10 Strange Deep-Sea Creatures You've Never Heard Of

The deep ocean is known for its strange and bizarre inhabitants like giant isopods, glowing anglerfish, and massive squid, but many other mysterious creatures of the depths fly under the radar.

10. The Giant Ostracod

Ostracods, also known as 'seed shrimp' or 'clam shrimp,' are found worldwide, even in tiny rainwater puddles. Typically no larger than a grain of sand, their abyssal relative, named Gigantocypris, can grow to the size of a marble or grape. Like all seed shrimp, they possess a fragile, multi-legged body encased in a translucent, bubble-like shell with a small opening to intake food and water.

What sets these deep-sea creatures apart is their unusually large eyes. Instead of lenses, their eyes are concave mirrors, which aren't great at forming detailed images but are exceptional at detecting light. This is why these creatures constantly rotate as they swim, their glowing eyes scanning the environment like sonar dishes. Predatory by nature, they are drawn to the bioluminescence of smaller crustaceans or fish, drawing them into their shells to feast. Although little is known about their reproductive behavior, their eggs are often visible within their shells.

9. Whalefish, Bignoses, and Tapetails

For years, a group of voracious deep-sea predators known as 'whalefish' were only known from female specimens and classified as Cetomimidae, meaning 'whale mimics.' Simultaneously, another fish group called Megalomycteridae, or 'bignoses,' was only found to be male. Strangely, these males seemed to lack complete digestive systems, suggesting they lived very briefly, just long enough to mate. A third category, 'tapetails,' was thought to be entirely juvenile, with long, delicate tails, typically inhabiting shallower waters.

It turns out that these three seemingly distinct creatures are actually all the same species. As tapetails mature and descend into the deep, they transform into either female 'whalefish' with large, pelican-like gullets used to catch prey, or male 'bignoses' that hardly feed and exist solely to reproduce.

8. The Toothed Seadevil

Also known as a 'needlebeard' or 'netbeard' seadevil, the female of this unique anglerfish species measures around 13 centimeters (5 inches) and lacks the typical 'fishing lure' found in other anglerfish. Instead, she has a streamlined, elongated body that allows for more active hunting, accompanied by a set of teeth that are disturbingly impressive even by anglerfish standards. Dozens of teeth grow at strange angles inside and even outside her mouth, crisscrossing each other like stitches. Each tooth ends in a tiny hook and can move independently, likely used to ensnare prey and guide it down her narrow throat.

Researchers are still uncertain about the species that get caught in these terrifying teeth. No direct observations of feeding behavior have been made, and specimens are too rare for stomach contents to provide much insight. Like other anglerfish, the male toothed seadevil relies on his large sinus cavities to detect a female by scent before attaching himself to her skin with his teeth. He remains there for life, surviving off her blood as their skin merges and most of his body withers away.

7. The Predatory Tunicate

Tunicates, commonly known as 'sea squirts,' are usually stationary, bag-like creatures that filter-feed. In the deep ocean, however, where plankton can be scarce, many abyssal animals must rely on slightly larger prey. The carnivorous deep-sea tunicate, Megalodicopia hians, has adapted by developing a large, gaping mouth to capture small crustaceans that are tricked into approaching due to its transparent body, making it resemble an underwater Venus flytrap. In 2009, another carnivorous sea squirt—featuring a horizontal, tubular mouth—was discovered off the coast of Tasmania.

Perhaps the most surprising aspect of these creatures is their close relation to humans. Tunicates are chordates, meaning that they begin their embryonic development with much of the same anatomy as vertebrates. One of the features we share in early development is the notochord, which becomes the spine in vertebrates. As a tunicate matures, it absorbs both the notochord and its brain, settling into a life of stationary feeding without much of a central nervous system.

6. The Holopus

Crinoids, also known as 'sea lilies,' are related to starfish and sea cucumbers and were once some of the most abundant creatures in the ocean. Today, only a few species remain, mostly found in the icy, dark waters beneath the Arctic ice or deep in the ocean.

The most peculiar of these ancient survivors is the holopus, which does not have the delicate, feathery, plankton-catching fan typical of other sea lilies. Instead, it has a smaller number of thick, segmented tentacles that fan out like a hand. These sturdy limbs can close into a tight 'fist,' and much like the predatory tunicate, this is likely an adaptation for capturing passing animals as prey. As if a predatory, alien starfish hand in the deep ocean wasn't eerie enough, these creatures often live upside down, hanging like bats from underwater ledges and cave ceilings.

5. Stygiomedusa Gigantea

While the giant squid is perhaps the most famous tentacled creature of the deep, jellyfish have their own abyssal horror that has rarely been observed by humans. Gliding through the water like something from J.K. Rowling’s world, the dark reddish-black Stygiomedusa gigantea doesn’t have the stringy, stinging tentacles seen in many other jellyfish. Instead, it trails five flowing, sheet-like muscular arms that can stretch over 6 meters (20 feet) behind it.

These thick, fleshy arms are actually massive versions of the 'oral arms' found in other jellyfish species, which they use to grab and hold onto prey. They serve as extensions of the digestive system, lined with microchannels that carry gastric enzymes and nutrients throughout the body. This means that prey are already being digested when they’re entangled in the monster’s suffocating arms, which blend so seamlessly into their surroundings that they are almost invisible. They resemble living, carnivorous shadows. As unsettling as it is, the jellyfish seems to have a companion. A small fish named Thalassobathia has been seen trailing it, though the nature of this relationship remains largely unknown.

4. Deep-Sea Snailfish

Snailfish, with their slimy, scaleless skin and almost gelatinous bodies, are closely related to the infamous 'blobfish' and the more vividly colored, finned lionfish. They inhabit various marine environments, ranging from warm kelp forests to the deepest parts of the ocean where fish have been observed alive at extreme depths.

Though these tadpole-like creatures may appear relatively harmless and even a bit cute, don’t be deceived—some species of snailfish exhibit a darker side, especially when it comes to their parasitic reproduction. Female deep-sea snailfish from the genus Careproctus use retractable, tentacle-like ovipositors to inject their eggs into the gill chambers of live king crabs, including those commonly eaten by humans. While the crab’s armor protects the eggs from predators, the presence of the eggs can impair the crab's breathing, stunt its growth, and in some cases, cause fatal necrosis in its gill tissue.

3. Pelagothuria Natatrix

Pelagothuria natatrix was first identified in the 1890s. At that time, it was frequently encountered during deep-sea expeditions, but sightings have since become exceptionally rare.

Based on anatomical records and a few preserved specimens, it’s known that this creature is a remarkably unusual type of sea cucumber that spends its entire existence floating in the water. Its transparent, umbrella-shaped swimming web is a highly specialized adaptation of the tentacles that typically surround a sea cucumber's mouth, greatly enlarging its small, bag-like body. It appears to feed on tiny particles that drift into its wide, funnel-like throat.

With its small, shrunken body and large, expansive 'fins,' Pelagothuria likely never needs to rest on the ocean floor, unlike other deep-sea swimming sea cucumbers that spend much of their time sifting through marine snow for food. It essentially leads a lifestyle similar to that of a jellyfish, without needing to settle on the seafloor.

2. Swima

Closely related to earthworms and leeches, marine polychaete worms are far more intricate annelids, often adorned with spines, armor-like plates, feathery gills, tentacles, fang-shaped mandibles, multiple eyes, and rows of foot or fin-like appendages. Some species even dedicate their lives to swimming gracefully, as indicated by the genus name 'swima.'

Certain species of these stunning creatures are also known as 'green bomber' worms because of their bizarre and unique defense tactic. Near their heads, these worms have rows of tiny green pods filled with light-emitting bacteria. When threatened, the worm can release one or more of these pods, which ignite with bright green light for a few seconds. As the worm swims away, predators may become distracted by the glowing 'light bombs' and lose track of the actual worm, a trick similar to how cephalopods use ink clouds.

1. Erenna

This jelly-like, worm-shaped being belongs to a group of jellyfish known as siphonophores. Like its famous relative, the Portuguese man o’ war, its body is actually a conglomeration of individual organisms, each functioning like an organ or appendage, forming something between a colonial organism and a singular body. A series of pulsating swimming polyps move it through the water, while a mix of feeding polyps and venomous stinging polyps trail behind. The stinging polyps of the erenna are distinct, with bright red, glowing tips that are reeled in and extended repeatedly.

The quick, coordinated movements of these appendages resemble tiny shrimp, attracting small fish that are swiftly paralyzed and drawn into the feeding polyps, which then distribute nutrients throughout the entire body. This makes the erenna the only known siphonophore with a luring strategy, though given how common and effective such tactics are in the deep, others might be out there waiting to be discovered.