The tenebrous oceanic depths are not exactly welcoming to land-dwelling creatures. In addition to the lack of light to see by, and air to breathe, the weight of all the water above creates crushing pressure.

But this lightless part of the world is teeming with life of its own; life that has evolved to thrive in these conditions, life that looks quite unlike anything you might find on drier shores.

Much of this life, for much of human history, has been inaccessible. It's just been down there, in the gloom, doing its thing. But the relatively recent invention of remotely-operated underwater vehicles, or ROVs, is finally opening our eyes to this dark, strange, silent world.

In October 2019, Schmidt Ocean Institute scientists aboard the research vessel Falkor came across just such a creature while piloting the ROV SuBastian: an otherworldly squid, rarely seen by human eyes. This genus inhabits the lower mesopelagic and bathypelagic depths of the ocean – up to 4,000 meters (13,123 feet) down, almost entirely beyond the reach of the Sun's rays.

The Planctoteuthis captured in the rare footage (now resurfaced by the institute on occasion of its two-year anniversary) looks very little like you'd expect a squid to look.

It has a long, ornate tail with many appendages; by contrast, its arms seem quite small. In this individual, the tail is decorated with long, blue streamers, and cells called iridophores that glitter as the squid shimmies through the water.

These iridophores are stacks of very thin cells that can reflect light at different wavelengths. Here, they glimmer golden, reflecting the light shining from the headlamps of the ROV. In the dim conditions under which the squid usually lives, these cells can harness what little light there is available to flicker and flash – although the purpose of this is unknown. It could be to attract prey, scare off predators, or communicate with other squids, or maybe a combination.

This small, delicate genus of squid is largely known from samples hauled up from the depths, and damaged in the process. We've only seen Planctoteuthis alive in its natural habitat very rarely, so there's a lot we don't know.

For example, scientists have found in the samples features usually only seen in juvenile squids. This suggests that the genus might be neotenic, or slow to mature. We also don't know the reason for the long, elaborate tails.

However, when a creature doesn't look like its species, there's a good bet that it looks like something else. This is called mimicry, and in the animal kingdom it often confers an advantage for avoiding predators. This specific Planctoteuthis is thought to resemble a siphonophore, a composite animal that possesses stinging cells, and flashes light to attract prey.

Planctoteuthis could be hijacking the appearance of a siphonophore to try and attract similar prey, but also to repel the predators that would normally be wary of being stung by a siphonophore.

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As we continue to dive deep into the ocean, we can only learn more about these mysterious squids. In 2014, Schmidt scientists caught a different species of Planctoteuthis on camera, and last year, Monterey Bay Aquarium Research Institute scientists also managed to snag a sighting.

Each expedition is bringing us rare sightings that tell us a little more of this eerie, beautiful, and mysterious world.