A case study on ghost crabs

Ghost crabs are a common intertidal species in tropical sandy beaches. Forget beach front digs with a sea view. They have literal digs on the beach. What’s more, Ocypode cordimana is a species of ghost crab that lives on K’garri (Fraser Island), Australia, so they have prime access to the largest sand island in the world in the luckiest country in the world.

On K’garri, O. cordimana burrows are distributed from the low tide mark to the high sand dunes. The size of the burrows are indicative of the size of the home owner. Larger holes and thus larger crabs are found further up the beach from the shore. Higher densities of smaller holes are found closer to the low tide mark and lower densities of larger holes are found further away from the shore.

A boring ecologist might hypothesise that this size-abundance gradient is explained by intraspecific competition or gradients of vegetation cover. But here is the socio-economic explanation:

Ghost crabs are nocturnal (hence, ghosty; not because they are terrible friends – or are they?). At night they head down to the low tide line and forage among the detritus. This is the Central Business District (CBD) where crabs do their biz and partay. Ghost crabs need to commute every day and, like all reasonable beings, they want to avoid the rush hour traffic least they succumb to road rage. And it would not do to be seen participating in such vulgar behaviour in this day and age. Contests are better left to the dishonest fiddler crabs – the cheaters. Rather, all crabs openly carry weapons (claws) as a deterrent via mutually assured destruction.

A schematic of a ghost crab. Not to scale.

Crabs could minimise their commute and live close to the CBD. But living here is dangerous. The sand is fine – poor digging quality so only small houses can be built. The close proximity to the CBD and the ocean means the area gets inundated at high tide, which makes insurance premiums go through the roof. Housing density is high and competition for space is fierce. No-one enjoys hearing their neighbours through the fine grain sand. The only crabs that can live here are small, young crabs (including grad students) who can only afford to live in these inner city slums and dream of living further away from the gangland crime.

In more recent times, the inner city has been undergoing gentrification. Young working professional crabs (Yuppies) and two-crab social groups with double incomes and no kids (DINKs) have been attracted by the convenience of the commute and short distance to local amenities. These crabs are larger than the typical inner city crab, have the income to create quality residences, and think the incoming tide adds character and charm to their property. These crabs enjoy an overpriced flat white with their avo on toast. You’ll often find them scurrying about to their barber appointments for their frothy bubble beards (plaid not included).

The socio-economic theory of ghost crab abundance on K’garri

Beyond the inner city transition zone lies the urban sprawl known as suburbia. Suburbia is where the hopes and dreams of the young go to die and are replaced with a well manicured lawn. Here, the sand is not as wet, infrequently gets inundated, and one could afford to build a large home. The commute to the CBD is a little longer than the inner city but there’s the best of both worlds as access to the world class foredunes is equidistant away. Perfect for the weekend escape from the mediocrity.

Finally, at the base of the dunes are the largest houses. The coarse sand and the roots of the foredune vegetation permit the largest burrows worthy of the largest and wealthiest crabs. Their commute is the longest but they don’t care about that, if they even need to commute at all. Their elevated position on the dune slope gives them the greatest vistas of the population and they live on the urban-rural fringe with easy access to silver-green, xerophytic spaces.

Beyond the dune crest lies The Sticks as the dune transitions to woodland containing, you guessed it, sticks. Not the kind of place for crabs so few crabs are found there.

And that is the socio-economic theory of animal abundance applied to ghost crabs.

Happy April Fools. There is no intellectual basis for applying the concentric zone model of cities to explain real ecological patterns. I initially conceptualised this during my undergrad ecology field trip to K’garri many years ago. This post is dedicated to Prof. Gimmie Walter, who heard it first – Happy retirement!

This post was originally published on EcoEvo@TCD on 1st April 2021.