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Tracking the World’s Largest Land Crab

Young Explorer Tim White marks a coconut crab (Birgus latro) before releasing it back into the forests of Palmyra Atoll. The crab has been fitted with a VHF tag (left) and a GPS tag (right) which will record the crab's position every two minutes for five days. (Photo by Ana Sofia Guerra)
Young Explorer Tim White marks a coconut crab (Birgus latro) before releasing it back into the forests of Palmyra Atoll. The crab has been fitted with a VHF tag (left) and a GPS tag (right) which will record the crab’s position every two minutes for five days. (Photo by Ana Sofia Guerra)

By Tim White

The decline of the dodo is a well-known tale: a large, slow-moving species was driven to extinction by human settlers on a remote island.

The comparable plight of the coconut crab (Birgus latro)—the world’s largest land crab, which can weigh over 4 kg (8.8 lbs) and span nearly a meter (3 feet) across—is much less familiar. This massive, alien-like land crab was once commonly found on many tropical Indo-Pacific islands, but its numbers have been greatly reduced by the exact same forces that doomed the dodo (namely overhunting and habitat destruction).

The two powerful claws of a coconut crab. Coconut crab use these claws to crush solid coconuts. (Photo by Ana Sofia Guerra)
The two powerful claws of a coconut crab. Coconut crab use these claws to crush solid coconuts. (Photo by Ana Sofia Guerra)

The coconut crab’s decline landed it on the International Union for the Conservation of Nature’s (IUCN) Red List of Threatened Species in 1981, but a shortage of research and monitoring led to the crab being classified as a Data Deficient species for the last two decades. We still have a poor understanding of this crab’s basic ecology, which is hindering efforts to protect the species.

How far do crabs travel in their daily movements? What are their habitat requirements? Which habitats must we protect to ensure the species’ long-term survival?  With these questions in mind, our research team travels to Palmyra Atoll—a small atoll located 1000 miles south of Hawaii.

The remote atoll is uninhabited except for research station staff and visiting scientists, so the crabs are plentiful in the absence of hunting.

Our goal is to attach 20 GPS loggers and radio tags onto coconut crabs so we can track their movements through the dense, tropical forests that blanket Palmyra. A GPS logger will record the crab’s position every two minutes for a week, while the VHF tag will emit a radio signal that lets us recapture the crab to remove tags at the end of the study.

Palmyra Atoll, located 1000 miles (1600 km) south of Hawaii, is an uninhabited protected area administered by the U.S. Fish and Wildlife Service. Scientists walk across long, shallow, and shark-filled flats in search of crab on remote islets. (Photo by Ana Sofia Guerra)
Palmyra Atoll, located 1000 miles (1600 km) south of Hawaii, is an uninhabited protected area administered by the U.S. Fish and Wildlife Service. Scientists walk across long, shallow, and shark-filled flats in search of crab on remote islets. (Photo by Ana Sofia Guerra)

Our 3500-mile (5600-km) journey to Palmyra begins at Stanford’s Hopkins Marine Station in Monterey, California, where I am currently a PhD candidate. After two days of bus rides, taxis, and flights, we land on a runway made of crushed coral at Palmyra Atoll. Once we settle into the atoll, I begin loading our gear onto a small boat with Ana Sofia Guerra, a graduate student at University of California, Santa Barbara (UCSB).

Palmyra Atoll is comprised of many smaller islets separated by stretches of water and shallow reefs, so the vehicle of choice is a motorized skiff. Since coconut crab tend to be most active in the evenings, we navigate these passages in the darkness using a compass and a spotlight to search for particularly shallow hazards. The lagoon is full of bioluminescent plankton that briefly glow bright blue as our boat churns the water.

Many coconut crab are more active in the night time, so researchers must navigate shallow coral reefs in the dark. (Photo by Ana Sofia Guerra)
Many coconut crab are more active in the night time, so researchers must navigate shallow coral reefs in the dark. (Photo by Ana Sofia Guerra)

After anchoring at our destination—a small islet named Sand Island—Ana and I lift drybags full of electronic tracking equipment over our heads and wade through chest-deep waters to reach the shore. Blacktip reef sharks quietly investigate the circle of light that our headlamps cast on the lagoon. Palmyra Atoll is home to one of the highest densities of sharks in the world, so we both glance over our shoulders every so often in case some of its larger friends become curious. As we approach the shoreline, dozens of nesting seabirds nervously announce our arrival, so we quickly dim our headlamps and hurry past their roosts to the interior of the forest.

Within seconds of stepping onto land, we are thrilled to see our first coconut crab of the trip—one of the largest that we have ever seen! The protected crabs on Palmyra Atoll are much larger than on the neighboring islands where they are hunted. A bright blue, ten-pound crab is casually munching on fallen plants on the jungle floor—it seems completely out of place among the green colors of the forest. We quickly get to work. I sneak up on the crab, grab a hold of its claws, and quickly lift it off the ground while Ana prepares a GPS logger and a VHF tag for deployment.

A coconut crab perched on a native tree at Palmyra Atoll - an uninhabited island 1000 miles south of Hawaii.(Photo by Ana Sofia Guerra)
A coconut crab perched on a native tree at Palmyra Atoll, an uninhabited island 1000 miles (1600 km) south of Hawaii. (Photo by Ana Sofia Guerra)

Coconut crabs must be handled with extreme care; they have two powerful claws that can easily crush bone. I have previously studied red king crab and snow crab in Alaska, but coconut crabs are in a league of their own in terms of brute strength. Ana and I secure the crab’s claws with a small piece of rope, and record the animal’s weight and length. We apply some glue to the crab’s carapace, or shell, and attach the two devices. Within a few minutes of capturing the crab, it’s safely released back into the forest—our first tag deployment is a success!

Tim numbers a crab with a temporary paint pen prior to release. (Photo by Ana Sofia Guerra)
Tim numbers a crab with a temporary paint pen prior to release. (Photo by Ana Sofia Guerra)

After some cheers and high-fives, we watch the crab and its expensive electronic jewelry walk off into a patch of tall terns.

We are thrilled that the tagging process went smoothly, but our work with this individual is only half done. In one week, we’ll have to track down this exact crab using a radio receiver, and then remove the tags in order to download the data from the GPS logger. Then we will repeat this process on 19 additional crabs! With a little luck, we will end the trip with a much clearer picture of how coconut crabs use the forest so we can better understand how to protect this enigmatic species.

Tim White captures a coconut crab in order to measure, weigh, and tag the animal. This individual weighed nearly 10 pounds with a legspan of over 3 feet across! (Photo by Ana Sofia Guerra)
Tim White captures a coconut crab in order to measure, weigh, and tag the animal. This individual weighed nearly 10 pounds (4.5 kg) with a legspan of over three feet (one meter) across! (Photo by Ana Sofia Guerra)