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The Secret Life of the Land Crabs from Rangitāhua

While we generally think of them as ocean creatures, it’s not unusual for crabs to venture on land, as seen by a species of land crab present on the remote archipelago of Rangitāhua. In this blog, Associate Curator, Te Mana o Rangitāhua Marina Klemm and Curator, Entomology Dr Leilani Walker share with us some of their secrets, and how these creatures are making a surprising comeback.

While we generally think of them as ocean creatures, it’s not unusual for crabs to venture on land or at least become exposed to the air during low tide. But the land crabs of Rangitāhua (Geograpsus grayi) take this much further. As adults, they live entirely on land, making them Aotearoa New Zealand’s only land crab species. What makes this taonga even more remarkable is their limited range: while this very species inhabits islands throughout the Indo-Pacific, in Aotearoa they exist only on Rangitāhua, a remote archipelago about 1,000 km Northeast of mainland. 

Land crabs can become core species on islands, heavily feeding on young vegetation and shaping forest understories. They also uniquely bridge two environments – beginning life as free-swimming larvae in the ocean, sometimes travelling far from where their eggs were first laid, before returning to islands as adults adapted to terrestrial life. When they reproduce, females journey back to the ocean to lay their eggs, creating a vital cycle of nutrients between moana and whenua. Although our researchers haven’t been able to capture this migration in Rangitāhua, we can imagine birds and sea creatures patiently awaiting the seasonal feast of fresh crabs eggs, larvae, and any struggling females making the perilous return journey. 

Land crabs of Rangitāhua feeding on liverwort (Geograpsus grayi) Photos by Marara Van Buuren

On Rangitāhua’s main island, the introduction of pests like goats, cats and rats devastated seabird populations and likely the land crabs’ too. Since 1910, settlers and rangers at Rangitāhua have described the crabs as occurring more “sparingly” compared to other islets that remained pest-free. However, following the confirmed elimination of Norway rats on Rangitāhua in 2002, the Department of Conservation began annual crab surveys, anticipating that, in the absence of mammalian pests, the crabs might start to make a comeback. This raises intriguing questions: what would a booming land crab population look like and how would it reshape the island ecosystem? 

It is unsurprising, then, that this enigmatic species captured the imagination of the Te Mana o Rangitāhua (TMoR) research team. For those daring enough to make the three-day voyage by sea, there is an additional impediment: these crabs are nocturnal. During the day, the only trace you can find of these crustacean residents are small burrows (sometimes no larger than a two-dollar coin) dotting banks along tracks. But within two hours of sunset, these enigmatic creatures emerge from their hiding spots. To find crabs, you sometimes have to look up as well as down. Besides roaming through the forest floor, these skilled climbers scale rock faces, trees and ferns – elsewhere, they even have been found at 916 metres above sea level. 

At Rangitāhua, TMoR researchers use standardized counting and measuring protocols to track population changes and reproductive patterns. Over the coming years, the counts will give some indication of whether the population is growing, dropping or remaining steady, while the other measurements will let us better understand their reproductive dynamics. But there is work that can be done back on the mainland as well. Historic museum collections indicate that females are most likely to be found carrying eggs between November and January. To understand their present-day ecological role, researchers can also employ lab-based techniques, such is the case for the metabarcoding of their gut contents and stable isotope analysis. But first, the crabs need to make back to the lab.

To run further analysis, a small number of crabs are collected and euthanized on the island before being carried down a steep hill to Fishing Rock; then loaded by bicycle-helmet-wearing researchers into dinghies right where the waves and rocks met; then back to the ship’s -20°C freezer for a 3-day journey back to Auckland Museum.

Look up, look down – Geograpsus grayi are everywhere on Rangitāhua! These adaptable land crabs forage in the canopy (left) and burrow on the forest floor (right). Photos by Leilani Walker.

In the museum lab, their gut contents are extracted into sterile tubes and preserved in ethanol to maintain DNA integrity. At University of Auckland’s, our partner marine biologists extract their DNA and take them to the University’s Genomics facilities. There, the extracted DNA is compared against a huge universal database, known as The Barcode of Life, revealing what these land crabs have been eating, a first for Rangitāhua.

While gut contents can tell us what they had been eating in the last 24h before collection, stable isotope analysis of muscle tissue can reveal what they’ve eaten throughout their lives. In short, stable isotopes are basically nature's way of stamping "I'm a top predator" or "I'm a vegetarian" right into every creature's atoms! So, to better understand their life-long feeding habits, TMoR researchers also sample muscle from the interior of the crabs’ carapace and send them down to the Stable Isotopes Analysis Facility at NIWA, where their nitrogen and carbon atoms can be studied from up close. 

Land crabs’ gut contents visual analysis reveals an ant’s head (bottom) and exoskeleton parts (right), possibly due to the cannibalism spotted on the motu. Photos by Marina Klemm.

Anything that lives or has ever lived on Earth is composed of carbon and nitrogen. Over the course of time, carbon and nitrogen atoms maintain the same number of protons but can have different numbers of neutrons, which makes their mass different. Since mass is something we can quantify, we can use these isotopic signatures to understand where species sit in the food web! 

Plants are the lightweights - they get their carbon straight from the air and keep things simple. But when a cicada eats the plant, it preferentially uses the lighter isotopes for energy and keeps more of the heavy ones in its tissues. Then when a spider eats the cicada, the same thing happens again: more heavy isotopes accumulate. Scientists can look at the isotope ratios in any animal's tissues and determine, based on how heavy the nitrogen atoms are, that the animal is eating things that are eating things that are eating plants! Likewise, looking at the carbon results, scientists can determine if an animal is preferentially eating land or ocean-derived food sources, as the carbon obtained from them also has different weights.
 

Our research team is currently preparing a stable isotopes article based on analysis of the wider Rangitāhua ecosystem, as well as a paper on the types of foods the land crabs had been eating based on their gut contents. As such, we won’t give the game away right now, but we can offer a small spoiler: Rangitāhua land crabs seem to be the top predators in all of the land, something that strongly hints that, whatever the future holds for Rangitāhua, these crabs will play an important part of it. 

References

Alexander, H. G. L. (1979). A Preliminary Assessment of the Role of the Terrestrial Decapod Crustaceans in the Aldabran Ecosystem. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 286(1011), 241–246.

Burggren WW, McMahon BR (1988) Biology of the land crabs. Cambridge University Press.

Chilton, C., 1911. The Crustaca of the Kermadec Islands. Trans. Proc. N.Z. Inst., 43: 544–573.

Gibson-Hill, CA. (1947b). Christmas Island. Field notes on the terrestrial crabs. Bulletin of the Raffles Museum 18: 43-52

Takeda, M., 1982. Two land-dwelling crabs from Minami-Iwojima Island. In: Conservation reports of the Minami-Iwojima Widlderness Area, Tokyo Japan, pp. 379–382. (In Japanese with English summary.)

Takeda, M., & Webber, R. (2006). Crabs from the Kermadec Islands in the South Pacific. Proceedings of the 7th and 8th Symposia on Collection Building and Natural History Studies in Asia and the Pacific Rim, edited by Y. Tomida et al., National Science Museum Monographs, (34): 191–237.