Olfaction and Sharks — Tester 
169 
Fig. 13. Activity index during successive 2-min periods, illustrating response of starved blacktip sharks 
to "quiescent mullet water,’’ "excited mullet water,” "distressed mullet water,” and "dead mullet water.” 
hunting response. Moreover it seems that shark 
activity, and thus presumably the amount of 
odorous material released, increases with in- 
creased agitation of the prey. 
It seems unlikely that the odorous material is 
associated with body juices released by direct 
injury to the prey on the part of the observer. 
The fish used were healthy aquarium specimens 
which in some cases, e.g., groupers, were used 
over and over again and yet suffered no obvious 
ill-effects from being repeatedly "agitated.” It 
is unlikely they would be damaged by rubbing 
against the sides of the smooth plastic container. 
They could, of course, rub against each other 
when excited. This may possibly have removed 
part of their mucous coating and enabled body 
juices to escape through the skin. 
That the results were not induced by the arti- 
ficial environment of the shark tanks was dem- 
onstrated in follow-up experiments with sharks 
in the natural environment of Eniwetok lagoon 
by Hobson (1963). Water siphoned into the 
lagoon from a plastic container in which large, 
living, agitated but apparently undamaged 
groupers had been placed, attracted both white- 
tip ( Triaenodon obesus ) and grey (C. menisor- 
rah ) sharks. They detected the "grouper water” 
from a distance and followed its path upstream 
to the source — a concealed plastic tube. 
If the substance which attracts the sharks is 
released by some subtle damage to the skin of 
the prey it might be similar to that demonstrated 
by von Frisch (1941) in the injured skin of 
the minnow ( Phoxinus laevis ) . As with von 
Frisch’s material, identified as a purine- or 
pterin-like substance by Hiittel (1941), it might 
produce an alarm reaction among the prey but 
still be attractive to the sharks. On the other 
