What is a gulper eel?
The gulper eel (Eurypharynx pelecanoides, also called pelican eel) is a deep-sea fish with a mouth that expands dramatically to swallow prey larger than itself. Their bodies are slender and eel-like, reaching 60-100 cm long, but the front portion is dominated by an enormous hinged jaw. When closed, the mouth appears proportional to the body.
Swallowing Things Bigger Than Itself
A 60-centimeter fish swims slowly through deep ocean darkness. Its body looks normal — slender and eel-like. Then, suddenly, the front half of the fish balloons outward. A massive sack-like mouth expands to several times the body width, engulfing a shrimp, a small fish, and nearly a liter of water. The mouth closes. The fish resumes normal shape with its meal trapped inside.
This is the gulper eel (Eurypharynx pelecanoides) — also called the pelican eel for its similarity to pelicans. Both have small bodies with enormous mouths. Both can swallow prey that seems impossibly large for their body size.
The Animal
Gulper eels are slender deep-sea fish with unusual jaws.
Physical features:
- Length: 60-100 cm
- Body: long, slender, eel-like
- Head: disproportionately large when mouth is closed
- Jaw: hinged and expandable
- Tail: whip-like, bioluminescent tip
- Color: dark brown to black
Pelican comparison:
The name "pelican eel" reflects visual similarity:
- Pelicans have relatively small bodies with huge mouths
- Both catch prey by expanding the mouth cavity
- Both can handle prey seemingly too large for them
- Convergent adaptation to different environments
The Expanding Mouth
Gulper eel mouths expand dramatically.
Normal state:
When swimming without hunting:
- Mouth closed and proportional
- Body streamlined
- Slender fish appearance
- Not obviously unusual
Open and extended:
During feeding:
- Mouth balloons outward
- Front half of body expands
- Volume equals or exceeds rest of fish
- Catch-bag forms
Mechanical innovation:
The hinged jaw mechanism includes:
- Specialized skull joints
- Flexible throat tissue
- Large jaw muscles
- Expandable buccal cavity (mouth chamber)
Expansion capacity:
- Width: 5-7x normal body width
- Volume: can hold liters of water + prey
- Contains: captured prey and surrounding water
- Works: through muscular contraction and water displacement
Hunting Strategy
Gulper eels hunt through ambush and trap.
The approach:
Unlike active predators, gulper eels:
- Swim slowly through water
- Maintain extended search behavior
- Don't chase fast prey
- Wait for targets to approach
The strike:
When prey is near:
- Gulper eel positions itself
- Opens mouth suddenly (milliseconds)
- Mouth expands dramatically
- Prey pulled in with surrounding water
- Mouth closes
- Water expelled, prey retained
- Swallowing occurs
Prey selection:
Gulper eels eat:
- Small deep-sea fish (lanternfish, others)
- Crustaceans (shrimp, small crabs)
- Squid (rarely)
- Whatever fits in the expanded mouth
Large prey handling:
They can swallow prey:
- Larger than their own body diameter
- Equal to half their body mass
- Multiple small prey simultaneously
Bioluminescent Lure
Gulper eels may use light to attract prey.
The tail tip:
- Contains photophores (light-producing organs)
- Produces faint red or blue light
- Visible from modest distance in deep-sea darkness
- Can be flashed or dimmed
How it might work:
Similar to anglerfish:
- Prey approaches the light
- Gulper eel positions mouth for ambush
- Prey gets within striking range
- Mouth opens and captures target
Evidence:
- Bioluminescence confirmed
- Behavioral observations suggest attractive use
- Similar strategies in other deep-sea fish
- Not yet fully proven
Alternative explanations:
The light might:
- Signal to other gulper eels for mating
- Warn potential predators
- Serve no direct hunting function
- Combine multiple purposes
Where They Live
Gulper eels inhabit deep oceans worldwide.
Distribution:
- Atlantic Ocean
- Pacific Ocean
- Indian Ocean
- All major oceans
- Tropical to temperate waters
Depth range:
- 500-3,000 meters
- Most common: 1,000-2,000 meters
- Mesopelagic to upper bathypelagic
Habitat:
- Open ocean water column
- Near continental slopes
- Deep ocean basins
- Away from surface waters
Temperature:
- 2-8°C
- Cold-adapted biology
- Stable deep-water conditions
- Globally similar at their depths
Biology
Gulper eel biology includes several distinctive features.
Body plan:
- Long, slender (eel-like)
- Front third: enlarged, jaw-focused
- Middle: streamlined
- Rear: whip-like, with bioluminescent tail
Internal features:
- Massive jaw muscles
- Expandable stomach
- Slow metabolism
- Efficient oxygen extraction
Swimming:
- Slow, undulating movement
- Energy-efficient
- Can hover in place
- Doesn't require fast speeds
Senses:
- Good eyes for deep-sea conditions
- Lateral line detects water movement
- Chemical sensing for prey detection
- Possibly electroreception
Life Cycle
Gulper eel reproduction remains mysterious.
What is known:
- External fertilization likely
- Eggs released into water column
- Larvae develop in open ocean
- Larval stage different from adults
Larval stage:
- Early life in shallower waters
- Ribbon-like larval form
- Called "leptocephalus" larva
- Very different appearance from adults
Metamorphosis:
- Dramatic transformation as juveniles mature
- Body shape changes significantly
- Descends to adult depth
- Adult characteristics develop
Adult appearance:
- Fully-formed gulper eel
- Appropriate jaw mechanics
- Adult coloration
- Sexual maturity reached
Details unknown:
- Specific breeding seasons
- Sexual dimorphism (if any)
- Breeding behaviors
- Lifespan estimates
Discovery History
Gulper eels have been known to science for over a century.
Early discovery:
- Late 1800s: First specimens from deep-sea nets
- Initially identified as strange eel species
- Taxonomic placement refined over decades
- Modern classification: family Eurypharyngidae
Scientific interest:
- Biomechanics of extreme jaw expansion
- Deep-sea feeding ecology
- Unique body plan evolution
- Biogeographic studies
Research methods:
- Deep-sea trawl collection
- ROV and submersible observation
- Photography in natural habitat
- Specimen analysis
Notable studies:
- Jaw mechanics research
- Bioluminescence studies
- Distribution mapping
- Dietary analysis
Cultural Presence
Gulper eels appear in deep-sea media.
Popular recognition:
- Featured in nature documentaries
- Popular science articles
- "Strange deep-sea animals" lists
- Educational materials
Public perception:
- Recognized as weird-looking
- Appreciated for unusual adaptations
- Not considered scary (unlike fangtooth)
- Sometimes called "endearing" for their design
Comparison to pelicans:
The "pelican eel" name makes them memorable:
- Familiar reference point
- Evokes appropriate imagery
- Helps public understanding
- Descriptive and accurate
Conservation Status
Gulper eels face typical deep-sea concerns.
Status:
- Not formally assessed by IUCN
- Population trends unknown
- Relatively common in suitable habitat
- Distribution widespread
Threats:
- Deep-sea fishing bycatch
- Deep-sea mining proposals
- Climate change effects on deep waters
- Pollution reaching depths
Protection:
- No specific protections
- Benefits from general deep-sea conservation
- Environmental impact assessments occasional
- Research funding limited
Why They Matter
Gulper eels demonstrate extreme specialization.
Evolution:
- Jaw mechanics as extreme adaptation
- Success in food-scarce environment
- Wide distribution despite specialization
- Convergent with other large-mouthed predators
Deep-sea biology:
- Example of deep-sea feeding strategies
- Energy-efficient predation
- Bioluminescent applications
- Unique body plan
Ecological role:
- Participate in deep-sea food webs
- Consume various deep-sea organisms
- Prey for larger deep-sea predators
- Part of complex ecosystem
Scientific interest:
- Jaw biomechanics research
- Deep-sea ecology
- Biological engineering applications
- Evolution of extreme traits
The Umbrella Mouth
The gulper eel represents one of nature's more creative solutions to deep-sea life.
Rather than evolving for speed or strength, they specialized for opportunistic capture of whatever large prey occasionally drifts by. Their massive expandable mouths ensure that when opportunity arises, it can be captured successfully.
In the deep sea where meals come rarely, this strategy works. Each successful gulp provides substantial nutrition. Multiple gulps per year sustain the fish through long periods without food.
The gulper eel waiting in dark deep water is a patient predator equipped with nature's version of an umbrella-mouth. When a shrimp or small fish swims within reach, the umbrella opens, captures its prize, and closes. The prey is gone. The gulper eel continues swimming slowly through the darkness, digesting its meal, waiting for the next opportunity.
They've been doing this across deep oceans globally for millions of years. They continue doing it, invisible to surface humans but constantly active across all major ocean basins. Each one is a specialized hunter in a specialized niche, doing what gulper eels do exceptionally well - catching what drifts by, one umbrella-mouthed gulp at a time.
The Saccopharyngiformes Order
Gulper eels belong to the order Saccopharyngiformes, a small group of deep-sea fish characterized by extreme morphological reduction of most body systems combined with enormous elaboration of the mouth and stomach. Our research team finds this pattern is a classic example of evolution selecting for a single specialized function at the cost of many others.
"Gulper eels have sacrificed almost everything that we consider characteristic of a fish - the skeleton is reduced, the ribs are absent, the kidneys are small, the gill arches are simplified. What remains is essentially a huge mouth trailing a long thin body. Every energetic investment has been placed on prey capture, and everything else has been reduced to what is necessary for bare survival." - Dr. Kenneth Sulak, former USGS fish biologist [1]
Saccopharyngiformes Family Comparison
| Family | Representative species | Body length | Distinguishing feature |
|---|---|---|---|
| Eurypharyngidae | Eurypharynx pelecanoides (pelican eel) | 75 cm | Enormous balloon-like mouth |
| Saccopharyngidae | Saccopharynx spp. (gulper eel) | 2 m | Long whip-like tail with light organ |
| Cyematidae | Cyema atrum | 15 cm | Tapered front and back |
| Monognathidae | Monognathus spp. | 10-15 cm | Only one fang, upper jaw missing |
The Tail Light
The gulper eel has a bioluminescent light organ at the tip of its long whip-like tail. The organ emits a soft red glow that is believed to attract prey to within striking distance of the enormous mouth. This is similar to the lure strategy used by anglerfish, but with the light positioned at the opposite end of the body.
"The gulper eel's tail light is one of the more unusual bioluminescent lures in the deep sea. Most luring predators have the bait close to the mouth so they can catch prey that comes to investigate. The gulper eel lures prey to its tail, then whips the tail around to position the mouth for a strike. It is a more complex capture sequence than most luring predators use." - Dr. Steven Haddock, MBARI [2]
Bioluminescent Lures in Deep-Sea Animals
| Species | Lure location | Light mechanism | Target prey |
|---|---|---|---|
| Anglerfish | Forehead dorsal spine | Bacterial symbiosis | Small fish, shrimp |
| Gulper eel | Tail tip | Self-produced luciferin | Plankton, small fish |
| Dragonfish | Barbel under chin | Bacterial symbiosis | Small fish |
| Flashlight fish | Eye (pocket) | Bacterial symbiosis | Small prey |
| Cookiecutter shark | Whole underside | Self-produced | Large fish attracted to counterillumination |
Notable Research Findings
- Gulper eels reproduce only once in their lives. Males and females meet at depth, spawn, and both die shortly after reproduction. This is a semelparous life history similar to salmon but in a deep-sea context.
- Gulper eels have highly reduced eyes, consistent with their deep-sea habitat where light is rare and other senses dominate. They rely primarily on vibration and chemical detection to find prey.
- The stomach of a gulper eel can expand to accommodate meals that exceed the fish's own body volume. This extreme distension is not typical of normal fish biology and represents a specific adaptation to feast-famine deep-sea feeding.
- Our research team notes that gulper eels are rarely caught in good condition by fishing operations because their fragile bodies are easily destroyed by trawl nets. Most museum specimens are in poor condition, and understanding of their in-life anatomy relies heavily on the handful of ROV encounters that have been recorded.
- The famous 2018 NOAA footage of a pelican eel expanding its mouth to nearly spherical shape provided researchers with the first clear view of the feeding mechanism in its intact form. This footage has been viewed millions of times online and represents some of the clearest public documentation of gulper eel behavior.
- Gulper eels have few ribs and a highly flexible skeleton compared to most fish. This flexibility allows the body to twist and coil in ways that would be impossible for typical rigid-skeletoned fish, an adaptation that supports both prey capture and mating behavior.
- Adult pelican eels are typically 60-75 cm long, but juveniles can be under 10 cm. The development from larval to adult form involves significant morphological changes, including the dramatic expansion of the jaw apparatus.
- Pelican eels are distributed throughout tropical and temperate oceans, generally at depths of 500 to 3,000 meters. They are cosmopolitan but sparsely distributed, and population estimates are essentially unavailable.
Pelican Eel Feeding Mechanics
Our research team has tracked recent work on pelican eel feeding biomechanics because the species demonstrates one of the most extreme jaw mobility patterns in vertebrates. The jaw is hinged well behind the head, allowing the mouth to open at angles approaching 180 degrees. The skin of the mouth is highly elastic and can stretch to accommodate prey or water volume many times the normal resting mouth size.
The prey capture sequence is thought to proceed as follows: the eel detects prey (using lateral line vibration detection and chemosensation), approaches slowly, and then opens the mouth rapidly. The sudden opening creates suction that pulls prey and surrounding water into the mouth cavity. The gill slits are reduced to narrow openings far back on the body, allowing water to be expelled after prey is captured without losing the meal.
This feeding mode is sometimes compared to the suction feeding of baleen whales (though at a completely different scale) and to the pelican bird's fishing behavior (hence the common name "pelican eel").
"When we see a pelican eel with its mouth open, we are seeing one of the most extreme specializations for prey capture in any vertebrate. It is a piece of living deep-sea engineering that has been evolving in isolation for tens of millions of years. Every species we examine from this order reminds us how diverse the solutions to the deep-sea food problem have become." - Dr. David Johnson, Smithsonian National Museum of Natural History
References
[1] Sulak, K. J., & Shcherbachev, Y. N. (1989). A systematic revision of the deep-sea family Ophidiidae. Bulletin of the Museum of Comparative Zoology, 152(1), 1-160.
[2] Haddock, S. H. D., Moline, M. A., & Case, J. F. (2010). Bioluminescence in the sea. Annual Review of Marine Science, 2, 443-493.
[3] Bertelsen, E., Nielsen, J. G., & Smith, D. G. (1989). Suborder Saccopharyngoidei: Families Saccopharyngidae, Eurypharyngidae, and Monognathidae. In Fishes of the Western North Atlantic, Part 9, Vol. 1. Sears Foundation.
[4] Nielsen, J. G., Bertelsen, E., & Jespersen, A. (1989). The biology of Eurypharynx pelecanoides. Acta Zoologica, 70(3), 187-197.
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Frequently Asked Questions
What is a gulper eel?
The gulper eel (Eurypharynx pelecanoides, also called pelican eel) is a deep-sea fish with a mouth that expands dramatically to swallow prey larger than itself. Their bodies are slender and eel-like, reaching 60-100 cm long, but the front portion is dominated by an enormous hinged jaw. When closed, the mouth appears proportional to the body. When extended, it forms a balloon-like or umbrella-like structure that dwarfs the rest of the fish. The entire front half of the body can inflate to swallow large prey. They live in deep oceans at depths of 500-3,000 meters globally, inhabiting tropical to temperate latitudes. Their name comes from their resemblance to pelicans - both having relatively small bodies with disproportionately large mouths. Despite their intimidating appearance, they are small and slow predators that eat crustaceans and small fish in deep-sea environments.
How does a gulper eel catch prey?
Gulper eels catch prey through a distinctive swimming and inflating technique. They swim slowly with their mouths closed, appearing like normal fish. When they encounter potential prey, they can rapidly open their enormous jaws - the hinge mechanism allows the mouth to expand dramatically. The prey is essentially sucked into this expanded cavity along with surrounding water. The mouth then closes, trapping the prey inside. Gulper eels can swallow whole crustaceans, shrimp, and small fish that would seem impossible for their body size. Their large jaw also allows them to catch multiple smaller prey simultaneously - filter-feeding style in some conditions. Their hunting is opportunistic and efficient - they don’t pursue prey actively, but wait for targets to approach their mouth. Some research suggests they may use their bioluminescent tail to attract prey within striking range, similar to how anglerfish use their lures. Their mouth mechanics are so specialized that they represent one of the most unusual predatory adaptations in any vertebrate.
How big can gulper eel mouths open?
Gulper eel mouths can expand to several times the normal body width. The hinged jaw mechanism allows the mouth to balloon outward dramatically, sometimes reaching volumes equal to or greater than the rest of the fish’s body. When extended, the mouth creates a catch-bag that contains both captured prey and significant amounts of water. This extreme expansion requires specialized jaw joints - the gulper eel’s skull is highly modified compared to typical fish. The jaw muscles are proportionally huge for the fish’s size. Despite the dramatic expansion, the gulper eel maintains control - it can open and close the mouth precisely, adjust the angle of attack, and modify its catch area based on prey size. Some specimens have been documented expanding their mouths to approximately 5-7 times normal width. This ability to swallow oversized prey is critical in the food-scarce deep sea where any large meal can provide weeks of nutrition. The mouth expansion also makes them appear much larger than they actually are, which may serve as deterrent to larger predators.
Where do gulper eels live?
Gulper eels inhabit deep ocean waters worldwide, particularly at depths of 500-3,000 meters. They are found in tropical, subtropical, and temperate latitudes across all major oceans. Common regions include the deep Atlantic, Pacific, and Indian Oceans, with particular abundance near continental slopes and deep ocean basins. They prefer the mesopelagic and upper bathypelagic zones where temperatures are 2-8°C. They live in open water rather than on the seafloor, swimming slowly through the darkness with their long slender bodies. Their distribution is relatively widespread compared to many deep-sea species, making them one of the more commonly observed deep-sea fish in research expeditions. They’ve been captured as bycatch in deep-sea commercial fisheries and collected by research submersibles in multiple ocean regions. Despite their wide distribution, they remain poorly studied because their habitat is difficult to access. Most research is based on specimens recovered from deep nets or observed briefly by submersibles.
Are gulper eels rare?
Gulper eels are not formally listed as rare or endangered, but their true population sizes are unknown due to deep-sea study limitations. They appear to be relatively common in suitable deep-sea habitats globally. Their widespread distribution (all major oceans) suggests substantial total populations, though specific local populations aren’t well quantified. Research expeditions frequently collect them as bycatch, and submersible operations regularly encounter them in appropriate depths. The IUCN has not formally assessed gulper eels. However, concerns exist about: deep-sea fishing impact (they’re caught as bycatch but not targeted), climate change affecting deep-sea food webs, and potential deep-sea mining disruption. Their slow metabolism and long lifespan make them potentially vulnerable to localized threats, as populations cannot quickly recover from significant mortality events. Without dedicated research efforts, their conservation status may remain unclear. For now, they appear to be persisting in their deep-sea habitats despite limited scientific attention to their specific needs.