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Look first for the fin pattern: a Greenland shark (Somniosus microcephalus) has no anal fin, two small dorsal fins of similar size without obvious spines, and a first dorsal fin set well behind the pectoral-fin base. Its heavy cylindrical body and rounded snout support the identification. Color, size, location and the parasite-covered eye are useful context, but none is conclusive by itself. Large sleeper sharks can look nearly identical, so an unusual or consequential record may require genetic confirmation.
The fastest reliable field identification
When you have only a photograph or a brief observation, work through the fins before judging color or size. The most useful combination is:
- No anal fin: the underside between the pelvic fins and the tail has no separate anal fin.
- Two small dorsal fins: both are approximately equal in size and lack conspicuous spines at their origins.
- Unusually rearward first dorsal fin: it begins well behind the base of the pectoral fin, around the middle of the body rather than close to the head.
- Second dorsal fin near the pelvic fins: its origin is close to the pelvic-fin insertion.
- Heavy, cylindrical body and rounded snout: these reinforce the identification but overlap with other sleeper sharks.
In the Fisheries and Oceans Canada key for Atlantic Canadian waters, the combination of an absent anal fin and dorsal fins without conspicuous spines leads to Greenland shark. That key covers 19 shark species, so it is a regional tool rather than a universal guide to every Arctic shark.
What a Greenland shark looks like
Body, head and snout
The body is thick and cylindrical, giving the animal a heavy appearance even when seen from above. The snout is relatively short and rounded, and the mouth is small compared with the body. These proportions differ from the more pointed profile of many fast-swimming sharks, but a single angle can be misleading, especially in turbid water.
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Dorsal fins
Greenland sharks have two small dorsal fins that are roughly the same size. They are spineless: there are no prominent rigid spines at the leading edges. The first dorsal fin starts well posterior to the pectoral-fin base. The second is positioned near the pelvic fins. Photographing the entire back, rather than only the head, is therefore important.
Anal fin
There is no anal fin. This is often the clearest mark in a side or underside view. Do not confuse the pelvic fins near the rear of the body with an anal fin farther back on the midline; the space between the pelvic and caudal fins remains without an anal fin.
Pectoral fins and tail
The pectoral fins are small and rounded. The tail is asymmetrical, with a relatively short upper lobe and a noticeable subterminal notch. Tail shape can support a diagnosis when the fins are clearly visible, but it should not override the more diagnostic fin arrangement.
Color and skin pattern
Color varies from grayish brown to blackish. Some individuals show dark or pale spots. Lighting, depth, camera white balance, abrasion and sediment can all change the apparent color, so color should be treated as a supporting clue only. A dark shark is not automatically a Greenland shark, and a lighter individual is not excluded.
The eye parasite
The eyes are relatively small and are often partly covered by the copepod Ommatokoita elongata. A conspicuous filamentous or cloudy growth over the eye can be a useful clue that you are looking at a Greenland shark or a close sleeper-shark relative. It is not a stand-alone species marker: parasite presence can vary, and poor image quality can make normal eye anatomy look similar.
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Teeth, only when already visible and safe
Upper teeth are slender and thorn-like. Lower teeth are broader and carry strongly oblique cusps. Tooth characters can help confirm an identification in a properly documented specimen, but they are not a reason to handle a live shark or approach its mouth. For a diver, beach observer or crew member, fin placement is safer and more useful than attempting to inspect teeth.
A step-by-step method for a photograph or sighting
- Establish a safe observation. Keep people, pets and equipment clear of the mouth and tail. Do not restrain, turn over or hold a live shark to obtain a better view.
- Find the underside or rear profile. Determine whether an anal fin is present between the pelvic fins and the tail. If that area is hidden, mark the character as unknown rather than assuming it.
- Count the dorsal fins. Look for two fins on the midline of the back. Note whether they are small and approximately equal in size.
- Check for spines. Prominent spines at the dorsal-fin origins point toward a dogfish or another spiny squaloid, not a typical Greenland shark.
- Locate the first dorsal fin relative to the pectoral fin. A Greenland shark’s first dorsal begins well behind the pectoral-fin base. A fin close to the shoulder argues against it.
- Locate the second dorsal fin. It should be near the pelvic-fin insertion. Record the relationship even if the first fin is partly obscured.
- Assess overall form. A heavy cylindrical body, rounded snout, small rounded pectoral fins and a short upper caudal lobe are consistent with Greenland shark.
- Record color and eye condition separately. Write down gray-brown, blackish, spotted or other observations, and whether an eye parasite is visible. Keep these as supporting evidence.
- Add geography and habitat. Note coordinates, depth, temperature, date and whether the animal was in a net, on the seafloor or near the surface. Use this information to assess plausibility, not to prove identity.
- Assign a confidence level. If the anal fin, dorsal-fin origins or full body profile are missing, report “Somniosus sp.” or “large sleeper shark” rather than forcing a species name.
How to separate Greenland sharks from common regional look-alikes
| Feature | Greenland shark | Spiny dogfish | Black dogfish | Prickly or bramble sharks |
|---|---|---|---|---|
| Anal fin | Absent | Absent | Absent | Not the primary distinction in the NOAA comparison; inspect the complete fin pattern |
| Dorsal-fin spines | No conspicuous spines | Prominent spines | Prominent spines | Use the guide’s full fin arrangement; the first dorsal is over or behind the pelvic-fin origin |
| First dorsal position | Well behind pectoral-fin base | More forward, associated with the first spine | More forward, associated with the first spine | Over or behind pelvic-fin origin in the NOAA description |
| Teeth, when safely documented | Upper teeth slender and thorn-like; lower teeth broad with strongly oblique cusps | Quadrangular teeth with a laterally directed cusp | Teeth with several triangular cusps | Not used here as a definitive field character |
| Body impression | Large, heavy and cylindrical | Generally more slender | Generally more slender | Can resemble other squaloid sharks |
The dogfish distinctions come from the Atlantic Canadian key, which is designed for sharks in that region. The NOAA-hosted guide describes prickly and bramble sharks as similar squaloid sharks whose first dorsal fin is over or behind the pelvic-fin origin, unlike the more forward first-dorsal position described for Greenland shark. Because that comparison does not cover every species or every angle, use it as a useful branch in the identification process, not as an exhaustive Arctic key.
Greenland shark versus Pacific and Southern sleeper sharks
The hardest cases involve other large members of the genus Somniosus. Pacific sleeper shark (Somniosus pacificus) and Southern sleeper shark (Somniosus antarcticus) can share the same broad body form, small dorsal fins, missing anal fin and subdued coloration. A photograph may show that an animal is a sleeper shark while failing to establish which species it is.
COSEWIC’s 2025 assessment states that no taxonomic character currently provides unequivocal separation among species within the large Somniosus group. Measurements that have been proposed, including interdorsal space and prebranchial length, vary and overlap. Some genetic analyses support Greenland and Pacific sleeper sharks as distinct sister species, while questions remain elsewhere in the group and hybridization has been reported.
What geography can and cannot do
Greenland sharks occur primarily in the North Atlantic and adjacent Arctic waters, including Atlantic Canada, Greenland, Iceland and parts of the Northeast Atlantic. Pacific sleeper sharks are associated with the North Pacific, and Southern sleeper sharks with Southern Ocean waters. This makes location useful for prioritizing possibilities, but range is not proof. Large sleeper sharks can occur outside an observer’s expectation, records can be mislocated, and hybrids complicate unusual cases.
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When to report a record as uncertain
Use a cautious label when the image lacks the anal-fin area, dorsal-fin origins or enough body detail to compare proportions. “Large Somniosus sleeper shark” is more informative than an unsupported species claim. For a stranding, fishery record, conservation action or other consequential identification, preserve original files and metadata and seek expert or genetic confirmation.
Habitat and environmental clues
Greenland sharks are cold-water, deep-dwelling animals. The Fisheries and Oceans Canada profile gives an Arctic and subarctic temperature context of approximately −2 to 7 °C and says they are commonly found below 200 m, with seasonal variation. The species can nevertheless be encountered in different depths and conditions, so depth and temperature are contextual evidence rather than diagnostic characters.
For a useful sighting record, capture the date and time, coordinates, water depth, estimated total length, water temperature if available, visibility, behavior and the fin characters you could actually see. A note that a feature was obscured is scientifically more useful than a guess.
Size, age and why they should not be used as identifiers
COSEWIC’s May 2025 Canadian assessment cites common lengths of 200–400 cm total length, adult males up to 375 cm and females up to 550 cm in cited records. Size varies with sex, age and measurement method, and a photograph rarely provides a reliable scale. A very large shark is therefore not automatically a Greenland shark.
The same assessment reports late maturity at approximately 150 years. NOAA Ocean Service explains that carbon dating can estimate age but cannot provide a definitive age; its explainer gives an estimated 272–512 years for the largest female in the cited carbon-dating study. That range applies to the examined shark and method, not to every individual or a guaranteed species-wide lifespan. Age and longevity are important for conservation, not practical field marks.
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Safe handling and documentation
Most observers should identify a Greenland shark without touching it. The animal is powerful, and stress or injury can result from restraint, lifting or crowding. Follow local wildlife and fishery rules, and keep a safe distance from the mouth, tail and fishing gear.
- Photograph the whole animal first, then the head, both dorsal fins, pelvic region and tail if this can be done without moving it.
- Include an object of known size only when it can be placed safely and legally; otherwise record an independent scale such as net dimensions or vessel equipment.
- Keep original, uncropped files and record who took each image, when and where.
- Do not cut fins, remove tissue or collect teeth unless an authorized scientific protocol permits it.
- For a live animal in fishing gear, follow the fishery’s release procedures and obtain professional assistance rather than improvising.
Common identification mistakes
Using color as the answer
Gray-brown, blackish and spotted individuals all fall within the reported variation. Color changes with light and water conditions, so it cannot replace fin characters.
Assuming every large Arctic shark is a Greenland shark
Large sleeper sharks overlap in form, and hybrids or mislocated records are possible. A northern location raises the probability but does not establish species.
Confusing pelvic and anal fins
The pelvic fins are paired and lie near the rear of the body. An anal fin would be a separate midline fin behind them. Greenland sharks lack that fin entirely.
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If obvious spines rise in front of the dorsal fins, examine dogfish alternatives. In Atlantic Canada’s key, spines are a decisive early branch away from Greenland shark.
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Trying to identify teeth on a live shark
Tooth shape is useful only when already visible in a safe, documented specimen. It is not worth approaching or handling an animal.
Overinterpreting one measurement
Interdorsal distance, prebranchial length and similar ratios overlap among Somniosus species. Measurements can support a specialist examination but do not guarantee a species-level result.
Conservation context for Canadian observations
COSEWIC assessed Greenland shark as Special Concern in Canada in May 2025. The Canadian assessment cites incidental capture in trawl, benthic longline and gillnet fisheries, an estimate of at least 1,700 Greenland sharks bycaught annually in Canada, very slow growth, maturity at approximately 150 years, exceptional longevity and possible effects from accelerated climate change.
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That designation is a Canadian status assessment, not a global status determination. Accurate records still matter: a well-documented observation can improve understanding of distribution and bycatch, while an overconfident label can contaminate monitoring data. Report captures and strandings through the relevant regional authority, including the photographs and uncertainty notes.
Quick Recap
A compact checklist
- Anal fin absent.
- Two small dorsal fins, approximately equal in size.
- No conspicuous dorsal spines.
- First dorsal fin well behind the pectoral-fin base.
- Second dorsal fin near the pelvic-fin insertion.
- Heavy cylindrical body and rounded short snout.
- Small rounded pectoral fins and a tail with a short upper lobe and subterminal notch.
- Color and eye parasite recorded only as supporting evidence.
- Geography and depth recorded as context, not proof.
- Large Somniosus look-alikes flagged for expert or genetic confirmation when the record matters.
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