The marine environment is full of unique and fascinating creatures. Whether it is a sandy beach, a coral reef or in the dark depths of the ocean, there are loads of bizarre and unusual beasts sporting all kinds of weird adaptations or odd lifestyles. I’m a marine biologist, so I thought I’d write a post about a different marine critter every Thursday, picking out my favourites and giving a brief blurb about why I like them. First up is the focus of my PhD thesis – sand bubbler crabs.
The common name ’sand bubbler crab’ is used for quite a few different groups of crabs, but I’m refering to the genus Dotilla. They can also go by the common name ’soldier crab’, which again can refer to a whole bunch of other sorts of crabs. Sand bubbler crabs can be found on tropical sandy beaches from the east coast of Africa, around the Red Sea, along the coast of India and into the Indo-Pacific. They are small crabs, no more than 2 cm wide. As the tide recedes down the beach every day, the crabs emerge from their burrows in the sand, and begin to feed. The crabs scrape sand into their mouth parts, where any organic matter that is adhered to sand particles is sifted out. As they feed, the crabs build up a round pellet of rejected sand at the top of their mouth parts, which they pinch off and throw back between their legs when it reaches a size of about 5 mm. These feeding pellets build up as the crabs feed in a radial pattern around their burrow, looking like the sand has bubbled up, hence the name sand bubbler crabs.
The patterns the crabs make are also pretty cool. The crabs are very sensitive to movement and generally quickly disappear back down their burrows when anything moves near them, be it human, predator or plastic bag blowing in the breeze. When the tide comes back in, the crabs retreat to the their burrows and plug them up, waiting for low tide the next day. Along with the more mundane adaptations, such as being a similar colour to the sand of the beach to avoid predation, sand bubbler crabs have some other excellent and unusual features that make them suited for intertidal life. One segment of each leg bears a thin flattened membrane. Originally though to act as a hearing organ, it was actually found to act as a gas exchange surface – these crabs can breathe through their legs. They also have a band of stiff hair on the underside of their body which they can stick into the beach to draw water out of spaces between sand grains, helpingthem to avoid desiccation from exposure to the sun. Sometimes they gather in large herds and move around en-mass (hence the other common name of soldier crabs). The crabs can be quite territorial, and as well as the time honoured display of using physical strength used to drive interlopers away, they also hop up and down on the spot, like an enraged child which can be pretty amusing. I’ve been studying these crabs for almost 3 years now, so I hold a bit of a soft spot for them. Despite being fairly ubiquitous in some areas, they tend to be overlooked due to their small size and lack of commercial importance. I find them quirky and intriguing, and I’ve spent many an hour patiently sitting still watching them pottering around on the beach.
