than the other. Three species were 
used: goatfish (Mullidae), 8 trials; surgeon fish 
( Acanthuridae), 3 trials; and squirrel fish (Ho- 
locentridae), 10 trials. The larger fish was then 
decharacterized by removing the head and all 
the fins, reducing it in size to approximately 
that of the smaller, still natural-appearing fish. 
A long, deep incision was then made along the 
ventral side of the natural-appearing fish to in- 
sure that olfactory stimuli emitted by both baits 
were essentially the same. The baits were both 
secured to a single line, one spaced approxi- 
mately 18 inches above the other, with their 
respective position being reversed on alternate 
trials. After assurance that no sharks were vis- 
ible, the baits were suspended motionless at a 
point midway between surface and bottom in a 
strong current. Observations were made from 
the cage and a record was kept which bait in the 
pair was taken first, along with the species of 
shark involved and the nature and direction of 
approach. 
It was assumed that if the characteristic mor- 
phological features which had been removed 
from one of the pair were significant as attrac- 
tive visual stimuli, then the natural-appearing 
fish would be favored in the initial choice made 
by the sharks. This in turn would demonstrate 
a high level of form discrimination ability in 
these sharks. However, no apparent preference 
was shown to either of the two bait types in 
21 trials. The decharacterized fish were hit first 
11 times, while the natural-appearing fish were 
hit first 9 times. Once both baits were hit simul- 
taneously. The relative positions of the baits on 
the line had no apparent effect, with the top 
bait being hit first 9 times and the bottom bait 
