Shark Ecology 107
ened; this is why they are known as shysharks or even doughnut sharks.
This behavior may make these small (60-cm, 24-in), slender sharks bigger
and therefore harder to swallow.
Swellsharks (Cephaloscyllium) go a step further in being able to increase
their size when threatened. Swellsharks are unique among sharks because
they can swell their entire body by filling their stomachs with water, a be-
havior better known in bony fishes such as pufferfishes. When threatened,
a swellshark bends its body into a U shape, grabs its tail fin in its mouth,
and swallows water, swelling its body to twice its normal size. This makes
the shark harder to grab, and if the shark happens to be tucked away in a
crevice, it would also make it difficult to extract. So, inflatable sharks are
more than pool toys.
Do sharks get sick?
Very few captured sharks are found to be diseased. Sharks have strong,
efficient immune systems. Their immune reaction to disease may be faster
than it is in other vertebrate groups such as mammals. In sharks, immune
cells are produced in the spleen, thymus, and other places, but not in bone
marrow as they are in mammals (remember: sharks don’t have bones). Im-
mune cells are already in the shark’s blood and can multiply quickly in re-
sponse to a challenge from a disease-causing substance or organism. But
this doesn’t mean sharks are disease-free. They suffer from a variety of ail-
ments and illnesses, including bacterial infections, liver disease, meningitis,
tumors, and the effects of a huge variety of internal and external parasites.
Parasites are discussed in the next question.
It was long thought that sharks were immune to cancer. As a result,
sharks were killed to extract potential cancer-curing remedies. Shark carti-
lage was promoted as an anticancer drug because laboratory studies showed
that cartilage extracts could slow the growth of blood vessels that often oc-
curs when cancer cells proliferate. Shark cartilage powder has sold for as
much as $145 a gram ($65,685 a pound), and at one time a single plant in
Costa Rica processed 235,000 sharks per month to make cartilage pills. But
sharks do get several kinds of cancer, including cartilage cancers. Also, real
trials with real people showed no beneficial effect from cartilage pills. Re-
gardless, sharks are still processed for their cartilage, and cartilage capsules
can still be purchased via the Internet to treat cancer and joint problems
(see “Are any medicines made from sharks?” in chapter 8).