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Killers of the deep

The underwater hunters are fast, efficient and deadly

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There is something spooky and frightening about killers in the water. I think it is because we rarely see them, unlike aerial or land hunters. They are arguably among the most unknown and undiscovered hunters due to the difficulties in researching them. 

This is a photo of a painting by Randy Berrett of an anglerfish, far right, from the 2005 Pixar movie ‘Finding Nemo’. Featured is its illuminated esca that it uses to attract unsuspecting fish, who are gobbled up when they touch the esca as they try to eat the ‘bait’. Notice its backward-facing teeth and how much Nemo, far left, and Dorry are fixated on the esca. AP/RANDY BERRETT AND THE NEW YORK MUSEUM OF MODERN ART

This article is merely a "drop in the ocean" of the hunters of the deep when you consider that we know more about the surface of the moon than we do about the bottom of the oceans. If these impressive hunters are relatively unknown to science, imagine what else may be lurking in the depths.

Sharks

An article on hunters of the deep would not be complete if sharks weren't mentioned, but I am not going to review the more familiar sharks like the great white, tiger and bull sharks. As there are about 400 known species of shark, with undoubtedly more to be discovered, and as they are all hunters (with filter feeders being included here), I will restrict my comments to a few specialist sharks.

The hammerhead shark is one of the most unusual looking animals in the world. Yet, despite its odd looks, it is one of the most specialised hunters. Called the hammerhead due to the shape of its flat, wide head, it is this bizarre feature that makes it such a specialist. hunter.

All sharks can detect electrical impulses given off by animals in the ocean due to sharks having electroreceptors on their heads. With the hammerhead, though, the electroreceptors are spread all along the front of the "hammer", giving it far more sensitivity to electrical impulses, allowing it to detect extremely low voltages in the water and thus enabling it to sense its prey based on mere muscle contractions, all of which make it a very efficient hunter.

Its eyes are also placed on the edges of the "hammer", giving it virtually 360 vision, which is actually more useful for avoiding predators than for hunting prey. Although attacks on humans by some of the larger species of hammerheads have been recorded, they aren't really a threat to humans. As with all sharks, the threat to them comes from humans in the fishing and finning trades.

Another specialised shark is the mako shark. Looking like a miniature great white shark (as it is in the same family) although more bullet-like in its body shape, the mako is one of the fastest fish in the sea. Growing up to 3m in length, the mako hunts by swimming under its prey, which comprise mainly bony fishes like the mackerel, tuna and other fast-swimming species.

Once a mako shark spots its prey, it attacks from below, hitting the prey at speeds of up to 50 km/h, disabling it, and then devouring it at its leisure.

It has been claimed that the mako can swim 70km/h and that they often soar out of the water after being hooked.

It is not uncommon to hear reports about makos jumping into fishing boats, where they proceed to thrash around, sometimes causing harm to people. They also have serrated, tooth-like scales that reduce drag in the water, enabling them to swim fast and, at the same time, making them extremely rough to touch. Handlers can easily get cuts in the process. An incredible hunter, but sadly, as with many shark species, the mako is threatened in the wild.

Speed killers

Continuing on the speed theme, the marlin species is made up of very agile hunters. Being the fastest fish in the sea, reaching speeds of up to 110km/h, with some species reaching lengths of over 5m, these giants are superb hunters.

Chasing down a prey, which could be a fast-swimming fish, it manoeuvres like a fighter jet under water, changing directions instantly, thereby leaving its prey little chance of escape. Due to their extreme adaptabilities, the members of the marlin family are among the most sought-after game fish, which, if sustainably conducted, can result in little impact on the populations. Nonetheless, marlin is a common food in many restaurants, meaning that their numbers are on the decline.

A Norwegian-led research team exploring the deep seas at depths up to 4km returned with a probable new species of anglerfish, also very ugly. TRACY SUTTON/INSTITUTE OF MARINE RESEARCH

Ugly but deadly

The anglerfish is undoubtedly one of the ugliest animals there is. It gets its name from its hunting technique, whereby it uses a protrusion called an esca growing out of its head to "fish" for prey, hence acting like an angler. It is, in fact, a modified part of the dorsal fin, and it can even wiggle the "bait" around to make it even more tempting to potential prey.

Some deep-sea anglerfish that live in very dark waters have a glowing end at the esca, due to the bacteria that live inside it. Bacteria in enough numbers glow in a process known as bioluminescence, which makes the esca especially attractive to prey.

Once a prey tries to eat the esca, the moment of contact triggers a reflex arc, and the enormous mouth, with its giant, backward-facing teeth, opens incredibly wide to engulf the victim, which can often be twice as large as the anglerfish itself! All of the anglerfish species are ugly, but, on the other hand, all of them are extremely effective ambush hunters.

My favourite

The most intelligent, most formidable and most powerful killer in the oceans has to be the mighty orca (killer whale). Being the largest species of dolphin, and therefore sharing the intellect dolphins are famous for, the orca hunts in packs. It has been termed the "wolf of the sea". After humans, it is the most widely-spread mammal species on the planet, inhabiting all the world's seas and oceans, eating a huge variety of different sizes of prey.

In Sir David Attenborough's incredible documentary entitled Blue Planet, the BBC team captured footage of a pod of orcas chasing down a huge grey whale mother and her calf. Despite the mother's best efforts to protect her calf, the orcas finally managed to drown the young whale, which was still far bigger than an individual orca.

They then ate the tongue and jaw and left the remaining carcass to the scavengers. At the time, I thought it must have taken a lot of effort on the orcas' part for such little energy reward, but I suspect they may have actually enjoyed the hunt. They are certainly intelligent enough to feel such emotion.

The really amazing behaviour exhibited by orca hunting pods off the coast of Argentina is stunning. The orcas also prey upon sea lions and elephant seal pups that are on the beach close to the water's edge. They then swim up the deep channels and actually beach themselves, a lethal action by any other cetacean.

They grab a pup in their mouth and then wriggle themselves off the beach, back into the water. They then toss the seal around often to other whales, slapping it with their flukes and even breaching and landing on the animal in what appears to be in playful fashion. They then eat it.

By far the most remarkable display of behaviour is when they catch a seal, take it out to sea, "play" with it and then return it to the beach with superficial injuries and don't eat it at all.

Quite why they do this is unknown, but it certainly is amazing behaviour exhibited by the most incredible killer of the sea.

Dave Canavan has an MSc in Behavioural Ecology and is the Head of Secondary at Garden International School. Dave is fascinated by science and loves animals, especially the dangerous kind! Contact Dave at davidc@gardenbangkok.com.

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