The Shaking Fiddler Crab, Uca seismella (Fiddler Crab of the Week)

This week we move to the northern half of Australia to find the Shaking Fiddler Crab, Uca seismella, so named because males shake and vibrate their entire bodies as part of their waving display (video unavailable, unfortunately).

Check me out! Photo from Pat Backwell.

The Shaking Fiddler Crab is a small species, fairly cryptic when not waving (when waving it is apparently hard to miss) and less colorful than most of the other fiddler crabs of northern Australia. It has a carapace that is more or less pale brown and gray, with gray sides and legs. The large claw contains a mix of white and pale salmon-pink, with bright white fingers. The fingers of the claw appear particularly smooth and flattened. It has yellow eyestalks that are close together. Females are more uniformly gray.

Photo from Pat Backwell.

Another species which hasn’t been heavily studied, Uca seismella is predominantly mentioned in the literature relative to other better studied species that it lives near or is somewhat similar to. It’s range in Australia coincides with at least half-a-dozen other species and it is often found in the same areas, if only intermixing on the fringes of their territories.

One of the more interesting tidbits about this species is it is one of the few in which female waving has been observed. Fiddler crabs are quite famous for the male waving displays, but it turns out that in a number of species females also occasionally wave their claws and limbs, or otherwise perform distinct behavioral displays. It is not clear how common female waving is in this species, but von Hagen (1993) filmed three females use waving and bobbing displays to fend off other females encroaching on their territories. Female displays are fairly understudied in fiddler crabs, but have been reported from a variety of species spread across the entire genus and may be more common than we realize.