160 
rinse.” As hand rinse doubtless contained sweat 
and as sweat produced a repellent effect on 
sharks, it was decided to investigate their re- 
sponse to L-serine. 
Three tests were conducted at the Hawaii 
laboratory on the tiger shark in Pond 2, during 
the winter of 1959-60, using 1.0 gm of L-serine 
per test. In the first, there was no overt response 
except an obvious sensing on encountering the 
material. In the second, the shark showed a 
sensing of the material, followed by rapid exits 
from the area and violent head shaking. In the 
third test, there was no noticeable response. In 
all three, however, the time spent in the test 
area during test conditions was less than during 
controls, as had also been the case with sweat. 
Three tests were conducted on blacktip sharks 
at the Eniwetok laboratory in I960, using 6 
ml of a solution containing 1.0 gm of L-serine 
(i.e., 0.12 gm per test). No repellent effects 
were noted other than a "wariness” in one test. 
Again, however, the time spent in the test area 
during test conditions was less than during con- 
trols. 
As definite repellent results had been noted 
in one test with the tiger shark, it was decided 
to run a third series at the Hawaii laboratory 
during the summer of 1961, using much larger 
quantities of L-serine despite its high cost. Three 
tests were conducted on the tiger and grey 
sharks co-inhabiting Pond 5, using the "curtain- 
drum” technique. 
In all three tests the tiger shark displayed an 
aversion to the chemical but only after intro- 
duction of the material had been completed. In- 
troduction required about 10 min (three to 
four 3-min periods). It seemed either that the 
response occurred after a threshold concentra- 
tion of the material had been reached, or that 
there was a latent period between exposure to 
the material and response. The overt response 
was a violent head shaking either while In or 
while leaving the area of concentration. At 
times this took place at the surface and caused 
considerable splashing. However, in only the 
first test (25 gm L-serine) was there frequent 
rapid exit from the area on encountering the 
material. In the second test (50 gm L-serine) 
there was swerving and head shaking after en- 
countering the material, but no turning-back 
on initial encounter. The response was less pro- 
PACIFIC SCIENCE, Vol. XVII, April 1963 
nounced than in the first test even though twice 
the quantity of material had been used. This 
may have been due to a higher tide and thus 
a larger volume of water and greater dilution 
of the material despite the larger quantity used. 
The shark frequently avoided the material by 
swimming in the shallows on the far side of the 
pond. In the third test (25 gm L-serine plus 
10 ml of a 99% nicotine solution), the tiger 
shark again displayed agitation and head shak- 
ing. The response, however, occurred less fre- 
quently than in the other two experiments. The 
nicotine had been added in the hope of a syn- 
ergistic effect; it had been our impression that 
the sweat of smokers was more repellent to 
the sharks than that of nonsmokers. Possibly 
it tended to inhibit rather than Increase the ef- 
fect of L-serine. 
The grey sharks, in contrast to the tiger shark, 
were not obviously agitated by L-serine; no head 
shaking or gill flexing was observed. In the first 
test, two grey sharks of the same species veered 
sharply on first encountering the material and 
returned to the end of the pond. Thereafter all 
three grey sharks circled in the end zone for 
the duration of the experiment. In the second 
test no veering was noted but there was re- 
peated circling in the end zone. It was uncertain 
whether this could be interpreted as a repellent 
effect, for the same habit was noted occasionally 
during control periods. In the third test, all three 
grey sharks passed through the test area without 
signs of awareness, agitation, or repulsion. 
Although there is no doubt that the tiger 
shark was actively repelled by L-serine, the phys- 
iological mechanism producing the response is 
unknown. We can offer no satisfactory expla- 
nation of the difference in response of the tiger 
and the greys to L-serine. It may have involved 
species differences in physiological effect or 
differences in the concentration of materials 
to which they were subjected. The latter is 
possible even though the tests were conductd 
simultaneously on the tiger and the greys, for 
uneven curtains of material were formed by 
tidal currents in all three tests and the shallow- 
swimming tiger shark may have encountered 
different concentrations than the deep-swim- 
ming grey sharks. It may be added, however, 
that directly opposite results were obtained with 
a highly irritating lachrimator which is presently 
