Japetella

Image: NOAA/MBARI

An octopus with two different kinds of invisibility cloak!

When you think "octopus" you probably picture a stout fellow with long tentacles clinging to rocks and hiding in crevices on the ocean floor. Or perhaps an enormous monster eating ships and batting aircraft out of the sky.

Image: NOAA/MBARI

Either way, Japetella octopods are different. They have a large, bulbous mantle and only the tiniest, daintiest tentacles. They look a lot more like Glass Squid, but without the obnoxiously massive eyes.

The thing with Japetella is, like Glass Squid, they're entirely pelagic. They spend the whole of their lives in the open ocean, never touching the bottom of the sea. They reach a mere 12 cm (5 in) in length (including tentacles) and presumably munch on whatever fish and crustaceans they bump into.

Japetella bears another similarity with Glass Squid... they're almost completely transparent! Or at least they can be.

These octopods spend much of their lives in the murky depths of the mesopelagic zone, some 400 to 800 metres (1,300 to 2,600 ft) deep.

At this depth there are two kinds of predators they have to be wary of. There are the ones with extremely sensitive eyes who swim around constantly looking upward in search of the silhouette of prey in the gloomy twilight. And there are the ones who use their bioluminescence as a spotlight, lighting up the area around them to find something to eat.

Image: Sarah Zylinski, Duke University

Japetella deals with the issue in a very cephalopod-like way.

Video: Stefano Di Criscio

They're covered in pigment cells called chromatophores. Shrinking them to tiny pinpricks means the octopus is almost entirely transparent. They now leave no shadow or silhouette to betray their presence to predators who use sensitive vision to find prey.

When the light of bioluminescence strikes them, the chromatophores instantly expand to cover the octopus in a dark red colour. This looks much like the blackness that surrounds them, so again, Japetella escapes detection.

Image: Michael Vecchione

When it's time to breed, females descend to a depth of 1,000 metres (3,300 ft) and develop a circle of light-emitting photophores around their mouths. Males have seldom been found, but presumably they go down and join them. His salivary gland is twice as big as that of the female's, and it's thought he uses it to produce pheromones to attract a mate. She can then light up her mouth to signal her presence when she's ready.

The female will now carry her eggs around in her tentacles, probably for several months, and it's likely she doesn't feed during all this time. She dies once the eggs hatch and the youngsters either ascend all the way up to a depth of 200 metres (650 ft) or their mother already carried them up there before hatching. Either way, the babies have a lot more food to eat up there, and will descend to ever greater depths as they grow.

It's nice to know that light in the deep sea doesn't have to be all about killing and eating stuff!