Watching crabs in a rock pool
AN APPRECIATION OF THE CRAB
Since ancient times, when people first traced the shape of Cancer in the stars, the crab has represented both strength and renewal. The crab’s ability to regenerate itself, stepping out of its old shell and starting anew as it grows, even re-sprouting lost limbs, is the reason that it is linked in folklore to the rebirth of the moon. Yet, for most of us, these creatures are associated with sunshine, ice-cream, sandcastles and blue skies. To feel the tickling of a baby crab scuttling across my toes in a warm pool, or to turn over an orange periwinkle shell and glimpse the pincers of a hermit crab tucked inside, is to step back to a time of long summer holidays and lemonade lollies.
The sound of the rocky shore at low tide is one of life’s most serene pleasures. Distant oystercatchers trill, competing to be heard over the ocean, while waves rushing in through kelp-filled gullies make their own music.
It’s only when I stop and pay attention that I hear strange creaks and rasping noises that seem to come from the rock itself. Hundreds of barnacles fizz and pop like breakfast cereal as they shut up their plates, while limpets grate down, scarring the slate to seal themselves against the drying air, but above all this there is a rhythmic ant-like clicking interspersed with harsh scraping. I crawl and slide over the rock to home in on the source. From within a dark crevice, two expressionless stalked eyes are watching,
framed by hefty claws. The crab backs further into the rocky recesses, grinding its olive-green shell deeper into the hole and clicking its mouthparts beneath a frothing pillow of foam to keep them damp.
The green shore crab, Carcinus maenas, is our most common intertidal crab, familiar to anyone who has paddled in rockpools or dangled a crab net off a pier. From its blue copper-based blood to its ability to live for up to a week out of water, this crab is an enigmatic champion of survival. Its emotionless exterior may not belie any cuddly interior – cannibalism is among its less appealing traits – but it commands our respect with its nutcracker pincers.
Naturalists are used to foreign invaders outcompeting native British wildlife, but the green shore crab is turning the tables. Arriving as plankton stowed away in ships’ ballast water or among shellfish catches, these crabs have inflicted catastrophic impacts on ecosystems and shellfish beds in places as far-flung as North America, South Africa and Australia. Little can be done to stop their spread.
For all its ruthless efficiency, the shore crab is a deeply familiar creature that brings endless delight to rockpoolers of all ages. I, for one, will never tire of stopping still on a quiet shore at low tide, listening for the unmistakable percussion of the crab’s long wait for the tide to return.
Heather is the author of Rock Pool: Extraordinary Encounters Between the Tides (September Publishing).