Wednesday, 18 June 2014

Who Eats Fiddler Crabs?

You would think that fiddler crab colonies would be a magnet for predators as the density of fiddler crabs is often really high.  However in all my years in the mangrove environment, I have rarely seen anything eating or attempting to eat fiddler crabs.  I suppose the lesson is that you often have to work hard to see predators in action.  It is also possible that fiddler crabs are not very good to eat.

Animals that are known/said to eat fiddler crabs include ibis, terns, possibly the giant mudskipper and the mangrove kingfisher.  At high tide, fish and rays will also eat any fiddler crab they can find. Mangrove kingfishers certainly eat crabs and recently I spotted one that appeared to be hunting fiddler crabs.  Being short of time, I could not sit and watch, however you can see several sets of fiddler crab eyes watching the kingfisher – look along the ridge line behind the kingfisher.  Mangrove kingfishers (Todiramphus chloris) are are much larger than other kingfishers and make a loud kak kak sound.

Todiramphus chloris
Mangrove kingfisher hunting crabs on the mudflats of Tingalpa Creek
Below is the would be prey of the kingfisher, Uca longidigitata, a fiddler crab which is common on the mudflats of Moreton Bay, near Brisbane.  They are mostly not very colourful, however some are a turquoise with sky blue nippers.

Uca longidigitata

Uca longidigitata

Of all the birds, the Great -billed Heron (Ardea sumartrana) would seem to be the ultimate fiddler crab catcher.  It is a massive and powerful bird which stands around 1.15 m tall and is only slightly smaller than a Jabiru.  They usually hunt the mangrove waterways near river mouths.

Ardea sumartrana
Great-billed heron hunting along waters edge
I watched the individual above as it slowly walked along a river bank and through 50 m of particularly rich fiddler crab habitat.  The bird showed no interest in the crabs and was only hunting something in the water, probably mullet but possibly other fish and prawns.  This does not prove that great-billed herons don't eat fiddler crabs, only that this individual was not interested at the time of observation.  It would require observations of many great-billed heron individuals for tens of hours and in many places and seasons before a properly substantiated statement could be made.  That said, all of my observations of great-billed herons suggest they hunt fish not fiddler crabs.

Fiddler crabs behind a great-billed heron.
The one animal I have seen attacking a fiddler crab was a small juvenile mud crab.  It had was wresting with a mature male two-toned fiddler crab of the same size that had wandered past the mouth of its burrow.  Unfortunately I could not stop and see what happened.

1 comment: