Suntanning in Juveniles Hammerhead Sharks
Taken with permission from
||Juvenile scalloped hammerhead sharks,
which are typically light tan in color, were
found to suntan when exposed to increases in
solar radiation. These sharks spend most of
their time in the deeper, murkier portions of
Kaneohe Bay, where little sunlight penetrates.
However, hammerhead shark pups held in a shallow
seawater pond at the Hawaii Institute of Marine
Biology subsequently darkened after being in the
clear waters of this pond for several weeks,
where UV levels are 600 times greater than those
at the murky bay floor. To determine whether
sharks were darkening in response to increases
in solar radiation, an opaque filter was placed
over part of the pectoral fin of untanned sharks
that were collected from the bay. Sharks were
then placed in the pond for 21 days after which
time the filters were removed.
|The cover image from Nature
(383(6602): 677; Oct. 24, 1996
"Areas of skin from under the opaque filter were
untanned, whereas all other skin exposed to
direct sunlight was considerably darker,
resulting in distinct "tan lines.
Picture by Gwen Goodman-Lowe
Our experiments demonstrated that the sharks
were truly suntanning and that the response was,
in fact, induced by the increase in solar
radiation, particularly UV. These sharks
increased the melanin content in their skin by
14% over 21 days, and up to 28% over 215 days.
This study represents the first documented case
of suntanning in lower vertebrates. To our
knowledge, the only other animals that have been
documented to suntan are mammals.
Because the pups may use the murky waters of the
bay as a refuge to avoid being eaten by adults,
it is most likely that this suntanning ability
better serves them after they leave the bay. As
adults, hammerhead sharks spend more time in
clear pelagic waters where UV penetrates to much
deeper depths. Thus, suntanning in these sharks
may play an adaptive role to their environment.
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