12 Things About Sharks: No Bones, Night Vision, Electric Senses and 35,000 Teeth

1. No bones

Sharks belong to the elasmobranchs, fish whose skeletons are made of cartilage, the flexible tissue in your ears and the tip of your nose, a group that also includes rays, sawfish and skates. Cartilage is much lighter than bone, and a shark's large liver is full of low-density oil; both help it float.

Sharks still leave fossils. As most sharks age, they lay down calcium salts in their cartilage to strengthen it, which is why dried shark jaws feel heavy and solid, almost like bone, and these minerals fossilize well. Shark teeth have enamel, so they turn up in the fossil record too.

A scalloped hammerhead shark. NOAA Fisheries.

2. Good eyesight

Most sharks see well in dim light, have excellent night vision and can see color. Behind the retina is a reflective layer, the tapetum, that makes the most of what little light there is.

A night shark's green eye. NOAA Fisheries.

3. An electric sense

The small dark pores around a shark's nose, eyes and mouth are the ampullae of Lorenzini, organs that sense electric fields and temperature changes in the water.

4. Skin like sandpaper

Shark skin is covered in tiny tooth-like scales called placoid scales, or dermal denticles. They point toward the tail and cut drag as the shark swims, which is why the skin feels like sandpaper.

Nurse shark skin, which, unlike most sharks', is fairly smooth. NOAA Fisheries.

5. A trance

Turned upside down, a shark goes into a calm, trance-like state called tonic immobility. That is why NOAA scientists often hold sawfish belly-up while working on them in the water.

Tagging a smalltooth sawfish in the Florida Everglades. NOAA Fisheries.

6. Very old

Fossil scales from Australia and the United States suggest sharks first appeared about 455 million years ago.

A grey reef shark. NOAA Fisheries.

7. Aged like trees

A shark's vertebrae carry pairs of opaque and translucent bands, counted like tree rings: 10 band pairs were taken to mean 10 years. Recent studies show that this is not always right, so each species and size class has to be checked to see how often a pair is actually laid down, a step called validation.

8. Blue sharks really are blue

The blue shark is a brilliant blue above and snowy white below. Makos and porbeagles are bluish too, but much less vivid; most sharks are brown, olive or gray.

A blue shark. NOAA Fisheries.

9. Whale sharks have their own spots

Each whale shark's pattern of spots is as unique as a fingerprint. The whale shark is the largest fish in the ocean, reaching 12.2 meters and by some estimates 40 tons; the second largest, the basking shark, grows to 32 feet and more than five tons.

A whale shark. NOAA Fisheries.

10. Breathing at rest

Most sharks must keep swimming to push water over their gills. Some have a spiracle, an opening just behind each eye that draws in water and supplies oxygen straight to the eyes and brain. Bottom-dwellers such as angel sharks and nurse sharks use it to breathe while lying on the seafloor, or while eating.

A nurse shark. NOAA Fisheries.

11. Different teeth for different sharks

Mako sharks have pointed teeth and white sharks triangular, serrated ones, and each leaves a telltale bite mark. A sandbar shark goes through about 35,000 teeth in its life.

A shortfin mako shark. NOAA Fisheries.

12. Eggs or live young

Sharks reproduce in many ways. Some species are oviparous, laying eggs that develop and hatch outside the mother's body, with no further care; others are viviparous and give birth to live young.

Sources

• NOAA Fisheries, "12 Shark Facts That May Surprise You"; rewritten in hubnx's own words. Photographs: NOAA Fisheries.