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Critters of the Twilight Zone

October 31, 2022

The dark and lovely critters of the mesopelagic zone, or ocean twilight zone, caught during the fall Bottom Trawl Survey aboard NOAA Ship Henry B. Bigelow. At-sea blog from biological science technician Katie Rogers.

 A profile perspective of a red fish with a large eye, pairs of leg-like appendages, and armor plate-like scales lying on a white table.

Leg 2 of Science Center’s Fall Bottom Trawl Survey is underway! We’ve had some lovely catches, including a handful that delve into deeper waters. Here are a few amazing critters we caught in the mesopelagic zone, or “ocean twilight zone”—an area of the ocean about 200 to 1,000 meters deep.

Atlantic Deep-Sea Red Crab

Left, a top-down perspective of a red crab resting on a white table. A pair of large claws are spread out in front of the crab’s head/body and four pairs of legs are spread out around its body. Right, a close up of a red crab’s face and mouth parts. A blue gloved hand holds the red crab.
Atlantic deep-sea red crab. Credit: NOAA Fisheries/Katie Rogers

You might think a bright red crab like the Atlantic deep-sea red crab would stand out to predators, but red light does not penetrate very deeply into the water. At these depths the red light is entirely filtered out, and these crabs actually appear black, blending with the darkness all around. There is a commercial pot fishery for these crabs.

Armored Sea Robin

Left, a profile perspective of a red fish with a large eye, pairs of leg-like appendages, and armor plate-like scales lying on a white table. Right, a close up of the underside of the red fish with pairs of leg-like appendages, a pair of fins on either side of the body, and armored plate-like scales running down the length of its lower half.
Armored sea robin Credit: NOAA Fisheries/Katie Rogers

Armored sea robins are another red fish found in the mesopelagic zone. This species has very sharp spines along its body and considerable bony plating on the head and pelvic girdle. Quite the formidable defense! Check out this amazing video of an armored sea robin moving around and searching for food. NOAA’s Office of Ocean Exploration and Research captured this video during the 2019 Southeastern U.S. Deep-sea Exploration survey. 

Blackbelly Rosefish

 Left, a profile perspective of a red and white mottled fish with a large eye. A pair of forceps helps to display the fish’s dorsal fin to show the black spot near the end of the fin. Right, a close up of a red fish’s face with its mouth open. The fish is held by its dorsal fin with a pair of forceps.
Blackbelly rosefish Credit: NOAA Fisheries/Katie Rogers

Blackbelly rosefish are another red fish that we frequently catch at these depths. It is a relative of the more common and commercially caught Acadian redfish. The black color inside its throat distinguishes this species from the others. Blackbelly rosefish use internal fertilization to reproduce, which is fairly uncommon in the fish world. 

Shortnose and Longnose Greeneye

Two fish lie one above the other from a profile perspective on a white table. A blue gloved finger is touching the top fish. The top fish is mottled silver/gray and yellow and has a large eye, small snout. The bottom fish is nearly all silver and has a large eye, longer snout.
Shortnose greeneye (top) and longnose greeneye (bottom) Credit: NOAA Fisheries/Katie Rogers

Shortnose greeneye and longnose greeneye have large lenses in their eyes that help to absorb what little visible light reaches the bottom. Under blue light, their eyes actually glow. These specialized lenses may allow them to see their prey better by absorbing the surface light so that only the light from their bioluminescent prey will pass through. These two species are also hermaphroditic—an advantageous reproductive trait among species in deep waters where it is often difficult to find fellow species members. 

Two greeneye fish held by a blue gloved hand in profile. There is a slight green glow to the two fishes’ eyes.
Close up of a shortnose greeneye (left) and longnose greeneye (right). Their eyes are glowing slightly green in these photos. Credit: NOAA Fisheries/Katie Rogers


Left, a profile perspective of a silver fish lying on a measuring board. It has two short fleshy appendages hanging from its chin. Right, a close up of the fish’s head and fleshy appendages.
Beardfish Credit: NOAA Fisheries/Katie Rogers

We don’t know a lot about beardfish behavior. The two prominent barbels on their chin distinguish these silver deepwater fish. They also have large eyes, typical for a deepwater species. 


 Left, a silver fish in profile lies on a white table. Its head and body are typically fish-shaped, but its back half is long and thin without a tail fin. Right, a close up of the fish’s head. A thumb pulls on the fish’s snout to reveal a complex, extendable mouth.
Grenadier, also called rattails, from the Macrouridae family. Credit: NOAA Fisheries/Katie Rogers

Grenadier are a unique looking fish from the Macrouridae family, a diverse and abundant group of deep-sea dwellers. Similar to their relative, the Atlantic cod, some grenadiers have specialized swim bladders that allow them to make “drumming” sounds, possibly to locate a mate for courtship. They also have large eyes to assist in low light and a complex, extendable mouth that helps them get a little more reach when trying to capture prey. 


A diamond-shaped orange/red fish in profile. These fish have large eyes, and its body is slightly taller than it is wide (snout to tail).
Boarfish Credit: NOAA Fisheries/Katie Rogers

Deepbody boarfish (are another red, deep-dwelling fish. Its laterally compressed body shape makes the boarfish easy to identify. Their eyes are also relatively large for their body size, a common adaptation for fish that live in deep, low-light conditions.


 Left, a profile perspective of white, silver, and gold mottled fish lying on a white table. Its snout is squat and thin. Right, a close up of the fish’s head. Its mouth is slightly open, and its lower jaw protrudes beyond its upper jaw. Its large eye is ringed in yellow/gold.
Duckbill Credit: NOAA Fisheries/Katie Rogers

Duckbills are aptly named for their broad, flattened snout. They have large mouths full of small, sharp teeth and, like many other deep water species, they have large eyes. You can see from the top-down photo below how the eyes are positioned near the top of the head, so the fish can easily see what is above them.

A top-down perspective of a fish lying on a white table. Its large eyes are close together and positioned near the top of its head.
Eyes of duckbills are positioned near the top of the head. Credit: NOAA Fisheries/Katie Rogers
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Last updated by Northeast Fisheries Science Center on November 14, 2022