Amusing Monday: Giant crab has amazing grip, but species is at risk

Coconut crabs are giant land-based crustaceans that can grow to 3 feet wide, claw-to-claw. The crabs, frightening to some, inhabit islands in the Pacific and Indian oceans.

These crabs, which grow larger than any other land-based arthropod, are known for their uncanny strength. They get their name from an ability to break through coconut husks with their powerful claws. They can also break a lot of other things, as revealed in a variety of amusing videos, some of which I’ve posted on this page.

Coconut crabs became a topic of discussion among scientists last month when a group of Japanese researchers reported that they had measured the strength in the legs and claws of coconut crabs. They found that these crabs could lift four times their weight, and their pinching power was greater than that of any other kind of crab, even greater than the jaw strength of terrestrial predators. The report was published in the online journal Plos One.

“We expected the force would be very strong, but the actual power exceeded our expectations,” said Shin-ichiro Oka of the Okinawa Churashima Foundation in a news release. “We were surprised that their pinching force was approximately 90 times their body weight. If I was a coconut crab weighing 65 kg (143 pounds), I could crush something with about 6 tons force!”

That’s actually the imaginary power of a person with the relative strength of a crab. The coconut crabs, although very large for a crab, grow to about 10 pounds. Still, that’s larger than other terrestrial arthropods, which are animals with exoskeletons, segmented bodies and jointed appendages. They include insects, spiders, scorpions and crustaceans.

Having an exoskeleton limits the size of these creatures, and gravity generally is more limiting for land-based arthropods than for those that live in the sea. Some people say terrestrial arthropods cannot get much bigger than the coconut crab, because they must shed their skeleton time after time as they grow. With each successive molting, their shell grows thicker and heavier. By the way, in their early years, the crabs live in a borrowed shell, like other hermit crabs.

After reading about coconut crabs, I could go on about their interesting food preferences and their habits of thievery, which is how they got the nickname robber crab. Instead I’ll refer you to “10 Ginormous Facts About Coconut Crabs” by science writer Rosemary Mosco or to Wildscreen Arkive.

While these crabs may seem strange and somewhat amusing, I wanted to mention that a lot of experts are worried that coconut crabs may be heading for extinction throughout much of their range. As is often the case with endangered species, the cause of decline is a combination of a high level of human exploitation, a loss of habitat and a low reproductive rate.

In 1981, the crab was placed on the “Red List of Threatened Species” by the
International Union for the Conservation of Nature. But actions to protect the remaining crabs have been limited by a lack of data.

To rectify the situation, researchers are increasing their studies of the crab. In National Geographic’s blog “Explorer’s Journal” (Oct. 28), researcher Tim White talks about a new research project that involves tagging coconut crabs with equipment that can track their movements on Palmyra Atoll, a cluster of islands about 1,000 miles south of Hawaii. The goal is to track the crabs to learn about their behaviors.

Because the giant crabs are not hunted by humans on the protected atoll, the crabs are able to live naturally, and researchers hope to learn about their needs. In a May episode of “NBC Dateline: On Assignment,” correspondent Harry Smith visits the remote atoll and talks about the coconut crab with Alex Wegmann of The Nature Conservancy, which owns the property. Go to 10:55 in the clip below.

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