The deep sea is one of Earth’s final frontiers. Given its crushing pressure, near-freezing temperatures and utter darkness, it seems as though it’s almost impossible for life to survive within this environment. Yet some fish still find a way to thrive there — often in forms so strange and well-disguised that scientists have only ever recorded them once.
These ephemeral creatures remind us that so much of our oceans are a mystery to us, with complete families of creatures existing largely beyond our understanding. Here are three of these rare and mysterious deep-sea fish.
1. The Ghost Fish
The ghost fish (part of the family Aphyonidae) is one of the most famous examples of a deep-sea creature that had previously only been known from trawl-caught specimens. In 2016, the National Oceanic and Atmospheric Administration’s Okeanos Explorer captured the first-ever live footage of this fish at roughly 2,500 meters in the Mariana Trench — as described in a special edition of the journal Oceanography.
Its body was roughly 10 centimeters (3.9 inches) long. Most curiously, it was also almost completely transparent and gelatinous; it had no scales, along with highly reduced, pigment-free eyes. Its almost ethereal appearance further confirmed that it is most likely a bottom-dwelling species, rather than an open-water drifter.
The discovery of the ghost fish illustrates just how elusive deep-sea life can be. Its delicate body makes it nearly impossible to collect intact with traditional nets, meaning live encounters — at 2,500 meters (8,200 feet) deep — are perhaps the only way for us to observe them. Researchers are increasingly relying on advanced remotely operated vehicles (ROVs) to observe these fragile creatures in their natural habitat.
2. Prince Axel’s Wonderfish
Prince Axel’s wonderfish (Thaumatichthys axeli) is only known from only a few specimens collected during the 1950s Galathea Expedition. As described in a Galathea Report, this species is a deep-sea anglerfish with a mouth like a trap and a built-in fishing lure. This creature is known to inhabit depths of 3,600 meters (11,811 feet).
Most astonishingly, it uses a strange forked light organ inside its mouth to attract prey. In turn, it uses its jaw as both a weapon and bait. This bizarre, trap-like design is what led it to be described as “unquestionably the strangest catch of the Galathea expedition, and altogether one of the oddest creatures in the teeming variety of the fish world.”
Prince Axel’s wonderfish highlights the ingenuity of evolution in one of the planet’s most extreme environments. Its specialized anatomy allows it to quickly and reliably source nutrition, which is a crucial trait for survival at depths of over 3,000 meters. But even with decades of research, scientists still know little about its reproduction and overall life cycle; there have been no recorded observations of this fish alive.
3. The Galathea Cusk Eel (The Deepest Known Fish)
If rarity is measured by depth, few can compete with the Galathea cusk eel (Abyssobrotula galatheae). First captured during the second Galathea expedition, this fish has since come to hold the record for being the deepest recorded in the world. Astonishingly, 2021 research from Marine Biology notes that it was trawled at approximately 7,965 meters (4.95 miles) deep in the Puerto Rico Trench.
Its body is soft, pale and nearly transparent at around 16.5 centimeters (6.5 inches), with tiny eyes adapted to life under crushing pressure. The combination of its extreme depth and delicate anatomy has made it impossible to observe in situ. As such, its behavior remains largely speculative; marine biologists have virtually no idea how it feeds, reproduces or survives in such extreme conditions.
The discovery of Galathea cusk eel is another that challenges many of the assumptions we have about how large vertebrates can adapt to extreme pressure and scarcity in the ocean. But above all, it also underscores the sheer scale of unexplored ocean depths.
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