Land Crabs on Ascension

The Land crab (Johngarthia lagostoma) is a species of which is found on Ascension. This species is only otherwise found on three small Brazilian islands – Trinidade, Atol de Rocas and Fernando de Noronha. Land crabs evolved from marine ancestors and now inhabit terrestrial environments. They are still linked to the ocean as they must migrate to the shore to release eggs into the sea and the eggs hatch in the water (spending a brief period as zooplankton) before emerging onto the land to spend the rest of their lives on terrestrial ground. 

The land crabs on Ascension are also the second largest native animal found here on Ascension. Land crabs are important keystone species of island communities as predators, litter controllers and seed dispersers. Through these functions they form a key part in the food web and help cycle nutrients through the environment. 

Land Crab Surveys 

 There are three possible origins of the land crabs on Ascension – America, Africa or other bygone Atlantic islands – it is likely their larvae were transported in ocean currents. Spawning occurs from January to June with peak numbers in March. Mating occurs in the residential habitat, along the migration route and at the shore as crabs travel down from the mountain to Northeast Bay which is the main site for spawning (reason for this is unknown). 

As part of a recapture monitoring survey with the AIG Conservation and Fisheries Directorate land crab surveys occur during spawning time to scan land crabs, after previous years of tagging studies where PIT tags were inserted into the front right claw of land crabs to make them identifiable (give them an ID number) to study growth rates meaning they could be recaptured in future years and measured with comparable data to previous years. Unfortunately tagging stopped last year, but recapture studies still occur now and I was able to get involved in this years first week of crab surveys. 

We met at the office at 18:30 to collect kit and head down to North East Bay. I was paired with Jonny and Sunitha with Simon. Jonny talked through how to grab a crab safely, how to handle them for measuring, made sure we were happy with how to use the callipers and where to take measurements on the carapace and how to determine the sex of the crab (shape of the abdomen flap). Jonny and I then made our way down to the furthest side of the beach armed with PIT scanners whilst Sunitha and Simon took the nearest side. We walked around the side of the beach up some rocky areas and also near the waters edge scanning any crabs we saw hoping for a beep to signal a crab that had a PIT tag. 

When one was found we would put it in a bucket and notify our partner. Then working as a pair one person would hold the crab in the sand in place so the other person could measure the crab and scribe and then the crab was checked for any injuries or missing appendages, its colour morph was noted, and it was then marked with pencil to stop double recapturing. 

More about Spawning… 

Females lay their eggs a few days after mating and incubate them beneath their abdomens for approximately 2 weeks. On average a clutch can contain approximately 72,000 eggs. The eggs begin an orange colour but by the time of spawning will be black in colour due to the pigmentation of the eye spots being fully developed inside them.

Land crabs spawn in phase with the moon (lunar entrainment) major spawning events occur over a few nights during the final quarter of the moon cycle in February, March and April. This is when the sea is at its warmest meaning the larvae develop more quickly and have less chance of being swept away from the island before settling. Why spawning is synchronised to the moon is not known – it could be to do with the tides and currents or because the mating is timed to coincide with the full moon (14 days earlier). 

Females can spawn more than once a year but for most it is probably an annual occurrence. Spawning can be a dangerous event for land crabs as adults are unable to breath underwater and can drown if they are swept away by a wave. 

Eggs hatch almost immediately after being released in the water and the larvae are planktonic and movements are influenced by current and tidal patterns. Planktonic larvae moult several times over a 2–3-week period before settling out into the seabed as ‘megalops’ – this is an intermediate stage between a free-swimming larva and a true crab, they have six walking legs instead of 8 and remnants of a tail.

Most larvae are probably swept away from the island by westward flowing currents but there are probably small numbers of megalops emerging every year (with mass recruitment events occurring in 1963, 1977 and 1980s with Georgetown being overrun by larvae). 

Colour morphs 

There are two colour morphs of land crabs on Ascension. The Yellow/Orange and the Purple/Red. When they emerge as megalops and moult into first stage crabs they are all purple, we don’t know what happens next but at about 30mm carapace width many crabs begin to take on an orange/yellow colouration that is most common in adults whilst others stay Purple. It is not known why this happens or why there are two colour morphs – purple and orange interbreed freely so they are definitely the same species and are found in the same habitats eating the same food. It appears to be genetic polymorphism like how humans have different eye colours etc. 

Their maximum age is unknown but PIT tagging studies and models from Exeter Uni have been used which are predicting living up to 40 years of age. 

They are mostly active at night and after rainfall as the risk of desiccation is lowest. Generally, they reach a maximum of 120mm carapace width. Usually, the land crabs reside above 200m on Ascension with the majority found above 400m and up green mountain. They also create and reside in burrows they dig into the substrate. 

Conservation

Land crabs were once harvested in large numbers, sometimes for food but mainly because they ate all of the crops grown on the military garrisons farms. Marines were offered rewards for killing crabs. Records from the 1870s to 80s indicate more than 335,000 were killed in 8 years alone. Nowadays invasive species is their major threat, with rats predating them.