The deep sea makes up almost 80% of the space on our planet available for life. This makes it easily the largest habitat on Earth. So far, we have explored a mere 1% of this vast space. This is hardly surprising when you consider that there are barely a dozen submersibles in the world that can dive to depths of more than a couple of miles down.
We’ve already begun our tour of this last great frontier by looking at the bizarre barreleye, but now we’ll be focusing on the other strange fish that call this eternal darkness their home.
We’ve already talked about anglerfish at Extraordinary Animals before (see here if you want to refresh your memory), but now it’s time to delve into some of the weirder species…
The wolftrap seadevil probably has the worst overbite in the ocean. Its much longer upper jaw is, uniquely, folded lengthwise, rather like the leaves of a Venus flytrap, allowing its upper ‘lips’ to close down around its lower jaw. This allows the fish to cage its prey before it is sucked into its throat. The rod – or illicium – of the wolftrap seadevil is long and articulated, and it looks like a stereotypically perfect fishing rod. The lure at the end – also called the esca – has hooks on it, which scientists are currently debating. Some authorities believe that these hooks ensnare prey that comes too close to the esca, and then the entire rod whips the prey into the seadevil’s mouth.
Enter another seadevil, this time the hairy seadevil. And you can see why it’s called that. This angler is covered in long, thin antennae, which in addition to making it look positively terrifying also assist in detecting the tiniest changes in water pressure that indicate the presence of prey. Why the hairy seadevil has lost its bioluminescent esca remains unclear.
The illuminated netdevil looks a bit like a sea-dwelling Christmas tree, for this fish has more luminescent organs than any other angler yet discovered. As well as a bioluminescent lure on top of its head, the netdevil also has several glowing barbels dangling from its chin, which resemble seaweed fronds. Unlike the esca, which contains luminescent bacteria, the barbels glow in the dark by way of complex photogenic granules.
The whipnose angler has a lure that is much longer than its body. A Japanese submersible that chanced upon one of these remarkable, little-seen fish revealed that they might swim upside down with the bioluminescent lure in front of them, dangling just above the bottom of the ocean. This behaviour is still not understood, but perhaps it helps to lure out invertebrate prey from the sea floor.
Animals that live in complete darkness their entire lives tend to be ugly and weirdly-shaped. If nothing else can see them, it seems, they can get away with it. And the gulper eel has done so unashamedly.
Just look at that mouth! Huge and expansive, its mouth can unfold like an umbrella to engulf prey that may be greater even than the gulper eel’s body. Having fastened onto its prey, the gulper must then pull itself over it like a python trying to swallow an antelope.
The rest of the gulper’s body seems to be made up of a long, thin, whip-like tail, which has a bioluminescent tip. Some species even hold their glowing tails in front of their gaping mouths to attract small fish, enabling them to easily snap them up. Between the giant mouth and the long tail, meanwhile, is what appears to be a relatively puny body, but its sac-like stomach can expand to cope with whatever huge meal is brought in from the maw.
And we end the section on a quick fact: the umbrella-mouth gulper has the biggest mouth, relative to body size, of any vertebrate – it’s a quarter of the animal’s total length.
The stoplight loosejaw is not only horrifically ugly, but it also looks like it has been viciously mutilated. Why has it got a gap between its jaw and its mouth? Well, that’s the reason it is called a loosejaw: its lower jaw has no floor to it – it is attached to the rest of the mouth only by a hinge and a modified tongue bone.
The jaws, being stripped down to skin, bone and tendon, allow for extremely rapid action, and it feeds with very quick lunges. Since the connection between the body and head is also significantly reduced, the cranium can be tilted further back and the jaws thrust forward with a wider gape. But even when it has wrapped its jaws around its prey, its floor-less mouth means that the victim is technically still outside the loosejaw’s body. Teeth in its ‘throat’ quickly latch onto the prey and then it is dragged backwards to be swallowed.
And now (assuming you have gotten over its unique mouth) it’s time to ask why it is called a stoplight loosejaw. Like many denizens of the deep, this fish can produce lights. Its bioluminescent organs are located beneath its eyes and they can emit red light. This is extremely rare (green and blue lights are much more common), and most deep sea creatures cannot even detect red light. But this works to the loosejaw’s advantage. By using a red beam of light, which is invisible to most other animals in its environment, the fish can illuminate potential prey in the complete blackness without being seen itself – it’s a bit like the night-vision scopes used by the military. Behind the organ that emits red light is another that emits green light; this resembles a traffic light, hence the fish’s common name.
‘Never bite off more than you can chew’. It’s good advice, and the fish in the picture below probably should have listened to it.
It seems almost incomprehensible that the fish would even have tried to eat an animal that is quite blatantly bigger than itself, but this fish is no ordinary fish: it’s a black swallower and, as its name suggests, it likes to swallow things, no matter their size. The black swallower can accomplish this not only due to its large mouth but also its highly distensible stomach. Admittedly, the individual in the picture seems to have picked a meal that was just a tad too big, but these fish can capably swallow prey over twice their length and ten times their mass.
Occasionally, though, black swallowers swallow such large prey that it cannot be digested before decomposition sets in, which results in the release of gases that not only kills the fish but also forces it – and its meal – to the surface. This is how most specimens, including the one in the image, came to be collected.
The second part of ‘Into the Abyss’ will look at the weirdly wonderful world of deep-sea invertebrates, but before that – tomorrow, in fact – we’ll be taking a look at an ecosystem in the ocean depths that can survive with no input from the sun at all…