The ghosts of Bird Island

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.

A typical Sooty Tern colony will cover nearly every inch of the grass.
A typical Sooty Tern colony will cover nearly every inch of the grass.

The rest of the bird life was pretty amazing too. I experienced my first Booby, several species of Terns, Tropicbirds, and Frigatebirds.

Frigatebirds are adept at flying and will regularly snatch food from terns in flight.
Frigatebirds are adept at flying and will regularly snatch food from terns in flight.
If there’s a cuter bird than the Fairy Tern, I have not seen it. This individual lost its nest during a storm, so the island’s guide made a new one using a coconut.
If there’s a cuter bird than the Fairy Tern, I have not seen it. This individual lost its nest during a storm, so the island’s guide made a new one using a coconut.

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.

This individual, named Raphael, was attacked by a dog when it was much smaller, hence the obvious missing scutes above his back-right leg. One can only imagine that the dog
This individual, named Raphael, was attacked by a dog when it was much smaller, hence the obvious missing scutes above his back-right leg. One can only imagine that the dog, and several generations of offspring, have succumbed to old age since the incident.

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).

Our study species was the Horned Ghost-crab, Ocypode ceratophthalmus, which have these awesome eye stalks.
Our study species was the Horned Ghost-crab, Ocypode ceratophthalmus, which have these awesome eye stalks.

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.

One of these was burrows was constructed by an adult, whilst the other was by a juvenile. I won’t reveal which is which.
One of these was burrows was constructed by an adult, whilst the other was by a juvenile. I won’t reveal which is which.

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.

I’m definitely going to win this year’s sand castle competition!

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.

Science!
Science!

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.

Martyn Drabik-Hamshare
About Martyn Drabik-Hamshare 3 Articles
Martyn is an ecologist, a birdwatcher and a keen herpetologist, with a particular interest in tortoises and turtles. Growing up in the United Kingdom, he worked with the few reptiles and amphibians around, but decided to travel to experience more of what the world could offer. He has now worked in the United States, the Seychelles and South Africa, studying tortoises, ghost crabs, and a host of other species.

3 Comments on The ghosts of Bird Island

    • Hello Dean. I was not on the island long enough to witness either of those, but I’m sure they do (the Sea Turtles at least). With regards to the tortoises, they are most likely introduced to the island. Most were adults, though I do remember seeing one subadult Aldabra Tortoise (which was still bigger than 99.99% of tortoises elsewhere in the world.

      The island also had two introduced radiated tortoises (originally from Madagascar); one apparently accidentally introduced with a crate of coconuts; and the other subsequently introduced as a mate. The male radiated tortoises is known to wander through the Sooty Tern colony chomping away at their eggs. Again, I did not witness this, but when I found the male, he was, perhaps not surprisingly, close to the tern colony.

      I had two interesting predation observations; one involved seeing an already dead sooty tern being dragged down a burrow by a single ghost crab, and subsequently getting stuck in the entrance; and a second involving a Caspian Tern with a meal.

      For the second instance, a tern had landed on the beach with a comparatively large fish; one that it could not fly easily with and could not swallow whole. The tern was being chased by probably 20-30 ghost crabs. The tern was forced to fly and land several times before giving up on its meal.

      • Dear Martyn,

        Thanks for your reply to my query. Both your observations were of considerable interest, and definitely publishable as a short natural history paper in one of the journals, particularly the second observation which is particularly notable as an example of group predation by an invertebrate species. However, my particular interest, and one of my research ‘themes’, is ‘predation on amphibians and reptiles by invertebrates’, so if you come across any examples, let me know if you would be interested in collaborating in natural history notes for one of the herpetological journals; you should have my email address. All the best,

        Regards, Dean (herpetologist, based in Sydney, Australia).

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