TO THE BEAT OF THE GHOST CRABS
The call of the sea starts as a chorus on the sand
On your beach, do you find tracks and holes like these, from high on the dune down to the tidal zone? Count yourself fortunate if you do. These are the burrows and tracks of ghost crabs properly known as Ocypode cordimana*. "Ocypode" is Greek for “fleet of foot” and a glimpse is all you will often get of these quick moving creatures. Their tiny pale bodies are about 30 mm across and hard to see against the sand. The word “cordimana” translates as “heart” and “hand” which refers to the shape of their large claws. You’re looking at one indicator species of the ecological quality of a subtopic Australian beach. You’re also looking at folk musicians of the intertidal.
The crabs live abundantly on those shores where there are still something of dunes and wild vegetation as opposed to walls, concrete and lawns. They do better where there are few if any vehicles and mechanical cleaners on the beach. Obviously compacting the sand or ploughing it up makes life impossible for them. The burrows they dig are not only their homes but, with the continued maintenance they do, these bring fresh oxygen deep into the sand. This helps other animals that live deep down and also fosters a certain community of microbes over other types. All this adds to what makes a wild beach feel and smell the way it does.
And the folk music? It’s mostly about sex and ties in with the phases of the moon. Male crabs differ from female ones sometimes in size but certainly in structure. Males have a thin abdominal flap tucked up against the underside of their body while the females have a rounder more broad flap which is where she will carry the eggs. When the male pulls his flap back, two hollow prongs pop up. He fits these into the female’s two holes, which are uncovered when she lifts her flap. Internal fertilization: a very good idea to protect the young but the act requires cooperation. So that’s why the serenades: the guys are wanting to please the ladies.
Twice a month, the females’ ovaries ripen. For reasons best understood by the females themselves, each selects one particular male over another. She visits a number of them before choosing. She finds them by listening to their song; a rasping sound, with a specific beat and pulse. She can pick what she likes by listening through the damp sand. The sound moves over a distance of 50-100cms.
The other males can hear too and they set each other off. Their competition makes a chorus. All the crabs hear this through a thin membrane called Barth’s organ, found in their walking legs. By happenstance, you can hear these crab songs on the night air, if there’s no wind and the sea is calm.
When a female crab makes her choice, the male puts her up in his mating burrow. This is her retreat for the few days she needs for laying eggs and hardening the clutch securely onto her abdominal flap. When the eggs hatch the larvae will all make out the sea, looking nothing like their parents, but that is another story.
The safety of mating burrows is obviously important to both crabs and the future of the beach. Given tide and lunar phase, if a male can manage to build another one and please another female, he will.
Males of different species of crabs also serenade. They rap or honk or rasp, all with their own signature style. I wonder if anyone has made any recordings: would be great to make a link to an mp3 of crab songs!
Meanwhile, as our society both destroys some dunes and protects and restores others, we are making our mark on the history of crabs which began in the Cretaceous (138 to 65 million years ago). Maybe, if we include their rhythms or even live recordings as a sample with some of our own music or as a soundtrack to a film clip, they could reach a wider audience who might appreciate them for what they are.
• Although both names “cordimana” and “cordimanus” are used interchangeably on a number of internet sources, I use the name selected by the Australian Museum.
Here's more pix and details about ghost crabs