How Long Do Japanese Spider Crabs Live? Age Claims Checked

Where does the 100-year claim come from?

Ask how long a Japanese spider crab lives and you will usually be told 50 to 100 years. The numbers appear in popular articles, children’s books and aquarium displays, often without any source. They seem to be estimates based on the crab’s huge size and slow life in cold water, repeated from one source to the next.

Scientific accounts of the species, by contrast, say little about lifespan, because the measurements simply have not been made. There is no long-term tagging study of wild Japanese spider crabs, and no accepted method has given a reliable age for one. A lifespan of several decades is plausible for such a big, slow-growing crab, but “100 years” should be read as a guess, not a fact.

Why crab age is so hard to measure

For many animals, age is written into the body. Trees have annual rings, fish have growth rings in their ear stones, clams lay down bands in their shells, and mammals can be aged from their teeth. Crabs have no such easy record.

The problem is molting. To grow, a crab sheds its entire hard outer skeleton, including the linings of its stomach and gills and the surface of its eyes, and builds a new, larger one. Any marks laid down in the old shell are thrown away with it. Size is a poor guide too, because growth depends on temperature and food: two crabs of the same size can differ in age by many years.

Japanese spider crabs add their own difficulties. They live 50–600 m (160–1,970 ft) deep, which makes tagging and recapturing them hard, and external tags are shed with the old shell at the next molt. Aquariums usually receive crabs as adults of unknown age, so even one kept for many years cannot reveal how old it was when it died.

New ways to estimate the age of crustaceans

Scientists have developed several methods for crabs, lobsters and shrimps, though none has yet been widely used on Japanese spider crabs:

• Age pigment: a fatty pigment called lipofuscin builds up slowly in nerve tissue, including the brain and eyestalks. Measuring it gives an approximate age, after calibration with animals of known age.
• Stomach bands: in 2012, a team led by Raouf Kilada in Canada reported growth bands in the hard, tooth-like plates of the stomach, called the gastric mill, and in the eyestalks of snow crabs, lobsters and shrimps. The bands appeared to form once a year. Later studies questioned whether these structures are kept through each molt, so the method is still debated.
• DNA changes: chemical tags on DNA change in a predictable way as an animal ages, and researchers have begun using them to estimate the age of lobsters.

None of these methods is yet precise enough to give the age of a very old wild crab with confidence.

Growth, molting and the final molt

What is known about the Japanese spider crab’s early life comes mostly from laboratory rearing. A larva hatches in late winter, passes through a brief prezoea stage, two drifting zoea stages of about 7–18 days each and a megalopa stage of about 25–45 days, then settles on the seafloor as a tiny crab. Colder water makes every stage last longer.

After that, the crab molts many times as it grows. Most spider crabs, the group to which the Japanese giant belongs, stop molting when they reach adulthood, after a final molt sometimes called the terminal molt. If the Japanese spider crab follows the same pattern, an adult’s size says nothing about how long it has lived since it matured. Each molt is also risky: aquarium observations show that freeing the long legs from the old shell can take more than an hour, and a freshly molted crab is soft and defenseless. Molting is also how crabs replace lost legs, as described in can crabs regrow legs?

How it compares with other crabs and lobsters

Crab lifespans vary enormously, from a couple of years in small, fast-growing species to many decades in slow-growing giants. Most published figures for long-lived species are estimates.

Lobsters face the same measuring problems and attract the same kind of myths, explored in do lobsters live forever? For animals whose great ages have been measured with confidence, such as the Greenland shark and the ocean quahog clam, see the longest-living animals.

What is actually known

Putting the evidence together gives a cautious answer. Japanese spider crabs grow from larvae weighing a tiny fraction of a gram to adults of up to about 19 kg (42 lb) in water close to 10 °C (50 °F), where growth is slow. That almost certainly takes many years, and adults probably live on for years after their final molt. A lifespan of several decades is reasonable.

Beyond that, the honest answer is that nobody knows. Until someone ages wild crabs directly, claims of a century-long life remain an appealing but unproven story.