Shark Behavior 83
movies). However, sharks do locate bony fishes by eavesdropping on their
prey’s sounds.
What is “tonic immobility”?
If you read popular articles about sharks or watch Discovery Channel
specials or YouTube videos, you’re likely to encounter instances of tonic
immobility (TI). For reasons that remain unknown (as with so many other
aspects of shark behavior), when some sharks are turned over and held up-
side down, or when their snout is pushed up and backward, or just squeezed
gently, the shark goes into something of a trance and stops moving. One
untested possibility is that a shark’s electroreceptors are concentrated in
the snout area, and perhaps overstimulating this area “shocks” them into
immobility (granted, this does not explain the reaction to being turned up-
side down). Whatever the explanation, researchers have used this reaction
to calm sharks that they want to measure or tag because it makes things
easier on both shark and researcher. Sharks in TI are less likely to injure
themselves, equipment, or people, and they recover from TI faster than
if an anesthetic is sprayed across their gills. See http://www.youtube.com/watch
_popup?v=WK2LpUoqX6A&vq=medium for a nice example set to music,
or watch a couple of scenes in the otherwise forgettable Hollywood block-
buster Dark Tide.
As a side note, many of the photos showing White Sharks lunging out
of the water with their mouths open are not instances of foiled attacks.
These are instead sharks that have been lured to the surface with bait and
had someone grab and push hard against the underside of the snout, induc-
ing a short-lived TI state, long enough to get the photo. We do not recom-
mend that you try this at home.