As a Shark Specialty Instructor making frequent trips to see bull sharks and white tip reef sharks on dives here in Costa Rica I would like to share some facts about these endangered and misunderstood creatures:
There are over 400 species of sharks.
The largest is the whale shark that can grow up to 59 feet. The smallest is the dwarf shark which is only 6 – 8 inches long.
Populations Dangerously in Decline
Hammerhead shark numbers, for example, have depleted 89% since 1986 in the North West and Central West Atlantic. Without sharks in our waters the marine ecosystem as a whole will drastically suffer impacting on the health of the world as a whole.
Sharks have Rows of Teeth
We find many shark teeth at Bat Islands on our bull shark dives (makes a nice souvenir necklace!) this is because sharks have replacement teeth arranged in rows that can sometimes replace old ones in just one day.
Sharks do not have Scales
A shark has tough skin that is covered by dermal denticles, which are small plates covered with enamel, similar to that found on our teeth
Different Sleeping Patterns
Sharks need to keep water moving over their gills to receive necessary oxygen. Not all sharks need to move constantly, though. Some sharks have spiracles, a small opening behind their eyes, that force water across the shark’s gills so the shark can be still when it rests. Other sharks do need to swim constantly to keep water moving over their gills and their bodies, and have active and restful periods rather than undergoing deep sleep like we do. They seem to be “sleep swimming,” having parts of their brain less active while they remain swimming.
Sharks are Cartilaginous Fish
The term “cartilaginous fish” means that the structure of the animal’s body is formed of cartilage, instead of bone. Unlike the fins of bony fishes, the fins of cartilaginous fishes cannot change shape or fold alongside their body.
Sharks have a lateral line system, which detects movements in the water.
A Lateral line system helps the shark hunt prey and navigate around other objects at night or when water visibility is poor. The lateral line system is made up of a network of fluid-filled canals beneath the shark’s skin. Pressure waves in the ocean water around the shark vibrate this liquid. This, in turn is transmitted to jelly in the system, which transmits to the shark’s nerve endings and the message is relayed to the brain.
Laying Eggs Vs Giving birth to live young:
Some sharks lay eggs others give birth to live young. Within these live-bearing species, some have a placenta like human babies do, and others do not. In those cases, the shark embryos get their nutrition from a yolk sac or unfertilized egg capsules filled with yolk. In the sandtiger shark, things are pretty competitive. The two largest embryos consume the other embryos of the litter! All sharks reproduce using internal fertilization. The fertilized ova are packaged in an egg case and then eggs are laid or the egg develops in the uterus.
While nobody seems to know the true answer, it is estimated that the whale shark, the largest shark species, can live up to 100-150 years, and many of the smaller sharks can live at least 20-30 years.
Sharks are not vicious man-eaters.
Bad publicity has lead to the misconception that they are vicious man-eaters. In fact, only 10 out of all the shark species are considered dangerous to humans. All sharks should be treated with respect, though, as they are predators, often with sharp teeth that could inflict wounds.
Humans are a threat to sharks.
Humans are a greater threat to sharks than sharks are to us. Many shark species are threatened by fishing or bycatch, amounting to the death of millions of sharks each year. While a shark attack is a horrifying thing, there are about 10 fatalities worldwide each year due to sharks. Since they are long-lived species and only have a few young at once, sharks are vulnerable to overfishing. One threat is the wasteful practice of shark finning, a cruel practice in which the shark’s fins are cut off while the rest of the shark is thrown back in the sea.
Fastest Shark Recorded: Shortfin Mako (60 mph and can jump 20 feet in the air).
Do you have any more interesting facts about sharks? Please comment below!
For more information about shark conservation issues please visit www.projectaware.org