Back in 2009, I was studying for my Zoology degree at the University of Southampton. One of my professors sent an email around to my year group to request applications for an interesting third year project involving crabs in the Seychelles. The email requested for students to apply in pairs, so I applied with my good friend Louise. We were extremely fortunate to be selected ahead of our peers.
We then arranged a meeting with the professor and were expecting a cool project already set up for us. As we sat down, we soon realised that was not the case. In fact the story was that our professor had gone on vacation to Bird Island, Seychelles the year before, saw some ghost crabs and thought it would be a good idea to send students there.
The next few months, my friend and I studied the literature of the island and everything that was known about ghost crabs. It didn’t take us long to find out that actually very little was known about the crabs on the island, but we did manage to find a paper that apparently did a full survey of biological life on the island.
Bird Island is the northernmost island of the Seychelles Archipelago, and is most famous (as the name might suggest) for its birds. I was told that Bird Island has the largest tourist-accessible bird colony in the world. Sooty Terns, a ground-nesting species (no natural predators on the island), carpeted parts of the island, with thousands and thousands of nests dotted around the northern part of the island. The sight was staggering.
The rest of the bird life was pretty amazing too. I experienced my first Booby, several species of Terns, Tropicbirds, and Frigatebirds.
Whilst the bird life was awesome, I was first attracted by some of its other inhabitants… the Aldabra Tortoise. There were a dozen or so Aldabra Tortoises living on the island. These tortoises were introduced to Bird Island, but some of the other islands in the Seychelles were once home to now-extinct species of giant tortoise. The Aldabra Archipelago is the only place in the Indian Ocean where the giants have not been re-introduced. My passion has always been with tortoises, so visiting this paradise island to see some free-roaming giants was something of a dream. As luck would have it, Bird Island is also home to Esmeralda, a male (yeah, I know) tortoise that is believed to be the oldest individual in the world, though this has been contested by some.
I realise I have slightly been side tracked by my intended topic. As a herpetologist and a birder, that is perhaps expected. The original document that my friend and I found detailing a full biological survey of the island reported two crab species. We spent 12 nights on the island, and we found four species; two species of ghost crab, a hermit crab, and a different species of land crab, whose name escapes me.
Our intended project was to investigate how ghost crab size affects their distribution. Ghost crabs are a semi-terrestrial species that build intricate burrows on beaches. Their name ‘ghost crab’ is not necessarily due to their camouflage (they can change their appearance through the use of chromatophores), though that may be a contributing factor. Ghost crabs are by far the fastest crabs in the world. The speed at which they can move and change direction is unexpected. One would have to see it to believe it (have a search on YouTube if you have time).
Fortunately, we did not intend on catching so many, as we set out to study their burrows. The burrows are used daily, with several ‘rooms’ underneath the sand that are used for different purposes; mating, sleeping, food storage. The diameter of the burrow entrance is proportionate to the size of the crab.
Like their cousins, the Fiddler Crabs (which mostly inhabit mangroves), Ghost Crabs have claws of different sizes. The smaller claw is used for feeding, whilst the larger claw is used for fighting, construction and signalling. Males also use their claws to carry sand and build mounds outside their burrows. They will sit atop these mounds and use their claws to communicate to females that they are fitter than their rivals.
A big mound is a good mound.
During our study, we found that larger crabs command larger areas of the beach than smaller crabs, with larger distances between them and their neighbours. We also found that juvenile crabs were usually found closer to the sea, perhaps to reduce chances of predation when feeding. Some of the beaches on the island had a tremendous density of burrows, with upwards of 10 burrows per square metre. However, our greatest discovery was that ghost crabs have a basic level of intelligence.
It’s entirely possible that many readers would have been to beaches where ghost crabs are present. They are found in Africa, Australia, the Americas, Asia and even the Mediterranean. If you find a burrow on a beach, it’s probably from a ghost crab. Unfortunately, many ghost crab populations are declining worldwide, mostly due to the tourism industry. Think of your typical beach in Florida or Brazil, and you will imagine a scene similar to the Sooty Tern photo from earlier, except it’s on a beach, and instead of birds, it is holiday-goers. Some countries have introduced laws to prevent construction and vehicle use on beaches, which definitely help, but it is unlikely that your average citizen or tourist will care so much about crabs. Perhaps fortunately for ghost crabs is they often share beaches with another group of animal that is loved by almost everyone; Sea Turtles. There are strict laws for disturbing nesting Sea Turtles in most countries across the world.
Since studying these ghost crabs, I developed a new-found respect for Crustacea. I always make a point in searching for various crab species in rock pools, mud flats or on beaches. Next time you are on a beach, I urge you to consider some of the smaller animals that might inhabit these habitats.