Roca Redonda, Galapagos

Arriving at Roca Redonda is disconcerting to say the least. As we approach the bare rock which forms the tip of this vast underwater volcano, the water is choppy and opaque. The boat glides to a stop, and I stare into the briny depths, trying to make out any shapes below. I watch the waves, looking for anything breaking them, and I realise I’m gripping the railing on the boat a little too hard.

Beneath me are sharks, swarming hordes of sharks, a huge congregation of dead-eyed killers, and in just a moment, I will be diving into their domain. The Roca Redonda is said to be the best place to see sharks in the Galapagos, and though I know there has never been a human death from sharks in the islands’ history, as I pull on my scuba gear, I prepare for a rocky introduction.

Sharks have some of the longest evolutionary histories of any invertebrates. First appearing in the Devonian, some 375 million years ago, they predate dinosaurs by entire geological periods. Their agile bodies, razor-sharp teeth and keen hunting instincts have given them a basic form that has remained unchanged for aeons. However, the natural world has provided some fascinating variations on the theme of the shark, and none is more striking than the hammerhead. 

As my eyes grow acclimatised to the underwater gloom, I see before me a vortex of hammerheads, perhaps a hundred strong, swirling in a great helix pattern. I watch as they move slowly, elegantly in unison, dodging the hot air bubbles that stream to the surface from the gas vents on Roca Redonda’s volcanic slopes. I tread water and absorb the scene; terrifying, chilling, wonderful, beautiful.

These are scalloped hammerheads (Sphyrna lewini), and like all hammerheads, by far their most striking feature is their large, mallet-shaped head. More properly known as cephalfoil, it can be thought of as a huge, multi-sensory organ which gives the shark a wide range of advantages. The head contains, strewn across it, numerous electroreceptors. This gives the shark a built-in biological antenna providing a sense unknown to humans, the ability to perceive electrical signals. With this, it can track the locations of hidden prey, such as stingrays buried in the sand.

Another advantage of the hammerhead is improved manoeuvrability. These hunters can turn and steer with greater speed and ease than other sharks thanks to the lift provided by the cephalphoil, which acts a bit like the hydrofoil of a plane. This allows them to pursue prey fish with deadly efficiency.

Hammerhead sharks, such as the scalloped hammerheads I’m watching, are unlike most predators, who tend to have eyes on the front of their head, giving them a good range of binocular vision (where the sight of the two eyes overlap) and providing the sharp depth perception that is invaluable for catching prey. Instead, the hammerhead has eyes on the sides of its head like a prey animal. This was a mystery until 2009, when it was discovered that, whilst all sharks have an almost 360-degree vertical field of vision, hammerhead sharks’ eyes also provide them with a horizontal visual field of up to 340 degrees, with a binocular overlap far exceeding that of normal-headed shark species. This allows them to see with clear depth perception almost everywhere around them, in almost all directions. 

I try to imagine what it would be like if I could see behind my own head, and I don’t think I can, so instead I turn my attention to the smaller, more streamlined, normal-head shape-bearing Galapagos sharks. 

These lightning-fast predators spend the daytime here, in the deeper water, before venturing out in the evening to the reefs to hunt. It's a highly social fish, often congregating in large groups, and they are quite territorial and aggressive, having even been known to charge and bother local fishermen. Like most sharks, they give birth to live young. But unlike mammals, which often care for their offspring, shark pups are left to fend for themselves and must quickly make their way to the safety of the shallow reefs.

I surfaced out of the water, captivated by what I just saw, but also a little relieved to be out of reach of these terrifying, albeit statistically harmless, fish. The boat zips away as I count my fingers, making sure no one’s come in for a snack, and prepare for my next encounter, this time with the inhabitants of the heavens, the raptors of Isla Marchena.