Hammerhead sharks, with their grossly misshapen noggins, have always inspired a sense of ominous dread and awe in humans.
They are just scary-looking. Thus they have to be dangerous.
But not all of them are.
Six members in the hammerhead shark family occur in waters off coastal North America. Two — the scoophead and the scalloped bonnethead — are found only in the Pacific Ocean. The other four species are Atlantic, which of course, includes the Gulf of Mexico.
Scalloped, smooth and great hammerheads are big fish, growing to 14 to 20 feet, with sinister-looking, anvil-angled cephalofoils (that’s what their hammer-shaped heads are called).
Our little guy, the one we are talking about here, is the bonnethead shark. It maxes out at 5 feet long and only averages 3 feet long.
The world record is only 32 pounds, caught in Apalachicola Bay, Fla.
Besides size, what separates the bonnethead shark from its much bigger cousins is the shape of its head. It is shaped much more like the business end of a garden spade or a ladies’ sun bonnet.
Compared to the other hammerheads, it’s almost cute.
The scientific name of the bonnethead is Sphyrna tiburo. Sphyrna translates from Greek as “hammer,” obviously in reference to its head. The specific name of tiburo is derived from a Taino word for “shark.” Tainos were the Indians native to the Caribbean Islands.
The Caribbean and the Gulf of Mexico are strongholds of this fish’s population, which stretches from the mid-Atlantic U.S. coast south along the entire coast of Brazil.
It is a warm-water species, preferring water temperatures of 70 degrees or warmer.
During winter months, bonnetheads from cooler regions migrate, sometimes in schools of thousands, toward more southerly, warmer waters.
Wherever they are found, they are not a blue-water species, preferring shallow coastal offshore waters, as well as shallow lakes and bays.
Bonnethead sharks tend to swim in small groups of five to 15 fish. They are not particular about the bottoms they hunt over: Mud, sand, grass and even coral bottoms are acceptable.
Their most preferred food items are crabs — particularly blue crabs — but also eat stone crabs, spider crabs and purse crabs. Other food items include shrimp, mantis shrimp (called king shrimp in Louisiana), clams, octopus, squid and small fish.
Occasionally, sea grasses are found in their stomachs. Bonnetheads living in coastal waters eat a wider variety of foods than those living in estuaries.
A bonnethead feeds by swimming over the sea floor, moving its head side to side like a metal detector, searching for tiny electrical pulses produced by creatures hiding in the sediment.
When they detect a pulse, they turn sharply and bite into the bottom where the disturbance was detected. Their jaws hold two types of teeth: small sharp teeth in the front of the mouth for grabbing their victim and broad, flat molars in the back of the jaws to crush their hard-shelled prey.
Scientists have debated whether the hammer-shaped heads of the sharks in this family are useful for detecting food by providing better electrical or olfactory reception.
Some experts once maintained the head shape assisted the shark in making sharp turns, before it was shown it was their vertebrae that allowed their maneuverability.
Most accepted now is the theory that the cephalofoil aids in keeping their bodies stable in the water.
Other fish control pitching (up-and-down body motion), yawing (side-to-side motion) and rolling with their pectoral (side) fins.
The large species of hammerhead sharks seem to almost totally control pitch and yaw with their cephalofoils. The smaller cephalofoils of bonnetheads are not as successful, and this species uses both their cephalofoil and their pectoral fins to get the job done.
Reproduction in bonnethead sharks is a little complicated. In the Gulf of Mexico, males mate with females in November and December. Sperm is transferred into the females by the male’s claspers, elongated extensions of the male’s paired pelvic fins that all male shark species have.
Females store the sperm in their bodies until mid-March to early April before releasing it to fertilize their eggs internally.
The eggs, with their tough but elastic shell, remain in the female until the young hatch.
Four to 14 baby sharks hatch with yolk sacs that provide nourishment to them. While feeding the young shark within the mother, the yolk sac attaches itself to the wall of the mother’s uterus, forming a placenta. Blood vessels in the placenta then nourish the young sharks (called “pups”) until they are ready for birth.
Gestation lasts 4½ to 5 months, the shortest known period for any shark species in the world.
Female bonnetheads move to extremely shallow water before giving birth. Pups are 12 to 13 inches at birth.
In an oddity, at the Henry Doorly Zoo in Omaha, Neb., a female bonnethead shark produced a pup without the presences of a male by parthenogenesis. Genetic tests confirmed the mother and pup were exact clones.