Bull Sharks

Watching two manatees in our condo complex boat channel this morning, my wife and I were startled by a large fish that partly surfaced near the sea wall, its prominent dorsal fin slicing through the calm water.  I initially mistook it for a dolphin but it never surfaced to breathe and disappeared with no further sightings.

While its identity remains uncertain, I suspect it might have been a bull shark, a species that favors shallow coastal waters, bays and brackish river mouths.  Found across the globe, these sharks are able to live in both saltwater and freshwater habitats and have been found far upstream from the ocean (near the junction of the Ohio and Mississippi Rivers in North America, for example).  Adults average 7-9 feet in length and generally weigh 300-400 pounds; however, much larger specimens have been encountered.  Breeding occurs in brackish waters, in late summer or fall, and females (which are larger than males) give birth to 4-12 free swimming young after a gestation period of almost a year.

Named for both their stocky build and aggressive behavior, bull sharks feed on a wide variety of aquatic animals, including fish, crabs, rays, sea turtles, small sharks, wading birds and aquatic mammals; they, in turn, may be victims of humans, larger sharks or saltwater crocodiles.  Since they favor shallow waters (and since they are especially aggressive), bull sharks likely account for more human shark bites than any other species.

Shark Attacks on Humans

The plethora of shark attacks along the Carolina Coast this summer has spawned headlines across the country and, I assume, around the globe; reading or hearing those accounts, one might think that cosmic forces have aligned to drive us from the beaches. After all, we humans look at ourselves as custodians of this planet, immune to the predator-prey relationship that lower creatures must endure.

Yet, during our 130,000 year history on Earth, we have been attacked, killed and/or eaten by a wide variety of predators, including big cats, bears, wolves, hyenas, snakes, bees and mosquitoes, to name just a few.  Once we entered marine environments, some 70,000 years ago, we became susceptible to sharks and other ocean predators as well.  Of course, in recent centuries, we have come to adore the beach, crowding there during the warmer months to lounge in the sun and surf like so many seals and walruses.

Entering the domain of sharks, we place ourselves at the mercy of natural killers that have inhabited the oceans for 350 million years.  While they may be drawn toward shore by other prey (schools of fish, sea turtles, etc.) they have no reason to ignore thrashing human beings, just another convenient source of food.  After such attacks, the media tends to report that the shark "mistook" the person for a seal or large fish; in fact, we humans, despite our superior intelligence, are just hunks of meat to a shark.  The only way to totally prevent shark attacks is to stay out of their habitat (or, perhaps, to swim in a suite of armor).

Evolution of Marine Animals

Life, itself, evolved in Earth's primordial oceans about 3.6 billion years ago; these initial unicellular forms of life were likely chemoautotrophic bacteria, that arose near deep ocean vents, and cyanobacteria, that evolved in warm, shallow seas.  Eukaryotic cells are thought to have developed by 2 billion years ago and, by the end of the Precambrian Era, 600 million years ago (MYA), complex, soft-bodied organisms (the Ediacaran fauna) and the first shelled marine life had evolved.

Based on fossil evidence, the diversity of shelled marine organisms exploded during the Cambrian and Ordovician Periods (550-440 MYA) and jawless fish had appeared by the end of the Ordovician; the first jawed vertebrates, including spiny fish and placoderms, appeared during the Silurian (440-400 MYA), when marine arthropods first crawled from the sea.  Primitive sharks, amphibians, lungfish and boney fish arose in the oceans of the Devonian Period (400-350 MYA), having since diversified into a vast array of species; some evolved to breed in freshwater streams while others, stranded in shallow, inland seas, became freshwater species.

Reptiles evolved from terrestrial amphibians late in the Paleozoic Era (some 300-250 MYA) and turtles appeared in the Triassic (225-190 MYA); some turtles, of course, have since returned to the sea.  In like manner, snakes evolved from lizards during the Cretaceous Period (about 70 MYA) and some became marine species. Primitive cetaceans (whales and dolphins) split from terrestrial ancestors and returned to the sea during the Eocene Period (about 50 MYA), as did sirenians (dugongs and manatees), the only marine mammalian herbivores.  Finally, pinnipeds (seals, sea lions and walruses) diverged from the bear lineage during the Miocene (25 MYA) and returned to the sea by the Pliocene (5 MYA).

The Largest Fish on Earth

Compared to its marine cousins, the whale shark, the largest fish on Earth, is a relative newcomer to the ocean ecosystems of our planet. While ancestral sharks and boney fish appeared in the Devonian Period, some 375 million years ago (MYA) and modern sharks arose in the Jurassic (160 MYA), current fossil evidence suggests that whale sharks did not evolve until the early Tertiary Period, about 60 MYA; ironically, this was the same time that their unrelated namesake, the mammalian whales, where returning to the sea.

Solitary for much of the year and known for their long migrations, whale sharks favor tropical and subtropical seas where they filter-feed near the surface, scooping in plankton, krill, small squid and small fish with their wide mouths. They do congregate at certain feeding grounds during specific times of the year, drawn by the seasonal spawning of coral or by plankton blooms; perhaps the most famous whale shark rendevous is at the Ningaloo Reef, off the west coast of Austalia.

Often exceeding 40 feet in length and weighing over 20 tons, whale sharks live for 70 years or more (perhaps as much as 120 years). Females give birth to hundreds of live young, each about 2 feet long; those that survive predation by other sharks and large fish will be sexually mature by the age of thirty. Despite bans, adult whale sharks are hunted by various human cultures, especially in Southeast Asia. Since they move slowly and feed near the surface, they are also prone to injury from motorized boats and, like many other marine species, have become the target of eco-tourism.

Great White Sharks

Famous for their power, massive size and occasional attacks on humans, great white sharks favor temperate waters of Earth's oceans and are less common in tropical and polar regions. Known to be migratory, they are most often observed in coastal areas where their prey is seasonally concentrated. Among their more common feeding territories are the Pacific Coast of North and South America, Southern Australia, New Zealand, Japan, South Africa and the Mediterranean region. Nevertheless, these wandering predators may turn up along any shore (as has occurred in New England this week) and have been observed in the Gulf of Mexico.

Slow to mature, great whites do not breed until the age of 15 and may live up to 100 years; births are thought to occur in early summer. Adults average 15 feet in length and often weigh 2000 lbs; records include individuals over 30 feet long, with weights exceeding 5000 lbs. To sustain their massive bodies, great whites feed on a variety of marine life, including fish, seals, sea lions, dolphins, sea turtles and penguins; they also consume carrion and seem to be especially fond of whale carcasses. Attacks on humans, while sensationalized in the media, are uncommon and likely occur due to mistaken identity (i.e. surfers in wet suits look like seals). Equipped with rows of serrated teeth, the shark's initial attack, usually from below, stuns and cripples the victim and, as it bleeds to death, the great white circles back to consume it.

Humans and orcas pose the only threat to great white sharks, which represent the lone surviving member of the genus Carcharodon. Inducing both fear and fascination, these large predators have become the victims of a wide range of human activity, from hunting to the modern bating and cage-watching industry. As with many other species of wildlife, we either fail to appreciate their value or can't resist invading their space.

The Arctic Shark

Of the many sharks that roam our oceans, only a few enter the Polar regions; of these, only the Greenland shark is known to permanently inhabit the Arctic Sea. Found across the North Atlantic, from northern Europe to Canada, this shark feeds at great depths (down 2000 feet or more) for most of the year, ascending to shallower water during the winter months.

Rivaling the size of a Great White, the Greenland shark has long been hunted for its liver oil and meat; however, the latter harbors a neurotoxin and must be boiled several times and then dried before eating. Human hunters, noticing its rather docile nature during capture, inspired the common title of "sleeper shark" for this largest member of the dogfish family. Nevertheless, it appears to be capable of catching agile fish, belugas, narwhals and seals; it has also been known to feed on polar bears and caribou, perhaps nabbing them as they cross open water. Of special interest is the susceptibility of these sharks to parasitic copepods, which attach to their corneas; past theories that these parasites aided the shark by attracting prey have not been confirmed.

So, if you're dreading two more months of winter and have begun to feel sorry for yourself, just think of the Greenland shark. You could be swimming in the cold, dark waters of the North Atlantic, with a copepod attached to each eye!