Wildlife and Science

Shark Tales: Keepers of the Deep Blue


PUBLICATION: The Beach Times, Costa Rica, print edition


Behind a rocky marine crag a small fish hides—quiet, motionless and out of sight. There is no sound, movement, or light.

Anywhere else on the planet the trembling creature would likely be safe, yet on the other side looms the shadow of a shark and—unfortunately for the tiny swimmer—its electro-receptors have kicked in.

Born of tiny, gelatinous pores under the snout (called ampullae of Lorenzini), sharks have a sixth sense that allows them to detect electric fields put off by movements as slight as a heartbeat. Moreover, a line of tiny receptors runs the length of their bodies letting them sense vibrations — something scientists call “distant touch,” like a long, invisible arm.

Add to that vision up to 30 meters (100 feet), a sense of smell spanning 300 meters (900 feet), and hearing that stretches for nearly a kilometer (half a mile), and one might say the rubbery, pelagic creatures look like machines crafted for the hunt, as if by intelligent design.

Fortunately for human beings, the chance of falling victim to that hunt is very small. In Costa Rica, it is almost zero.

“(In Costa Rica,) it is 100 times more likely that a human will die being struck by lightning than from a shark,” says Dr. José Rodrigo Rojas, a biologist with the World Conservation Union’s Shark Specialist Group.

“The probability is much less than being struck by lightning, hit by a train, or stung by a swarm of bees,” he says. “Marine animals in general are timid and passive. Some species attack, but as a defense response.”

According to Dr. Moises Mug with the World Wildlife Fund (WWF), when sharks do bite humans here it is generally because they have confused them with another prey. The shark expert, who acts as WWF’s senior programming official for Latin America, says humans are not part of their intended diet. A swimmer, according to Rojas, without a surfboard or other devise has “very little” chance of ever being bitten.

He says some species congregate near the coast to feed, reproduce or bathe. It would be in these areas where the “remote possibility” of an attack would be most likely.

Of the 52 species that have been registered off Costa Rica’s shores, the tiger shark, he says, is perhaps seen as most dangerous. In part however, he says that is because they tend to give fishermen a bigger fight.

Mug says juveniles often inhabit the shoreline, especially in rocky reefs and mangroves where there are more hiding places and different kinds of food sources.

Although there is variation from species to species, he says, sharks are by and large solitary creatures. Large congregations are uncommon except around ocean islands, which they sometimes use as reference points.

Isla del Coco, for example, off the southwestern tip of Costa Rica, is known as one of the world’s most important shark habitats and research is frequent there. In general however, he says, there are huge expanses of open sea with little or no sharks.

“The ocean is a very vast and empty place,” says Mug. “If you were to group all the animals in the ocean it would take up a small space…the biomass is not homogenously placed. It’s patchy.”

Sharks have many unique features not found in any other creatures. Their skin, for example, is made up of millions of dermal denticles—small outgrowths that look like tiny teeth—that channel water over the body.

The result produces a flow that reduces friction. Rojas says it is an advantage in hunting, making them “hydro-dynamically silent.”

A shark’s skin is ten times thicker than an elephant’s, the biologist says, noting females—who are bitten by males during copulation—have even thicker patches in those regions.

In their mouth they have layers and layers of teeth, which are constantly being lost and replaced. Some species go through 30,000 to 50,000 teeth in their lifetime.

“They have countless teeth,” says Mug. “The jaws have several rows of teeth and they are continuously growing more.”

The creatures, which live at depths of up to 3000 meters, range in size from the pigmy shark that stretches just 22 centimeters (less than a foot) and weighs less than 500 grams, to the whale shark spanning 20 meters (65 feet) and 15 to 20 tonnes.

A unique respiratory system forces them to constantly keep moving. In this, water enters through the mouth and passes through the gills, which retain part of the dissolved oxygen before it exits through small openings.

“Sharks don’t sleep,” says Rojas. “If they stop moving they go deep and would simply drown. They can achieve a certain amount of circulation pumping water through rhythmic contractions in the mouth and the gills, but this wouldn’t be sufficient to stay alive.

“Because of this, sharks must be in a constant state of motion.”

He says they exhibit strong communication skills, often displaying struggles for power. The phenomenon is best observed during mating rituals and among siblings where some species kill each other before birth. The sand tiger sharks, for example, often eat one another in the uterus. The biologist says they are “defining who is the strongest—who will survive and who will die.”

Rojas says the threat to sharks is “real and growing,” pointing to uncontrolled fishing, shark- finning, and environmental changes.

Shark meat is used in food industries and their teeth are sometimes sought in jewelry and charms. Certain parts are hunted by the pharmaceutical industry for their belief to fight cancer, despite most studies showing the benefits to be nil.

Like many sea creatures, sharks can be traced to eras long before humans or other creatures inhabited the land. Fossil records go back more than 450 million years and modern sharks are linked to the Lower Cretaceous period.

The ocean dwellers figured in to countless tales of Hawaiian and Pacific island mythology and sharks were widely considered to be gods and keepers of the sea.

Like the sea turtles, which nest on the Pacific and are also endangered, prior to human influence, sharks endured the scientific test of time.

“They are extremely intelligent animals that have the capacity to capture prey from hundreds of meters away and swim great distances,” says Rojas. “They have survived thanks to the genetic plasticity of adaptation to changing conditions of the aquatic eco-systems.”


This article was originally published in The Beach Times newspaper. Guanacaste, Costa Rica. Photo with permission from Pixabay.