A hammerhead hunts the seafloor

The water off Gulfport is warm and green right now, thick with suspended sediment carried in from the bay. Visibility drops to a few feet in places. From the surface, you can see the bottom in the shallows, but farther out the water closes over itself and you are looking at nothing. This is where the great hammerhead works.

The great hammerhead is the largest of the hammerhead sharks, reaching lengths of fourteen feet or more, and it is present in these Gulf waters through the summer months. The head structure that gives it its name is called a cephalofoil — a wide, flat, rectangular extension that spans up to three feet across in large adults. Most people assume this shape is about vision, giving the shark a wider field of view. That is part of it. But the more important function is sensory in a way that has no human equivalent. Distributed across the underside of the cephalofoil are hundreds of tiny gel-filled pores called ampullae of Lorenzini. These pores detect electrical fields. Every living animal generates a weak electrical signal through muscle movement and nerve activity. The hammerhead sweeps its head low across the seafloor and reads those signals the way you might run your fingers across a surface looking for texture.

The animal it is most often hunting in waters like these is the stingray. Stingrays spend most of their time buried under a thin layer of sand, which hides them from most predators. Their body outline disappears. Their coloring matches the bottom. But they cannot hide their heartbeat. The electrical field from a buried ray is detectable to a hammerhead from roughly a meter away, and the wide cephalofoil effectively increases the detection area the shark can sweep in a single pass. When the shark locates a ray, it pins it to the seafloor with the flat of its head before turning to bite. The rays do defend themselves — great hammerheads are frequently found with stingray spines embedded in their jaws and mouths, sometimes dozens of them — but the strategy works often enough that stingrays are a dietary staple.

The great hammerhead is listed as critically endangered by the IUCN. Populations have declined sharply over the past several decades due to bycatch and targeted fishing for fins. The species reproduces slowly, with females giving birth to litters of roughly twenty pups after a gestation period of eleven months, so populations recover slowly when they are reduced. That this animal is still moving through the murky water off Gulfport in summer is worth noting. It follows prey, and the Gulf still holds prey worth following.

The warm water matters here in a specific way. Stingrays are more active in summer, and the Gulf's shallow coastal zones concentrate them in the kinds of soft-bottom habitats the hammerhead hunts best. The turbidity that reduces visibility for everything else is no obstacle to an animal that is not primarily hunting by sight. The murk that makes the water off Gulfport look impenetrable from a pier is functionally transparent to the hammerhead's sensory system. What looks featureless from above is, for this animal, a readable surface.

If you are near the water right now, the Gulf is probably flat or nearly so in the mid-summer heat. The surface reflects sky. Somewhere past the point where you can see the bottom, the seafloor is soft sand and silt, and something may be moving just above it, head angled down, reading the ground.