An “invisibility cloak” keeps these squids safe in the twilight zone.

Glass squids (family Cranchiidae) live in the boundless waters of the twilight, or mesopelagic, zone. With no protective shell and nowhere to shelter, they need to get creative. Transparency is one way to thrive in a home with few places to hide.

Like other cephalopods, glass squids are covered in tiny pigment sacs called chromatophores. They often keep their chromatophores closed so their skin is basically see through. This invisibility cloak hides them from both predators and prey.

Glass squids have a large internal cavity they fill with ammonium, a chemical that is lighter than seawater. Building a more buoyant body means the squid does not have to swim as hard to stay afloat. They maneuver slowly through the midwater with their fins, constantly on the lookout for danger or a delicious meal.

Special light organs called photophores mask the shadow of their opaque body parts, like their eyeballs. These organs glow at the same intensity as the dim sunlight from above to hide the squid’s silhouette from predators hunting from below.

When the glass squid’s cover is blown, they expand their chromatophores to darken their appearance. Some may fill their body cavity with ink instead, presumably to blend into the darkness. And when danger still looms, a glass squid may squirt ink into the water and jet away. A ghostly shroud of ink creates a distraction so the squid can escape.

More than 60 species of glass squids live in deep twilight waters around the world. Some are little more than 10 centimeters (four inches) long, but others are giants. In fact, the largest of all squids—the colossal squid (Mesonychoteuthis hamiltoni), nearly 10 meters (33 feet) long and weighing up to 495 kilograms (1,091 pounds)—belongs to the family Cranchiidae. We typically see smaller glass squids about 30 centimeters (12 inches) long, likely because the larger ones are faster swimmers that can easily elude our slow-moving submersibles.

But the future of these fascinating midwater animals is in jeopardy. 

The deep seafloor holds buried treasure: nodules of precious minerals critical to modern technologies. Mining these metals will release plumes of wastewater that will cloud the ocean’s twilight zone.

Most deep-sea habitats have very low concentrations of naturally suspended sediment, even near the seafloor. Glass squids and other midwater animals have extremely large eyes and keen eyesight. Many communicate with living light, or bioluminescence. Investigating how deep-sea animals sense their surroundings will help us predict how much harder mining will make their day-to-day lives. We urgently need to identify the impacts deep-sea mining will have across all ocean habitats, from the midwater to the seafloor.