In 36 years of deep-sea exploration, MBARI’s remotely operated vehicles have completed more than 7,300 successful dives and recorded more than 28,400 hours of video, yet we have only encountered the dreamer anglerfish nine times. MBARI’s last observation of Oneirodes was in 2016.

“Dreamers are stealthy ambush predators who lie secretly in wait for their prey. Instead of actively hunting for food, they let it come to them, which is one reason we see them so rarely. Coming upon a lurking anglerfish is an exciting experience for anyone exploring the deep water column,” said Senior Scientist Bruce Robison, who leads MBARI’s Midwater Ecology Team and has conducted extensive research on deep-sea fishes.

This deep-sea anglerfish is so named because, when specimens were first collected by scientists in the late nineteenth century, the fish seemed too fantastical to be real and resembled something out of a dream. The unusual appearance of Oneirodes—ultra-black skin, a luminous lure, and sharp teeth—ensures survival in one of the most challenging environments on Earth. 

In the deep sea, temperatures hover just above freezing, distance from the surface extinguishes all sunlight, and pressure mounts from the weight of the water above. Food and mates can be hard to find in this dark, expansive environment. But deep-sea animals have adapted in remarkable ways to thrive in these conditions. Like other deep-sea anglerfishes, Oneirodes has an elaborate lure that produces bioluminescence to attract prey, such as small crustaceans. When a curious crustacean comes close, the anglerfish’s large jaws rapidly open wide and snap shut to trap a meal.

When the team first encountered this dreamer anglerfish, the luminescent lure was extended and “fishing” for food. However, as the ROV moved a little closer, the fish stowed the lure away. During several of our past encounters with Oneirodes, we have observed individuals deploying their lure. 

An ultra-black appearance helps Oneirodes hide from unsuspecting prey. The dark coloration absorbs the light cast by the fish’s luminescent lure so prey cannot see the large mouth waiting just beyond.

“In a habitat where even a single photon can give you away to your ever-hungry predators and would-be prey, ultra-black skin ensures that any light that hits you, even the bright light from your own streetlamp-like lure, is completely absorbed—nothing reflects back to expose your location in the inky black, wide open expanse of the deep, open ocean,” explained MBARI Adjunct Karen Osborn, research zoologist and curator of annelids and peracarids at the Smithsonian National Museum of Natural History. 

Many fishes have evolved dark coloration as camouflage for life in the ocean’s midnight zone. MBARI researchers and their collaborators have revealed the unique way that deep-sea fishes structure their black pigments to achieve ultra-black coloration for camouflage. Their work found the dreamer anglerfish is the blackest of any fish studied to date.

“While other animals such as butterflies and birds of paradise have achieved ultra-black coloration, none have done it in such an efficient system—simply changing the size, shape, packing, and location of the same melanin we have in our skin to absorb every last photon that hits their skin. It really is ingenious and is already inspiring the design of synthetic ultra-black materials used in telescopes, camera lenses, and protective gear,” said Osborn. “This discovery came from basic, curiosity-driven exploration of our ‘backyard’ deep-sea canyon—who knows what else we will find out there as we keep looking.”

As with most deep-sea anglerfishes, there is a dramatic difference in size between females and males. Female Oneirodes are much larger than the males. They can grow up to 37 centimeters (almost 15 inches) in length. MBARI submersible pilots estimate the anglerfish they encountered was about the size of a grapefruit. Male dreamer anglerfish grow to just 16.5 millimeters (about half an inch) in length. While some deep-sea anglerfishes have parasitic males, male Oneirodes are free-living.

Dramatic differences in size and appearance between the sexes is called sexual dimorphism. Such extreme dimorphism is one adaptation to the challenging conditions of the deep sea. Producing eggs requires more energy than producing sperm. The extreme dimorphism reflects the differing reproductive roles of the two sexes—males evolved to seek out females for spawning, and females evolved as hunters to find food to fuel egg production.

MBARI researchers did not collect this fish, but this video observation still provides a valuable glimpse into the lives of deep-sea anglerfishes.

Most of what scientists have learned about deep-sea fishes has been based on specimens caught by nets. Video observations from robotic submersibles, however rare, provide rich details about the behavior and ecology of fishes and other animals that dwell in the deep sea. 

This encounter marked MBARI’s first opportunity to film a deep-sea anglerfish in 4K. The fish was filmed with an ultra high-definition camera developed by MBARI engineers in partnership with DeepSea Power & Light. 4K resolution allows researchers to see details of deep-sea animals in remarkable clarity, and this footage will no doubt be of interest to anglerfish taxonomists and ichthyologists.

Video is the backbone of MBARI science, as well as our education and outreach.

The robotic submersible that recorded this observation recently completed 4,500 dives for deep-sea science. The trove of deep-sea video filmed by MBARI’s submersibles has helped scientists document the diversity of life in the ocean’s depths and better understand humanity’s connection to the deep sea.

The ocean is fundamental to Earth’s climate—the ocean and its inhabitants soak up carbon dioxide at the surface, then marine life transports it down to deeper waters. MBARI’s Carbon Flux Ecology Team is working to understand the role of midwater organisms in this carbon superhighway. 

A healthy ocean is one of the best ways to fight climate change. The ocean has buffered us from the impacts of climate change, but at a cost—it is getting warmer, more acidic, and losing oxygen.

The deep sea needs our protection from climate change and other threats. MBARI hopes the stunning images and videos we share on social media raise awareness about the deep sea and inspire ocean stewardship. The future of the amazing animals of the deep is in our hands.

Learn more about deep-sea anglerfishes and other amazing denizens of the deep in MBARI’s Animals of the Deep gallery. For additional information or images relating to this article, please email pressroom@mbari.org.