100 Sharks: The Animal Answer Guide
gesting avoidance of colder winter conditions. Atlantic Stingrays leave the
Chesapeake Bay during the fall when surface waters cool and head south or
offshore to warmer water. Cownose Rays move up and down the Atlantic
coast of the United States, going north in the late spring and south in late
fall. Clearnose and Winter skates move inshore in the spring and offshore
in autumn.
How do sharks survive droughts?
Many bony fishes are “intertidal,” meaning they live in tide pools or
along shorelines that dry up once or twice daily at low tide. These fishes
have many behavioral and physiological adaptations to help them through
these relatively dry spells. Few sharks live in water so shallow that it goes
dry on a daily or seasonal basis. An exception is the Epaulette Shark of
Australia and New Guinea, which has the amazing ability to survive low
oxygen conditions by lowering its blood pressure and heart and breathing
rates, and switching off non-essential brain functions. Under laboratory
conditions, Epaulette Sharks have survived up to an hour without oxygen
at 30°C (86°F). (Recall that warmer water holds less oxygen at the same
time that metabolic rates usually increase with increasing temperature.)
This is apparently an adaptation for hunting in tidal flats at night when
oxygen levels fall.
Some bony fishes such as lungfishes and Salamanderfishes dig into the
mud when ponds lose their water in the dry season. We don’t know of
any freshwater sharks or rays that do this (on freshwater species, see “How
many shark species live in rivers and lakes?” above). However, some sharks
do live in places where rivers dry up seasonally, as in the Northern Terri-
tory of Australia. There have been instances in this region of Bull Sharks
becoming stranded in pools as the former river dried up. An Australian fish
biologist almost fell victim to a Bull Shark when he went to cool off in such
a pool after a hot day of fish collecting. This fairly large Bull Shark had
survived by eating animals that came to the pool to drink; dismembered
kangaroo and camel skeletons on the shoreline were a clue that told him to
exercise caution.
Are there sharks in the desert?
Deserts by definition are short on water, especially salt water. Where
water occurs, it usually emerges from the ground at springs, which besides
being too fresh for sharks also lack a necessary connection to the ocean that
would allow invasion by sharks. So desert sharks are unlikely (although the
movie Sand Sharks might lead you to think otherwise).
http://www.ebook3000.com