Sand Tiger Shark

Despite its alarming appearance, it is one of the least dangerous large sharks. It is a slow, placid, mostly nocturnal hunter of fish, and it shows little interest in divers, who regularly swim among aggregations of them without incident. The handful of recorded bites on humans have almost all involved spearfishing, where a shark went for a speared fish, or a shark being provoked or caught. Its resemblance to more dangerous species, and those long visible teeth, have given it a worse reputation than it deserves, and it does far better in an aquarium than most sharks precisely because it is so calm.

The sand tiger does not breathe air in the sense of using it for respiration; it still takes oxygen from water over its gills. What it does is swim to the surface, take gulps of air into its stomach, and burp out the excess, using the trapped air as an adjustable float. Sharks lack the gas-filled swim bladder that bony fish use for buoyancy and normally rely on a large oily liver and constant swimming to keep from sinking. By storing air in its gut, the sand tiger can achieve near-neutral buoyancy and hang still in mid-water, which lets it approach prey slowly and quietly instead of chasing it down. No other shark is known to do this.

Yes. The sand tiger shark practises the most extreme known form of prenatal cannibalism among sharks, called embryonic cannibalism or intrauterine cannibalism. A female has two uteruses, and several embryos start to develop in each. Once the largest embryo in a uterus reaches about 10 centimetres and has developed teeth, it kills and consumes its smaller siblings, then feeds on the steady supply of unfertilised eggs the mother keeps producing. The result is a single large, strong, well-nourished pup born from each uterus, up to a metre long, ready to hunt immediately. It is a strategy of quality over quantity taken to its limit.

Because it breeds more slowly than almost any other shark. A female produces at most two pups every two years, so a population simply cannot absorb losses. Sand tigers have been heavily fished for meat, fins, and, historically, as a supposed threat to be culled, and large numbers drown in the nets set off swimming beaches for shark control and as bycatch in other fisheries. Their habit of gathering in predictable groups at reefs and wrecks makes them easy to target. Populations have collapsed across much of the world, and the species is now listed as Critically Endangered, with Australia and the United States among the countries giving it legal protection.