Extreme habitats call for extreme adaptations, many of which look mind-bogglingly strange to us. And the deep sea has got to be one of the richest seams for a biologist wanting to seek these oddball beasts – there are a lot of weirdoes down there.
The Pacific barreleye (Macropinna microstoma), which lives in the depths of the North Pacific, is a prime example.
This fish, around 15cm long, has several slightly strange features, including large, cartoonish scales on its flanks, an oversized pelvic fin for stability and a really tiny mouth. But its showstopper feature is that the front part of its head is transparent.
Early scientific drawings of this intriguing specimen depict it as an odd, duck-billed beast, with stiff, tubular eyes pointing upwards and a thin, shallow, drawn-out snout. But the barreleye doesn’t actually look like this at all.
It turns out that the trauma it went through while being captured, then the rapid reduction in pressure as it was brought up from deep waters, meant that by the time scientists got to see it properly for the first time, in 1939, the fish had, in effect, exploded.
It wasn’t until 2004, 60 years later, thanks to a massive improvement in deep-sea tech, that the first specimens of Macropinna were seen alive and in context – but this didn’t make the fish seem any less odd.
We knew these deep-sea dwellers had relatively large, tubular eyes, but what we hadn’t seen in those early specimens was that, unusually, they were not on the surface of the animal, but inside its head.
And what a head it is – a rounded, transparent, fluid-filled dome, like a fighter pilot’s cockpit. This means that the fish’s brain, nervous systems and all the internal workings of the head are also visible.
To understand the advantages of this peculiar arrangement, you need to picture where this fish hangs out. At around 600–800m below the surface, in the mesopelagic or twilight zone, light is at a premium.
Here, having tubular, rather than spherical-shaped eyes, is the most efficient adaptation for harvesting the little light that makes it this far down: a large lens at one end and a retina loaded with a vast number of light-sensitive rods at the other acts like a camera’s telephoto lens to increase the focal length and create a larger image on the retina.
The eerie lenses, which look out from inside the fish’s see-through head, are thought to be green to help it see its bioluminescent prey.
Many organisms that live in the ocean use adjustable bioluminescence to disrupt their silhouette, changing their glow to match the surface light and confuse predators.
But this bioluminescence has a broader spectrum than the ambient light, and the barreleye’s peepers are thought to allow this greener light through while filtering out some of the rest, allowing it to pinpoint its prey.
As for the upward stare, this could be because the barreleye hunts either siphonophores (colonies of tiny animals that look like a jellyfish linked together in chains) or prey that have become entangled in these creatures’ curtains of stinging cells. The weak light coming down from above reveals them in silhouette.
But why the need for that transparent head? It wasn’t until 2008 that scientists saw that Macropinna’s eyes could rotate.
While its tiny mouth is where you’d expect it to be – about 90º away from the direction its eyes are normally looking in – once the fish has spotted its prey, it turns its body up to face it, mouth first.
Those ghoulish eyes then rotate forwards by up to 75º so the fish can accurately pluck at its meal. Its see-through head enables this extraordinary visual mechanism while also protecting the delicate sensory apparatus from the stinging cells of its prey.
Now it all makes perfect sense.
