Blog: Ten Things I Love about Sharks [3]

“Sharks are actually very sensitive.”

To what animal group do sharks belong?… Everybody that is even slightly interested in biology and sharks is now throwing me a sarcastic look, but you’d be surprised how often I get asked this exact question. A shark is a fish and not a mammal. Whales and dolphins, those are marine mammals. But there is something different about the shark, compared to other fish. Sharks and the closely related skates and rays are what are called cartilaginous fish, whose entire skeleton is made up of cartilage instead of bone. Sharks also developed a different way to control their buoyancy in the water by regulating the fat content in their liver, instead of inflating and deflating a swim bladder like other fish do. A third difference is how shark reproduce. Sharks do not spawn thousands of eggs, most are even live bearing, their lives are long, their growth is slow, and the number of offspring they produce is very low.

As for some examples: The spiny dogfish is a small-bodied shark that occurs all over the world, including in the North Sea. At birth, the shark measures around 20-30 cm. and doesn’t start mating until it reaches 80 cm., which takes it over ten years. Subsequently, it has a gestation period of two years, before giving live birth to a maximum of 20 pups. The giant manta ray is a similar story. It is the largest ray living on this planet and yet it only has 1 or 2 offspring at a time while delivering only once every 2-5 years. But the frilled shark takes the cake. This prehistoric looking deep-sea species has one the longest gestation periods of all animals; it takes them 3,5 years to develop a single set of embryos.

These slow reproductive strategies, which are more comparative to those of large whales and dolphins than to most fish, make sharks more sensitive to human threats and put them at higher risk of going extinct.



Missed the previous posts of this series?

Ten things I Love about Sharks [1]
Ten things I Love about Sharks [2]

By Linda Planthof