A Brief History
Sharks are among one of the oldest groups of animals that still exist today. They started showing up in
the fossil evidence around 400 million years ago. The first sharks bore very little resemblance to what
the modern sharks look like today. The world’s modern sharks started showing up around 100 million
years ago compared to 35,000 years ago in which we saw the appearance of modern mankind! Most of
the fossil evidence comes in the form of fossilized shark teeth which are found spread across the ocean
Sharks are different than bony fish; they have a skeleton made of cartilage instead of bone. Shark
skin is made up of a series of small overlapping scales called denticles. They provide the shark with
aerodynamic movement through the water. If you rub a shark with the denticles it is smooth but
rubbing it the opposite way is like rough sandpaper.
Sharks use a variety of senses. Their eyesight is not great and they only rely on this when they are very
close to their prey. They also have a lateral line which primarily is a line that runs along the body that
picks up vibrations in the water at a distance. Their sense of smell is also incredible as it is proven that
a shark can detect one drop of blood in 1,000,000 drops of water! All sharks also have sensory organs
around their head, making it possible for them to detect electrical fields. The hammerhead shark is a
great example of this where the hammer is loaded with these organs to enable it to hunt its favorite
prey, stingrays, which are buried in the sand.
Sharks also have the most varied type of reproduction in the animal kingdom. While all sharks have
internal fertilization, there are three different ways to bear the young. Many of the bottom dwelling
sharks release a capsule with the fertilized egg inside. The capsule is often spiral in shape which makes
it stick in crevices and it also protects the developing embryo. The most common form is that young are
hatched within the uterus and continue growing there until the yolk sack is used up. They are then born
as a true miniature copy of the adult shark. There are also a few sharks, like tiger and blue sharks that
have the young attached to a placenta, similar to that of mammals. The pups are born fairly large and
All sharks are hunters and eat anything from crabs to sea lions depending on the species, although fish is
the staple diet for most sharks.
There are a handful of sharks that are potentially dangerous to humans. We are not on the menu of
sharks but because we engage in water activities like spear fishing, surfing and swimming in areas where
there are sharks, there will always be the possibility of a not so nice encounter. A yearly average of 100
or so attacks and less than 10% of this resulting in death on a world basis is statistically nothing. More
deaths are attributed to people slipping in the tub (341), get struck by lightning (50) or attacked by dogs
(26). These numbers are from the National Safety Council and are for only the US and for the year 2000.
We however make that up with interest as humans are responsible for killing over 100 million sharks a
year, mainly for the growing shark fin soup for Asian countries. Many of the shark populations are down
by as much as 90% compared to 15 years ago and measures must be taken if we are to keep sharks
around for future generations. They play a vital role in the ecosystem and removing them can have dire
More than 450 species of shark exist today and of these only a handful poses a potential threat to
Sharks in Costa Rica
Costa Rica has a diverse marine habitat ranging from rocky islands and outcrops, sandy beach areas, to
calm bays and two large gulfs. The rocky islands and its outcroppings are favorite hangouts for sharks
like the bull shark, black tip and white tip reef sharks and hammerheads.
The chance of encountering a shark by accident is quite rare. You have to be extremely lucky to see
one. If, however, you are diving in Costa Rica there is a good chance you will see some sharks. The most
common ones seen around the Pacific coast are white tip reef sharks and nurse sharks. The Bat Islands
in Guanacaste have a collection of bull sharks in the area as well. Hammerheads are mostly seen at Coco
Island along with white tips, tiger sharks, Galapagos and silky sharks and the occasional whale shark.
Cocos Island is ranked among the top 10 diving sites on the planet! Click here for more info on Sharks of Cocos Island.
About the Author
Thomas Jones is a marine biologist with a masters degree in shark reproduction. He moved to Colorado at the age of 6 and returned to his native country, Norway after high school to study. He took his masters at the University of Oslo and now lives in Costa Rica where he runs Bahia Rica Fishing and Kayak Lodge with his wife Vigdis.