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Greenland sharks grow and reproduce on an unusually slow timetable because they live in cold, deep water and appear to have an exceptionally slow life history. However, science can describe the pattern more confidently than it can identify a single cause. A few mark–recapture observations indicate growth of no more than about 1 centimetre per year, while radiocarbon modelling of eye lenses suggests that some individuals live for centuries and may not reach sexual maturity until well over a century old.
What makes the Greenland shark’s life history unusual?
The Greenland shark (Somniosus microcephalus) can exceed 5 metres in total length, yet it does not reach that size quickly. Unlike many familiar sharks, it appears to add only a very small amount of length each year. The available direct growth estimate is no more than about 1 centimetre annually, and that figure comes from a small number of marked sharks that were caught, released and later measured again. It should therefore be treated as a limited estimate, not a universal rate for every Greenland shark or every stage of its life.
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Its reproductive schedule is equally unusual. Radiocarbon-based age modelling indicates that sexual maturity may be reached only after more than a century. A separate study of reproductive specimens estimated the body length at which half of the animals are mature, with very different values for males and females. These findings describe two related but distinct facts: one concerns estimated age, while the other concerns size associated with reproductive condition.
Nielsen and co-authors captured the scale of the animal’s life history in their 2016 Science paper: “The Greenland shark (Somniosus microcephalus), an iconic species of the Arctic Seas, grows slowly and reaches >500 centimeters (cm) in total length, suggesting a life span well beyond those of other vertebrates.” The wording “suggesting” is important. The shark’s extreme longevity is inferred from a model, not read directly from a known birth date.
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How slow is its growth?
The direct evidence is sparse
The often-cited figure is no more than 1 cm of annual growth. It comes from mark–recapture observations summarized by Nielsen et al. (2016) and the 2025 assessment by Canada’s Committee on the Status of Endangered Wildlife in Canada (COSEWIC). In a mark–recapture study, researchers record an animal’s length, mark it, release it and measure it again when it is recaptured. The difference divided by the elapsed time gives an observed rate.
Only a few Greenland sharks have provided this kind of repeated measurement. That makes the estimate valuable but narrow. It does not establish that a newborn, a mid-sized shark and a 5-metre adult all grow at exactly the same rate. Nor does it rule out periods of faster or slower growth caused by food availability, illness, pregnancy or seasonal conditions. The defensible conclusion is that observed growth is exceptionally slow, with the available evidence placing it at no more than roughly 1 cm per year.
Why a centimetre matters at this scale
A rate of 1 cm per year would require many decades to add a metre of length. Even that simple calculation should not be mistaken for a complete growth curve: the estimate is based on few individuals and may not apply uniformly across the lifespan. It does explain why the species’ large body size is compatible with very old ages rather than rapid juvenile growth.
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Why ordinary growth bands do not work well
Many fish and sharks can be aged by counting regularly deposited bands in bones or hard tissues. Greenland sharks are difficult to handle this way. They lack the fin spines used for ageing some other sharks, and their vertebrae are too poorly calcified to produce dependable bands of the kind used in several better-studied species. Researchers therefore needed a different biological clock.
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The eye-lens nucleus as a time capsule
The 2016 study examined the nuclei of the sharks’ eye lenses. These nuclei contain crystalline proteins formed during prenatal development. Once formed, the proteins are considered metabolically inert: they do not renew in the ordinary way, so the carbon incorporated while the shark was developing can remain preserved.
Researchers measured radiocarbon in that tissue from 28 female Greenland sharks between 81 and 502 cm in total length. Radiocarbon levels provide a timestamp because atmospheric carbon changed markedly during the era of nuclear-weapons testing. The resulting “bomb pulse” is especially useful for identifying relatively young animals born after that signal entered marine food webs.
From radiocarbon to an age estimate
For smaller, younger sharks, the bomb-pulse signal supplied a time marker. For larger, pre-bomb animals, the researchers combined radiocarbon calibration with biological and environmental constraints to model when the sharks were born. This is an indirect inference. It is not equivalent to counting 392 annual rings or finding a historical record of a shark’s birth.
The model also depends on assumptions about how radiocarbon moves through the deep ocean. COSEWIC’s 2025 assessment notes that deep-ocean radiocarbon dynamics before and after nuclear testing affect estimated birth dates. It also states that the age estimates have not been validated by a viable alternative method. Those limitations do not erase the evidence for exceptional longevity, but they do mean that the reported ages must be presented as estimates with broad uncertainty.
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What ages did the studies estimate?
| Finding | What was measured or modelled | How to interpret it |
|---|---|---|
| At least 272 years | Estimated lifespan in Nielsen et al. (2016) | A model-based lower bound, not a directly observed maximum age |
| 392 ± 120 years | The 502 cm female in Nielsen et al. (2016) | The uncertainty represents the extent of a 95.4% probability range; it is not a precise birthday |
| At least 156 ± 22 years | Inferred age at sexual maturity in Nielsen et al. (2016) | An age inference linked to the maturity-size relationship and the radiocarbon model |
| No more than 1 cm per year | Limited mark–recapture growth observations | A sparse direct-growth estimate, not a rate measured across the whole species |
The safest wording is that Greenland sharks are estimated to live for centuries and to mature exceptionally late. Calling the largest sampled female definitively “392 years old” overstates what the data show. The plus-or-minus value describes a broad probability range generated by the model, not measurement error from a known date of birth.
When do males and females become mature?
Nielsen et al. (2020) assessed reproductive condition and modelled the total length at which 50% of animals would be mature, often called the length at 50% maturity or TL50. Their estimates differ substantially by sex:
| Sex | Estimated length at 50% maturity | Evidence represented |
|---|---|---|
| Males | 284 ± 6 cm total length | Size associated with reproductive condition in sampled males |
| Females | 419 ± 4 cm total length | Size associated with reproductive condition in sampled females |
These are size estimates, not direct age readings for every shark at those lengths. The 2016 study’s estimate of at least 156 ± 22 years at sexual maturity relies on the radiocarbon age model, whereas the 2020 estimates rely on reproductive specimens and statistical modelling of size. Keeping those evidence types separate prevents a common error: treating a maturity length as if it were a guaranteed age.
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The sex difference also has practical consequences for population recovery. If females generally need to reach a much larger size before half are mature, they may spend longer in a non-reproductive stage and have fewer opportunities to replace animals removed from the population.
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Why might cold, deep water slow growth?
The leading explanation is metabolic, not proven
Greenland sharks occupy cold Arctic and North Atlantic waters and can descend into deep habitats. NOAA describes very slow metabolism, potentially adapted to cold, deep conditions, as one theory that could account for slow growth and long life. In a cold environment, biochemical reactions generally proceed more slowly, and a low-energy lifestyle could reduce the rate at which an animal grows, ages and uses resources.
This is a plausible physiological explanation, not an experimentally settled cause. The available evidence does not show that cold water alone imposes a specific growth rate on every individual. Nor has it demonstrated that a single metabolic pathway explains both longevity and delayed maturity. The observations establish the life-history pattern; the mechanism remains a hypothesis under investigation.
Energy allocation may reinforce the pattern
A shark that grows slowly may allocate energy cautiously among maintenance, movement, immune function and reproduction. Delaying maturity allows more time to reach a large body size, which can improve survival and reproductive potential, but it also means a long interval passes before an individual contributes offspring. This kind of trade-off is consistent with an exceptionally slow life history, yet consistency is not proof that one particular allocation rule causes the shark’s schedule.
Large size and deep-water ecology
Deep habitats can offer relatively stable temperatures and periods of low food availability. A predator adapted to those conditions may benefit from conserving energy and using food efficiently rather than maintaining the high metabolic throughput seen in fast-growing species. Greenland sharks are also known for slow swimming and scavenging behaviour, traits compatible with energy conservation. These ecological observations help explain why the metabolic hypothesis is considered reasonable, but they do not replace direct physiological tests.
How does late maturity affect reproduction?
COSEWIC describes Greenland sharks as lecithotrophic viviparous: embryos develop inside the mother and are nourished by yolk rather than by a placenta supplying ongoing maternal nutrition. Important details remain uncertain. The assessment says uterine fecundity or litter size, gestation duration and the length of the reproductive cycle are not established with confidence.
That uncertainty matters because population productivity depends on more than age at maturity. Scientists also need reliable estimates of how many young a female produces, how often she reproduces and how many offspring survive. It would be misleading to attach a precise gestation period or litter size to the species when the current assessment does not establish those values.
What the evidence can—and cannot—tell us
Strongly supported conclusions
- Greenland sharks grow extraordinarily slowly compared with most large vertebrates; limited mark–recapture observations indicate no more than about 1 cm per year.
- Radiocarbon measurements from eye-lens nuclei support estimated lifespans of centuries.
- Sexual maturity occurs very late, and estimated maturity size is much larger in females than in males.
- The eye-lens method preserves an early-life carbon signal and provides an indirect age estimate rather than a direct count of annual bands.
Claims that require qualification
- The 502 cm female was not proven to be exactly 392 years old; 392 ± 120 years is a model estimate with a broad 95.4% probability range.
- The 1 cm growth figure is based on few recaptured sharks and should not be presented as a species-wide constant.
- The 156 ± 22-year maturity estimate depends heavily on the radiocarbon age model, while the 284 cm and 419 cm figures are size-at-maturity estimates from reproductive observations.
- Cold-water, low-metabolism physiology is a proposed explanation, not a demonstrated single cause.
Why the uncertainty does not make the finding unimportant
Ageing a deep-water shark without dependable skeletal bands is inherently difficult. The absence of an independent validation method means the exact numerical ages may change as researchers improve ocean-carbon models or develop another ageing technique. Even so, the results from eye-lens radiocarbon, the observed slow growth and the late maturity estimates point in the same broad direction: Greenland sharks operate on a timescale far longer than that of most studied sharks.
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How to read the “oldest vertebrate” headline responsibly
Greenland sharks are often described as the longest-lived vertebrates known, but the scientifically careful version is that radiocarbon modelling has produced estimates in the several-century range, including a lower-bound lifespan estimate of at least 272 years. Because the model has broad uncertainty and no viable alternative validation method has yet confirmed it, “estimated to live for centuries” is more accurate than an unqualified claim that every large shark reaches a particular age.
The same discipline applies to the word “why.” The shark’s slow growth and late maturity are well documented patterns. A slow metabolism associated with cold, deep habitat is a leading theory, but current evidence does not establish a single causal mechanism. The most accurate answer is therefore both striking and qualified: Greenland sharks appear to have evolved an exceptionally slow life history suited to their environment, while the precise physiological explanation remains unresolved.
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