Leopards of the sea
Leopard sharks are one of the most common sharks along the coast of California. They’re beautiful, slender fish with silvery-bronze skin, patterned with dark ovals that stretch in a neat row across their backs. (Look closely at the dark spots—the older a leopard shark is, the paler the interior of the spots.) Sturdy, triangular pectoral fins are matched by two dorsal fins, and a long, tapered tail swishes gracefully back and forth.
Leopard sharks live in shallow waters of bays and estuaries and occasionally patrol the kelp forest, usually staying near the bottom. They are rarely found in water more than 65 feet deep, although they have strayed as deep as 300 feet. At the other extreme, they often follow the high tide to feed on shallow mudflats, then move back out again as the water recedes.
Baby sharks are called pups. Unlike most fish, which lay eggs, mother leopard sharks keep their eggs inside their bodies until they hatch. After 10 to 12 months, she gives birth to a couple dozen wriggling shark pups, each about 7 inches long.
Keeping the eggs safe inside her is a good way to make sure they all hatch, though it’s also a reason why leopard sharks have so few young—at least compared with the thousands of eggs many other fish lay . Female leopard sharks are usually about 10 years old when they have their first litter, but after that they generally mate every year.
Swim or sink
Leopard sharks are at home on the sea floor, just a foot or so above the sand. This is because they, like all sharks, lack the swim bladders that other fish use to fine-tune their buoyancy. Instead, leopard sharks store oil in their enormous livers. All that oil helps to counterbalance the shark’s own weight, but they usually remain slightly less buoyant than the water around them…so they tend to sink whenever they’re not swimming.
Leopard sharks are made to feed on the bottom—a good thing for a shark that sinks. Their mouths are on the flat underside of their heads, and they open downward. Almost all sharks have sub-terminal mouths except for the whale shark and megamouth sharks. Skimming above the sandy surface, the sharks pluck up crabs, clam siphons, fish eggs, and the burrowing, hot-dog-shaped fat innkeeper worm.
As leopard sharks get older, they start eating more fish and fewer crabs. Leopard sharks have been found with smoothhound sharks, bat rays, and even octopuses in their stomachs.
Shovel for your supper
How do leopard sharks get at buried prey? Divers have seen them swimming stealthily just above the sand, looking for the fleshy siphon of a clam sticking two or three inches above the bottom. If the shark is quick enough, it can grab the siphon in its teeth and yank it out of the sand, occasionally getting the whole clam in the process.
Other times the clam senses the predator and yanks its siphon back to safety. In this case—as well as when the shark goes after innkeeper worms—the shark shovels its nose into the sand. With a mighty twist of its body, the shark unearths a pile of sand and, if it’s lucky, gets a clam or worm for its trouble.
Leopard sharks live close to shore and have firm, light meat that tastes good, so they’re popular with fishermen. Anglers and spearfishers catch around 140 tons of leopard sharks per year in California. There’s also a small commercial fishery in California that brings in an additional 30 tons per year. Though this is a fairly light level of fishing for a common species, leopard sharks take a long time to grow to maturity—about a decade.
Studies indicate that populations could be vulnerable to too much fishing. Setting a size limit for "keepers" helps—right now leopard sharks must be 36 inches (91 cm) long for anglers to keep them. Because of their looks, leopard sharks are also popular aquarium fish—just remember that fish below this size limit are illegal if they came from California waters.
Polluted water can pollute fish
In 1975, scientists dissected five leopard sharks caught in San Francisco Bay to see what they ate. On a hunch, they also had the fishes’ livers analyzed. In all five they found high levels of mercury, a toxic element. In 1997, another study investigated pollutants in a variety of fish from San Francisco Bay, including eight leopard sharks. All eight had levels of mercury in their tissues that were over the accepted safe limit for humans.
Mercury is an industrial pollutant that gets into coastal waters through rain and runoff. Because leopard sharks spend so much time feeding in sand and mud, they probably get exposed to more pollutants than other fish. They also live a long time, giving mercury levels longer to build up in them. We don’t yet know whether such high mercury levels harm leopard sharks. But the evidence suggests it could be a bad idea for people to eat them—at least ones caught in San Francisco Bay.