Sunday, 22 June 2014

Catching some Winter Sun

Fiddler crabs wave their nippers for various reasons. Some crabs seem to get stuck halfway through a wave and hold their nipper out for minutes on end. When I saw that the little feeding hand was also held out, it became clear that the crabs were sun baking. The crabs are also oriented to catch the most sun. Being Cairns on a warm day, it was not cold by any stretch of the imagination and I can only guess how warm these crabs were getting. Down in the boundary layer next to the ground, where the wind is almost non-existent, the crabs may get quite hot. Years ago, on a mangrove planting project, many of the mangrove droppers/seeds (Rhizophora hypcotyls) got so hot at the point the dropper entered the ground, it killed the droppers. There would be a ring of dead material at ground level, with the below ground part and the above ground part being fine. Based on that experience I would think that a temperature of forty degrees or more would be possible. These sorts of temperatures are often dangerous for invertebrates and it would be interesting to know why the crabs are warming them selves up. A thermal imager would have been handy to see how hot the crabs were getting – they are now only $350 or the price of a good compact camera.  
Uca vomeris
Uca vomeris in the sunbaking pose
Uca vomeris
All dried out

1 comment:

  1. I am also seeing fiddler crabs of another species sunbaking in the middle of the day in the height of summer. Some other crab species also sunbake but it is not as obvious.