How big is Japanese Spider Crab? | Scale of the Universe

The Japanese spider crab, Macrocheira kaempferi, has the widest leg span of any crab, and of any arthropod, the animal group that also includes insects and spiders. The largest one measured by 2015 spanned 3.7 meters (12 feet) from claw to claw, with its legs stretched out. It lives on the seafloor off the Pacific coast of Japan.

Size

A spider crab's size is usually given as its leg span, because its legs are so much longer than its body. The hard shell covering the body, called the carapace, grows to about 30–37 centimeters (12–15 inches) across. Males grow larger than females.

Few of these crabs have been measured carefully. A 2015 survey of the largest ocean animals found that the largest confirmed leg span was 3.7 meters, from a crab on display at the Sea Life center in Scheveningen, part of The Hague in the Netherlands. That crab weighed more than 13.6 kilograms (30 pounds). Larger spans of 3.8 and 4 meters appear in some references, but the survey's authors could not confirm them.

Size comparisons

A leg span of 3.7 meters is a little more than twice the height of a person 1.7 meters (5 feet 7 inches) tall.
The legs span 10–12 times the width of the carapace.
The carapace is only one and a quarter to one and a half times as wide as a basketball.
One leg of a captive crab called Big Daddy measured 1.43 meters (4 feet 8.5 inches) in 2013. That single leg was more than four-fifths as long as a 1.7-meter person is tall.
The largest measured leg span is about one and a half times the height of a 2.5-meter (8.2-foot) sunflower.

Legs that keep growing

A Japanese spider crab has ten legs: eight for walking and two front legs with claws. In adult males the clawed legs are much longer than the walking legs, while in females the clawed legs are shorter than the walking legs. Once a crab is adult, its carapace stops growing, but its legs keep getting longer.

The crab grows by molting: it sheds its hard outer skeleton and forms a new one. A leg lost to a predator or a fishing net can grow back at the next molt.

Researchers suggest that molting may also limit the crab's size. In crabs as small as 11–14 millimeters, a new outer skeleton takes 8–13 days to form. It may take far longer in a giant, which would leave the crab exposed to predators for longer.

Life on the seafloor

Adults live on sandy and rocky seafloor 200–550 meters (660–1,800 feet) deep, and young crabs live in shallower, warmer water. Adults move up to water about 50 meters (160 feet) deep to spawn, and a female can lay up to 1.5 million eggs in a season. Few of the young survive.

Japanese spider crabs are mainly scavengers. They eat dead or decaying fish and other animals, along with algae, and sometimes pry open the shells of mollusks. They are thought to live 50–100 years.