66 Sharks: The Animal Answer Guide
Nurse Sharks pay attention to color differences when choosing mates, at
least to the extent that they avoid lighter-colored males during mating ac-
tivities. The role played by absolute male size in these observations may
complicate the issue.
Do a shark’s colors change at different times of the day
or in different seasons?Bony fishes, especially tropical marine and freshwater species, change
color on a moment’s notice, or exhibit very different colors during the day
than at night, being mostly blotchy at night regardless of their daytime
hues. Background matching is well known in bony fishes, a classic example
being a flounder’s ability to almost mimic a checkerboard when placed on
that kind of surface.
Most sharks, however, remain the same basic color throughout the day,
at night, and in different seasons. A few can change color slowly, perhaps
to better match their background, becoming darker or lighter or more
blotchy. Obviously, this is more useful among benthic sharks than in spe-
cies that swim actively in the water. Parascylliid Collared Carpetsharks
(Parascyllium collare) are reported to change their skin color to better match
seafloor coloration. Juvenile Nurse Sharks held in tanks lightened in skin
color over a period of minutes when the tank they were kept in was covered
and darkened. Little Skates held in illuminated tanks with white or black
walls also changed color, becoming light brown in white tanks and dark
brown in dark tanks. The color change took on the order of 9 to 12 hours,
much longer than in color-changing flounders and other bony fishes. Spiny
Dogfish also get lighter when kept in white-walled tanks, but the color
change takes even longer, about two days. Physiological studies in skates
and bottom-living sharks such as triakid smoothhounds and Small-spotted
Catsharks showed that the animals sensed outside light using their eyes and
the so-called third eye or pineal organ on the top of their heads. Skin color
changes were due to a melanophore-stimulating hormone secreted by the
pituitary gland that caused melanophores in the skin to expand or contract.
Is there much geographic variation in the color of a
shark species?Geographic variation in color occurs in sharks, although many species
that were thought to be geographic variants turn out to be different spe-
cies. Our increase in understanding comes from additional study, includ-
ing improved genetic techniques that reveal differences that weren’t obvi-
ous from external anatomy alone. For example, the Puffadder Shyshark of
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