The Insane Biology of: Hammerhead Sharks

The world’s first predators emerged during the Cambrian explosion 540 million years ago in the form of giant shrimp-like creatures like the Anomalocaris which trapped its prey in its mouth lined with hooks or the five-eyed Opabinia which caught its victims using a flexible clawed arm attached to its head where within a few million years this simple ecosystem would disappear replaced by an ocean full of diverse mobile and highly effective animals.

By 450 million years ago the ocean was populated by the ancestor of what is now the most fearsome predator of the sea sharks where the first modern sharks arrived in the late Devonian 370 million years ago taking the iconic shape we know today where they were six feet long with a streamlined body five to seven gill slits and dorsal fins where soon sharks dominated the oceans.

Around 20 million years ago evolution created the newest shark to enter the water arguably the strangest one of them all with a mallet-shaped head full of sensory organs and eyes set on either end where the hammerhead is one of the most recognizable and downright bizarre looking animals on the earth where their body plan a drastic departure from the other sharks that roam the sea.

The iconic hammer of the hammerhead shark is called a cephalofoil and the size of it varies from species to species where it’s easy to assume this weird shape is a rubbery extension of flesh but it’s actually a flattened and stretched out skull where the smallest cephalofoil is the modest bonnet head also known as the shovelhead where the largest is the winghead shark whose wing-like head is so big its width is nearly 50 percent of its total body length.

In humans our forward-facing horizontal visual field is around 190 degrees where our binocular vision where the vision of each eye overlaps giving us depth perception covers 120 horizontal degrees where in fact most predators have large binocular fields to help quickly scan the environment for prey an ability made possible by having eyes that face forward where by contrast most prey animals have eyes on the sides of their heads to help them be aware of danger coming from any direction where pigeons for example have a visual field of around 310 degrees but a very narrow binocular portion in the front.

In 2009 researchers compared the visual fields of three hammerhead shark species to two sharks with more typical head morphology where the lemon shark had a mere 10 degrees of binocular overlap the black nose just 11 where the modest bonnet head had a bit more with 13 and the scalloped hammerhead had 32 degrees of overlap and the winghead shark the one with the widest head had 48 degrees of binocular overlap nearly four times that of the typical sharks where this gives them an advantage when hunting for prey by giving them exceptional depth perception.

Elasmobranchs like the sharks and rays don’t have a swim bladder so they have to constantly swim to avoid sinking to the bottom where for a long time it was thought that the cephalofoil indeed acts like a wing producing lift forces that help the hammerhead stay vertically positioned in the water column.

Researchers laser scanned the heads of eight species of hammerhead where each digitized head was then placed in a virtual underwater environment allowing them to measure water pressure drag and flow where surprisingly they found that the cephalofoil does not create lift when the shark is swimming in a regular forward motion but when the head is tilted up or down strong forces quickly come into play where when the angle of attack changes the shark can rapidly ascend or descend where the hammer is not for lift but for maneuverability.

Unlike mako sharks that chase down prey in long pursuits hammerheads swim just above the sand looking for bottom dwelling prey where once detected these prey animals like stingrays or squid will erratically dart away to escape zigzagging up down left right and the hammerhead follows suit where the hammerhead swims above the sand it waves its head like a metal detector looking for treasure its treasure being a buried stingray.

Supporting this hypothesis is the winghead shark who has the biggest head of all the hammerheads where compared to its body it has the largest amount of drag but also shows the greatest change in lift as the attack angle changes where of all the hammerheads it has the best maneuverability and when you look at its diet you can see why evolution would create something so extreme where most hammerheads eat crabs or stingrays but the winghead diet consists of about 93 percent teleost fishes like herrings which are very fast and very agile.

Sharks receive tiny electrical signals from their environment via a series of pores peppered over their heads where these pores are filled with an electrically conductive jelly and lead to tiny bulbous cells called ampullae of Lorenzini and this is the key to their amazing power where all animals generate electricity around them as their muscles contract in movement and their heartbeats and this current radiates away from them in the water where when these electrical currents travel towards the shark and through the jelly they stimulate cilia hair-like projections on the ampullae which then trigger the sensory neurons.

Researchers found that newborn bonnethead sharks can detect electric fields less than one nanovolt per square centimeter where this is around the equivalent to the intensity of a voltage gradient that would be created in the sea by connecting one end of a 1.5 volt double A battery to the Long Island Sound and the other end in the waters off of Florida where theoretically a shark swimming between these places could tell when the battery was switched on or off where such incredible electrical sensitivity is over 5 million times greater than anything we could ever feel where even our best technology struggles to detect something that minute.

Sharks have dominated the seas since the end Cretaceous and as a group have survived all five mass extinctions so far in large part due to their ability to fill many varying ecological niches but the hammerheads as the newest species of shark have not yet faced such an event that is until now where many believe we’re currently living through the sixth mass extinction due to human activity.

Sharks especially the hammerheads are among the most at risk where one study in Australia reported that 80 percent of scalloped hammerheads have been lost where they’re threatened by commercial fishing mainly for the shark fin trade where the fins are cut off and the rest of the body is discarded where their numbers like many sharks are dwindling.