Expert Take | Mark Valderrama — AquariumStoreDepot
None of these fish belong in a home aquarium, and none of them ever will. I’ve been keeping fish for over 25 years and this is the one category where even I am just a spectator. The pressure adaptations, the bioluminescence, the biology that falls apart the moment these animals reach the surface: the deep sea is genuinely humbling. These 15 species represent what happens when life evolves in total darkness, crushing pressure, and near-freezing temperatures for millions of years. They’re fascinating precisely because they’re so far outside anything we can replicate.
Deep sea fish are the one category where even someone with 25 years in the hobby just gets to be a spectator. None of these species can be kept in captivity, and most have never been seen alive at depth. What draws me to them is how extreme the adaptations are: bioluminescence, hinged jaws, pressure-resistant biology that we still don’t fully understand. As someone who’s spent decades obsessing over what makes fish thrive in controlled environments, the deep sea is a reminder of how much of the ocean is still completely outside our reach. Here are 15 of the most unusual species ever documented.
Unfortunately, none of these fish will ever be able to make their way into the home aquarium. Still, they’re some of the most interesting species ever discovered. Let’s look at 15 of the most unusual deep sea fish ever documented.
Key Takeaways
- There is an incredible amount of life at the bottom of the ocean.
- The better majority of the ocean has yet to be discovered and new species are regularly found.
- Deep sea organisms have had to make extreme adaptations to their environments.
- New technology is allowing for deep sea observation, collection, and preservation.
Avoid If
- You’re looking for aquarium fish. Every species on this list is impossible to keep in captivity.
- You assume “rare” means “available.” Deep sea collection would kill these animals before they reached a surface tank.
- You confuse shallow-water anglerfish (which some hobbyists do keep) with their abyssal relatives. They’re from completely different environments.
An Overview
What is the deep sea and how deep is it actually?
It’s believed that the deepest part of the ocean is about 35,876 feet (10,935 m) below the surface1.
There are many layers to the ocean which can be divided into 5 different pelagic zones based on depth below sea level: the Epipelagic (0-660 ft / 0-200 m), Mesopelagic (660-3,300 ft / 200-1,000 m), Bathypelagic (3,300-13,000 ft / 1,000-4,000 m), Abyssopelagic (13,000-20,000 ft / 4,000-6,000 m), and Hadalpelagic (20,000-36,000 ft / 6,000-11,000 m) zones.
The deep sea is largely considered the part of the ocean without light, where continental shelves start to turn into continental slopes. Light begins to fade at about 660 feet (200 m), which falls in the Mesopelagic zone, also known as the twilight zone. This means the majority of the ocean is considered the deep sea. We’ve learned a great deal from Alan Jamieson, a well-known marine biologist who has participated in over 65 deep sea expeditions and runs the Deep-Sea Podcast. Definitely check it out after reading this article if you want to learn more about these creatures.
What fish live in the deep sea?
Many fish live in the deep sea. It is impossible to say exactly how many species are living at the bottom of the ocean since the ocean is largely unexplored. Some of the most recognizable deep sea species are anglerfish, sleeper sharks, and lanternfish.
What fish goes the deepest?
As of now, the deepest fish ever recorded is a Mariana snailfish (Pseudoliparis swirei) at 26,135 feet (7,966 m).
What is the newest species discovered?
An estimated 5,000 new species of deep sea organisms were found in the Pacific Ocean in recent years. One of these new species is a gummy squirrel (Psychropotes longicauda), which is a type of sea cucumber2.
Some Crazy Facts About Creatures Here
As of now, only about 20% of the ocean has been documented and understood. Even within that 20% are numerous microcolonies and ecosystems that have yet to be fully explored. As marine science technology advances, scientists are finally able to get a glimpse into the world that exists at extreme depths.
One of the major factors preventing further research is pressure, which makes exploration and collection extremely difficult. As depth increases, temperature decreases while pressure increases.
Interestingly, the bottom of the ocean always remains just above freezing at about 39 degrees F (4 degrees C). While this temperature may be adaptable for some creatures, the other extreme factors experienced in the ocean’s depths have caused many animals and bacteria to become highly specialized.
Light begins to dissipate after about 660 feet (200 m) below sea level. Even in complete darkness, life found a way. Many deep sea fish still have eyes. While it is not fully understood why the deepest fish have eyes that would otherwise be used to sense visible light, it is strongly believed that they interpret bioluminescence instead.
Difficulty Tiers | Can These Fish Be Kept?
- Impossible (All abyssal and hadal species): Anglerfish (deep sea varieties), Mariana Snailfish, Faceless Cusk Eel, Barreleye, Rattail Fish, Lanternfish, Viperfish, Stoplight Loosejaw, Black Seadevil, Deep-Sea Lizardfish, Chimaera. None survive depressurization during collection.
- Theoretically possible but not in practice (mesopelagic species): Atlantic Wolffish and Sarcastic Fringehead live in reachable depths but have never been established in captivity. Specialized public aquariums occasionally attempt them, with mixed results.
- A related species IS aquarium-kept: Shallow-water anglerfish from the genus Antennarius are available in the hobby and can be kept. They’re a completely different animal from the deep-sea varieties shown here.
Bioluminescence

Bioluminescence, a chemical reaction between luciferin and oxygen that generates internal light, is a common method of both predation and protection in deep sea organisms. It is believed that up to 75% of deep sea organisms generate their own light.
For example, some species of anglerfish (Lophiiformes order) are able to light up their fishing lure appendage to attract prey in front of their large mouth. Other fish, like the marine hatchetfish (Sternoptychidae family), use bioluminescence for counter-illumination. This is a method of camouflage where the fish lights up the bottom of its body to better blend in with any light that a predator below might perceive.
Absence of Light
What happens to the organisms that depend on light? Photosynthetic organisms do not exist past the sunlight zone of the ocean. However, there are known species of coral and sponge that thrive outside of this layer.
Deep sea corals grow extremely slowly and are incredibly old as a result. Instead of using light for energy, these organisms rely on consuming other organisms. Like so many deep-sea creatures, a lot of their nutrition comes from organic material that falls from the ocean surface above.
Chemosynthesis
Even at the bottom of the ocean, life depends on bacteria. The nitrogen cycle looks a little different down here, though, and organisms chemosynthesize instead3.
Chemosynthesis is the process that organisms use to create energy from inorganic materials. For example, giant tube worms (Riftia pachyptila) contain symbiotic bacteria that use oxygen and hydrogen sulfide to provide the worm with essential nutrients.
Extreme Pressure
As ocean depth increases, pressure increases and conditions quickly become uninhabitable for most species. Most terrestrial and shallow marine organisms have gas-filled cavities, like lungs. Under high pressure, these cavities would collapse. Deep sea species have had to evolve to survive this.
One way organisms do this is by being comprised mostly of water. This way, internal pressure matches external pressure. Other adaptations include flexible bodies, specialized lungs, slower movement, and reduced metabolic processes.
One instance of extreme pressure adaptation is the sperm whale (Physeter macrocephalus). These whales regularly exhibit vertical migration, sometimes spanning 1,000 feet (305 m). Pressure varies enormously across this distance. In response, sperm whales have collapsible ribs and lungs that can adjust as needed.
Collecting deep sea fish is difficult for scientists precisely because of these pressure gradients. If an organism is brought to the surface, things that were compacted under pressure are left to expand. Organs are sometimes forced out of the animal’s orifices, resulting in death. This is why so many deep sea specimens can only be studied from washed-up carcasses or real-time observation on the sea floor.
Unique Ecosystems
While the ocean floor is a unique ecosystem in itself, there are self-sustaining environments with species found only in those locations. One of these ecosystems surrounds hydrothermal vents.
Hydrothermal vents are fissures in the deep ocean where seawater cycles through the sediment and gets geothermally heated. The water is then released back into the ocean, filled with important minerals and gases. These vents are often located near areas with high volcanic activity.
Hydrothermal vents are rich in minerals and gases. The intense heat can sometimes reach 700 degrees F (371 degrees C). Despite this, some deep sea organisms are found only in these ecosystems, including the yeti crab (Kiwa spp.) and the scaly-foot gastropod (Chrysomallon squamiferum).
Another mini ecosystem that emerges in the deep ocean is known as whale fall. When a deceased whale sinks to the ocean floor, it becomes a temporary ecosystem that can last a few decades. Whale falls are very important for local ecosystems as well as those above. Some common deep sea fish seen around whale falls are hagfish (Myxinidae family) and sleeper sharks (Somniosus spp.). Many other invertebrates and small crustaceans also make their homes in whale falls, including mussels, clams, and octopuses.
Top 15 Deep Sea Fish
These are some of the most recognizable and remarkable deep sea species documented. None can be kept in home aquariums, but each one shows what millions of years of deep-sea evolution actually looks like. We have a video from our YouTube Channel below. Check it out alongside the article for more context, and subscribe if you enjoy our content.
1. Anglerfish

- Scientific Name: Lophiiformes order
- Diet: Carnivorous
- Size: Up to 3 feet (91 cm); most individuals stay under 7 inches (18 cm)
- Origin: Worldwide
- Depths: Epipelagic to mesopelagic (under 3,300 ft / 1,000 m)
- Unique Features: Bioluminescent fishing rod lure
There are over 200 species of anglerfish. A select few shallow-water species are available in the aquarium hobby, specifically species from the Antennarius genus. The deep-sea varieties on this list are a completely different story.
Deep sea anglerfish are easily identified by the modified fin ray that dangles in front of their mouths and acts as bait. Most times, the lure is bioluminescent. Another fascinating fact: most anglerfish species display extreme sexual dimorphism. In some species, the male becomes a parasite that permanently fuses to the female’s body purely for reproduction.
2. Black Seadevil
- Scientific Name: Melanocetus spp.
- Diet: Carnivorous
- Size: 1-7 inches (2.5-18 cm)
- Origin: Atlantic, Pacific, and Indian Oceans
- Depths: Mesopelagic to bathypelagic (660-13,000 ft / 200-4,000 m)
- Unique Features: Pitch-black body coloration
One notable genus of anglerfish is the group of black seadevils (video source). These fish have pitch-black skin that allows them to blend in across the twilight zone and deeper. Within this genus is one of the most recognizable species, the humpback anglerfish (Melanocetus johnsonii).
3. Viperfish

- Scientific Name: Chauliodus spp.
- Diet: Primarily carnivorous
- Size: Up to 12 inches (30 cm)
- Origin: Atlantic, Pacific, and Indian Oceans
- Depths: Mesopelagic to bathypelagic (660-13,000 ft / 200-4,000 m)
- Unique Features: Vertical migration; bioluminescent lure; transparent body
At first glance, the viperfish is genuinely intimidating. These fish lack scales and appear almost transparent. They have large bottom teeth and an extendable jaw that opens wide for larger prey.
Like many other fish in the meso- and bathypelagic zones, viperfish use bioluminescence. To attract prey, they have a modified bioluminescent fin ray they dangle in front of their mouths, similar to anglerfish. They also use counter-illumination to hide from predators. Viperfish display diel vertical migration, living in deep water during the day and moving to shallower conditions at night to hunt.
4. Sleeper Shark
- Scientific Name: Somniosidae family
- Diet: Carnivorous
- Size: 20+ feet (6+ m); most individuals average 12 feet (3.6 m)
- Origin: Arctic Ocean
- Depths: Mesopelagic to bathypelagic (660-13,000 ft / 200-4,000 m)
- Unique Features: Anti-freeze proteins; extreme longevity
The sleeper shark (video source) is one of the more well-known species, namely the Greenland shark (Somniosus microcephalus) which is known to live up to 500 years. These sharks live in deeper polar and subpolar waters and have adapted to extreme cold using anti-freeze proteins that prevent their blood from crystallizing. Their slow movement and low metabolism are believed to be key factors in their remarkable longevity.
5. The Sarcastic Fringehead

- Scientific Name: Neoclinus blanchardi
- Diet: Carnivorous
- Size: Up to 8 inches (20 cm)
- Origin: Pacific Ocean
- Depths: Epipelagic (0-660 ft / 0-200 m)
- Unique Features: Enormous hinged jaw
The sarcastic fringehead doesn’t come from the deepest parts of the ocean but still lives towards the edge of the epipelagic zone at an average depth of about 300 feet (91 m). These fish make this list because of their remarkable giant mouths that open up triangularly from the sides. They live in tight crevices in shallow waters and regularly defend their homes against other males using jaw displays that are genuinely hard to believe the first time you see them.
6. Stoplight Loosejaw
- Scientific Name: Malacosteus spp.
- Diet: Carnivorous
- Size: Under 1 foot (30 cm)
- Origin: Worldwide
- Depths: Mesopelagic to bathypelagic (660-13,000 ft / 200-4,000 m)
- Unique Features: Open hanging jaw; red photophores; fang-like teeth
The stoplight loosejaw (video source) is aptly named for its hanging, bottomless lower jaw and fang-like teeth. To help catch prey, these fish use red suborbital photophores that emit from their head. Most fish living in low-light conditions cannot perceive red light, which makes this essentially an invisible flashlight for hunting. Despite all this predatory equipment, stoplight loosejaws prefer relatively small foods like copepods and crustaceans.
7. Chimaera
- Scientific Name: Chimaeriformes order
- Diet: Carnivorous
- Size: Up to 5 feet (1.5 m)
- Origin: All oceans besides the Antarctic
- Depths: Epipelagic to bathypelagic (under 13,000 ft / 4,000 m)
- Unique Features: Cartilaginous skeleton; pectoral-fin “flight”
Not to be confused with the mythological chimera, chimaeras (video source) are cartilaginous fish related to sharks and rays. They have a unique way of swimming that makes them appear to be flying through the water, propelled by large pectoral fins with surprisingly little effort. Like many cartilaginous fish, chimaeras use electroreception to sense prey in the dark. They mainly eat crustaceans.
8. Mariana Snailfish
- Scientific Name: Pseudoliparis swirei
- Diet: Carnivorous
- Size: Under 1 foot (30 cm)
- Origin: Pacific Ocean
- Depths: Hadalpelagic (20,000-36,000 ft / 6,000-11,000 m)
- Unique Features: Transparent skin; deepest fish ever recorded
The Mariana snailfish holds the record as one of the deepest fish ever documented, named after its home in the dark caverns of the Mariana Trench. This transparent fish is actually a top predator in its stretch of the trench, feeding on small crustaceans and other fish. An interesting discovery about the Mariana snailfish is its adaptation to laying unusually large eggs, though the exact evolutionary reason for this is not yet understood.
9. Lanternfish
- Scientific Name: Myctophidae family
- Diet: Carnivorous
- Size: Under 6 inches (15 cm)
- Origin: Worldwide
- Depths: Epipelagic to bathypelagic (under 13,000 ft / 4,000 m)
- Unique Features: Bioluminescence; vertical migration; enormous biomass
Arguably the most prevalent deep-sea fish in all the world’s oceans, lanternfish are believed to make up about 65% of the total biomass of all deep sea fish. That makes them an extremely important food source for their immediate ecosystem and those above and below. They were recorded on video for the first time in 2007, reported by National Geographic.
Lanternfish are named after their prominent use of bioluminescence. These fish emit blue, green, or yellow light depending on species and sex. They also display diel vertical migration, staying in and around the bathypelagic zone during the day and moving to the epipelagic zone at night to feed on zooplankton.
10. Red Handfish
- Scientific Name: Thymichthys politus
- Diet: Carnivorous
- Size: About 3 inches (7.5 cm)
- Origin: Tasmania
- Depths: Epipelagic (under 660 ft / 200 m)
- Unique Features: Red coloration; hand-like pectoral fins
The red handfish (video source) isn’t a true deep sea species, but their appearance and biology put them in the same conversation. Found only in very specific reef ecosystems around Tasmania, red handfish have a distinctive light red body speckled with darker spots. They use their hand-like fins to scoot across the sea floor searching for worms and small crustaceans. The red handfish is currently recognized as critically endangered.
11. Rattail Fish
- Scientific Name: Macrouridae family
- Diet: Carnivorous
- Size: 4-60 inches (10-152 cm)
- Origin: Worldwide
- Depths: Epipelagic to bathypelagic (under 13,000 ft / 4,000 m)
- Unique Features: Large head; tapering rat-like tail
Also known as grenadiers, members of the family Macrouridae are very abundant in the deep ocean (video source) and are believed to make up about 15% of the deep sea fish population. There are many different species, with some reaching impressive sizes and some forming schools. They get their common name from their large head, big eyes, and sharply tapering tail.
12. Faceless Cusk Eel
- Scientific Name: Typhlonus nasus
- Diet: Carnivorous
- Size: About 11 inches (28 cm)
- Origin: Pacific and Indian Oceans
- Depths: Bathypelagic to abyssopelagic (3,300-20,000 ft / 1,000-6,000 m)
- Unique Features: Reduced face; enormous nostrils; bottomless mouth
The faceless cusk eel (video source) is rare and unmistakable. Despite the name, it is not related to true eels (Anguilliformes). These fish have a large face that often lacks visible eyes, with enormous nostrils that are frequently mistaken for eyes, and a mouth positioned at the very bottom of the face. Once seen, impossible to forget.
13. Deep-Sea Lizardfish
- Scientific Name: Bathysaurus ferox
- Diet: Carnivorous; sometimes cannibalistic
- Size: Under 28 inches (71 cm)
- Origin: Atlantic Ocean and Indo-Pacific
- Depths: Mesopelagic (660-3,300 ft / 200-1,000 m)
- Unique Features: Upward-pointing mouth; teeth on tongue and jaws
The deep-sea lizardfish is a fearsome ambush predator (video source). It lies flat on the sea floor, completely still, waiting for other fish to swim above. Then it strikes upward with a large mouth lined with teeth on both the jaws and the tongue. It will eat other lizardfish when the opportunity arises.
14. Atlantic Wolffish

- Scientific Name: Anarhichas lupus
- Diet: Carnivorous
- Size: Up to 5 feet (1.5 m)
- Origin: West and east coasts of the Atlantic Ocean
- Depths: Epipelagic to mesopelagic (under 3,300 ft / 1,000 m)
- Unique Features: Anti-freeze proteins; protruding canine teeth
The Atlantic wolffish is a large, recognizable species often caught in bycatch. These fish prefer rocky habitats and caves toward the deeper end of their range. They have powerful jaws and very large protruding teeth that help them crush sea urchins and green crabs, making them an important population regulator in their ecosystem. Like the Greenland shark, wolffish carry special anti-freeze proteins to keep their blood from crystallizing in cold Atlantic waters.
15. Barreleye
- Scientific Name: Opisthoproctus soleatus
- Diet: Carnivorous
- Size: 4-5 inches (10-13 cm)
- Origin: Eastern Atlantic Ocean, South China Sea
- Depths: Mesopelagic (1,600-2,300 ft / 490-700 m)
- Unique Features: Transparent head; tubular eyes that rotate upward
The barreleye (also known as the spook fish) is one of the strangest fish in any ocean. Its head is completely transparent, which allows it to soak in more ambient light. The tubular eyes inside that transparent dome can rotate upward to detect the silhouettes of prey above them. No teeth, no spines, fairly small at 4-5 inches (10-13 cm). The transparent head is not just a novelty: it’s a precision hunting adaptation built over millions of years.
Mark’s Pick | Most Fascinating Species on This List
After 25 years in the hobby, my answer is the barreleye. The transparent head and rotating eyes are hard to accept as real even when you’re watching footage of one. A close second is the Mariana snailfish: a small, translucent fish holding the record as the deepest vertebrate ever documented, thriving in conditions that would crush most life forms. Both of these species remind me why the ocean is still so much bigger than anything we fully understand. If any of these ever became somehow keepable (they won’t), those two would be the ones I’d want to observe up close.
Deep Sea Species at a Glance
| Species | Max Depth | Size | Bioluminescent? | Keepable? |
|---|---|---|---|---|
| Anglerfish | 3,300 ft (1,000 m) | Up to 3 ft (91 cm) | Yes | No (shallow species only) |
| Viperfish | 13,000 ft (4,000 m) | 12 in (30 cm) | Yes | No |
| Lanternfish | 13,000 ft (4,000 m) | Under 6 in (15 cm) | Yes | No |
| Mariana Snailfish | 26,135 ft (7,966 m) | Under 1 ft (30 cm) | No | No |
| Barreleye | 2,300 ft (700 m) | 4-5 in (10-13 cm) | No | No |
| Sleeper Shark | 13,000 ft (4,000 m) | Up to 20 ft (6 m) | No | No |
| Sarcastic Fringehead | 660 ft (200 m) | Up to 8 in (20 cm) | No | No (not established) |
| Atlantic Wolffish | 3,300 ft (1,000 m) | Up to 5 ft (1.5 m) | No | No (not established) |
Closing Thoughts
Marine life doesn’t stop past the reach of light. The depths of the ocean are full of life, though a very different kind of life than what we’re used to. Animals have had to make extraordinary adaptations to these extreme environments: bioluminescence, anti-freeze proteins, transparent bodies, eyes that rotate inside fluid-filled domes. Some migrate hundreds of feet every night just to feed. None of them will ever be in a home aquarium, and that’s fine. Some things are worth appreciating from a distance.
If reading about extreme fish sparked something for you, the aquarium hobby has plenty of its own fascinating species that actually can be kept. Shallow-water anglerfish from the Antennarius genus are genuinely weird and are available through specialist suppliers. If you’re looking for something more accessible but still visually striking, both Flip Aquatics and Dan’s Fish carry a solid selection of unusual freshwater and marine species worth exploring.








































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