We went for quite a few walks on the beach while we were in Port Douglas, usually in the early morning before things got too hot! We were surprised by the near-total lack of shells washed up on the sand (the copious cuttle-fish ‘bones’ didnt’ count). And fascinated by the way that the sand between high & mid-tide was usually covered by little balls of sand.
On closer inspection we could see that among the soft sandy beads were holes dug down into the substrate:
When we first saw all this, we thought we were looking at the wastes expelled by buried shellfish, but when we watched carefully we saw that it was all produced by well-camouflaged, sand-coloured crabs, scurrying around among the sand balls and diving down into the holes at the first sign of danger. Some of those holes were quite large; neither of us was game to put a finger down to seek for an inhabitant.
And while there were definitely crabs of many sizes living in the intertidal zone, we suspected that there could also be crabs of more than one species. Some of the sand balls were quite poorly formed while others were very precisely rolled, and they also differed in how they were arranged around the hole. In some cases they were arranged into neat circular walls, while other crabs seemed to hurl their balls out any old how. You can see why we found them so fascinating. (The locals probably thought to themselves, ‘crazy Kiwi tourists, spending so much time noses down & bums up on the beach!’)
A quick google found out that this behaviour is common in fiddler crabs. These little animals are detritiovores, bringing gobs of sediment to their mouths & sifting through this to find anything edible, The left-over sand or mud is formed into balls & deposited around the entrance to the crabs’ home burrows. But we weren’t seeing fiddler crabs, because ‘our’ crabs had symmetrical pincers (in fiddler crabs one of the front claws is much enlarged, & used to signal to the neighbours, marking territories & attracting mates.). ‘Our’ crabs turned out to be ghost crabs, & our suspicion was correct – there is more than one species living on Port Douglas’s Four Mile Beach.
As for the differences in sand-ball construction & distribution patterns, in some species at least this may be related to courtship & territory displays. (It would be safe to describe the various walls & heaps & scatterings of sand as a visual display.) I couldn’t find a lot of easily accessible material on ghost crabs. But there’s some experimental data on fiddler crab ball-building – & some questions to get you thinking about how to interpret this – at this site by the Association for the Study of Animal Behaviour. It certainly looks as though female fiddler crabs find big piles of balls extremely attractive