Catsharks occur in warmer seas all over the world and are the most common and numerous sharks in many tropical and warm temperate regions. Catsharks are generally small, usually less than 2 ½ feet long (80 cm), and have cylindrical bodies, tapered at the ends, with two small dorsal fins set far back down the body. Their elongated eyes have a cat-like appearance and sit high on the sides of their head; a rudimentary nictitating lower eyelid helps protect those eyes from damage. Slow-moving carnivores, catsharks use their teeth they have from 40 to over 110 rows—to snag and feed on bony fish and small invertebrates like octopi and squid. Although some bear live young, about 90 percent lay oddly shaped eggs with almost fully developed young. The egg cases, known as “mermaid’s purses,” have distinctive colors and shapes, unique to each species.

On the Agulhas Bank, catsharks have taken that basic model and gone wild. More than 11 species of catshark inhabit the Bank, a broad, 155-mile-wide (250 km) swath of continental shelf off the southeast coast of South Africa, where the Indian and the Atlantic Oceans meet. The waters here are shallow (no more than 330 feet (100 meters)) and temperate, and the most fertile fishing grounds in Southern Africa. In other words, perfect for catsharks.

Most catsharks lay egg cases, known as “mermaid’s purses.”

Among the many species are tiny tiger catsharks (Halaelurus natalensis), barely 1 ½ feet long (45 cm), which have a prominent up-turned knob on their snout. The sharks are yellow-brown in color, with pairs of broad, vertical, dark-brown stripes on the body. Also wearing a suit of stripes is the much larger pajama catshark (Poroderma africanum), a nocturnal predator with seven longitudinal stripes down its back. Other sharks have gone for a polka-dotted theme. The yellowspotted catshark (Scyliorhinus capensis), which can be up to 4 feet long (1.22 meters), has bright yellow or golden spots on a dark grey body, with irregularly shaped blotches forming “saddles” down the back. And the stocky leopard catshark (Poroderma pantherinum), a finback catshark, has highly variable color patterns, with black spots, rosettes, and lines in horizontal rows on a grey to whitish background.

As its name suggests, the smalleye catshark (Apristurus microps) has peculiarly small eyes. Unlike its colorful companions, this stout, 2 foot-long shark is plain, uniformly black or dark brown or grey, with no markings. Other catsharks on the Bank are more unusual for their behavior. True to their appellation, The puffadder shyshark (Haploblepharus edwarsii), brown shyshark (Haplobelpharus fuscus), and Natal shyshark (Haploblepharus kistnasamyi), will cover their heads with their tail when threatened, as though they are hiding. The largest of the three shysharks, the brown (up to 27 inches (69 cm))—which is actually more yellowish than brown—inhabits the close inshore waters of the Bank, often in shallow, rocky areas, where it feeds on lobsters and crabs. Its kin the puffadder shyshark, a somewhat smaller species (up to 23 inches long (59 cm)), lives farther offshore, in deeper waters. The sandy brown shark has seven reddish brown saddles bordered in black and numerous dark brown and white spots. The smallest of the trio, the less-than-20-inch-long (50 cm) Natal shyshark, was once considered a geographic variant of the puffadder; it lives even closer to the shore, sometimes venturing into the surf zone.

Among the oddest of the catsharks of the Agulhas Bank is the balloon shark, Cephaloscyllium sufflans, one of a group of catsharks known as “swell sharks.” This odd, pale grey-brown creature, which can be over 40 inches long (110 cm), lives in areas with sand and mud bottoms. When disturbed, the balloon shark shows off its parlor trick: the shark pumps large volumes of water into the forward part of its stomach, and expands in diameter up to two times its normal size.

While still in their mother’s uterus, sand tiger shark embryos develop teeth–and an appetite. The largest of the babies in each of the sharks’ two uteruses attacks and eats its smaller siblings, leaving just two pups to be born.

It’s a particularly gruesome and extreme example of siblicide, the killing of one’s siblings, and the a rare form of cannibalism within the womb. And yet the practice of cannibalism itself, once thought to be an aberrant behavior, actually isn’t as rare in nature as we might like to believe.

Animals usually avoid cannibalism, but not because of any ethical or moral imperative; although consuming the flesh of one’s species mates offers a useful source of nutrients, it is also likely to contribute to the spread of pathogens, and, therefore disease. However, a number of studies over the past decade have revealed that cannibalism is surprisingly widespread across the animal kingdom. Groups that occasionally eat members of their own species range from lowly protozoa, slime molds, and sea slugs to insects, spiders, fish, reptiles (including dinosaurs; dinosaur bones have been found bearing the teeth marks of their kind), amphibians, birds, and at least 100 species of mammals, such as hamsters, rats, squirrels, bats, seals and sea lions, otters, polar and grizzly bears, lions, tigers, and chimpanzees. Researchers in New Zealand have even found the remains of a giant squid tentacle within the stomach of another giant squid-evidence that it either ate its compatriot, or accidentally ingested itself. Either option, scientists say, is cannibalism.

Male redback spiders are willing participants in sexual cannibalism. Photo © Fir0002

Like sib-eating sand tiger sharks, birds such as the bearded vulture and the northern goshawk have been observed eating their nest-mates. Other species, especially fish, eat their own young. For example, egg-eating is rampant among male fish guarding egg nests. Researchers have found that while the nests often contain eggs sired by other dads, the fish aren’t terribly discriminating in the eggs they munch, which means they’re often eating their own babies. It may be worthwhile to the fish, experts suggest, because the nutrients supplied by the eggs improves their odds of producing viable eggs and offspring in the future, which increases their lifetime reproductive fitness. Alternatively, the practice may perpetuate because it has a “neutral” effect, neither harming nor helping the lifetime reproductive success of the animal. Egg-eating may simply be a behavior that can occasionally come in handy.

Among small mammals like hamsters, mice, and rats, mothers often do the eating; after delivering a litter of pups, a new mom may realize that food and water are scarce, and will kill and eat her litter for self-preservation. Cannibalism can also occur to cull a an overly large litter, or when the animals are stressed. In many animal species including snakes, stillborn or deformed babies are often immediately consumed by their mother, which lets her get back at least some of the energy investment she put into her pregnancy (while also protecting any viable babies from being exposed to diseases from sick or decaying offspring).

Many other species generally try to avoid eating their own kin-but feed nonetheless on members of their own species. In some cases, it’s simply for food; male polar bears, for example, have been witnessed hunting and eating unrelated cubs when resources are scarce, and komodo dragons will feed on smaller conspecifics. Large crocodiles feed on smaller crocs to keep down the population size and preserve their food supply. This same behavior–large individuals eating small ones–occurs among many other animal species. Meanwhile, the males of other species, such as lions, grizzly bears, many rodents, and chimpanzees, will kill–and sometimes consume–infants to force their mothers into estrus, so they can sire the next litter.

Far more common is a practice known as sexual cannibalism, in which a female eats her mate during or immediately after having sex. The behavior has been observed among female green anacondas, who will mate with a number of males in a “breeding ball,” then eat one of her suitors, perhaps beefing up to survive a long pregnancy. Sexual cannibalism is rife among insects and arachnids, like black widows, praying mantises, and scorpions. In the redback spider, a relative of the black widow, males are actually willing participants in the feast; they flip onto the female’s mouthparts during copulation, encouraging her to dine. Because the spiders transfer their sperm to her in the process, their reproductive success is actually higher after this act of self-sacrifice.