The Flame-backed Fiddler Crab, Uca flammula (Fiddler Crab of the Week)

The Flame-backed Fiddler Crab, Uca flammula, is a particularly striking species whose name derives from its color. In adults, the carapace is usually more-or-less solid black, except for a pair of whitish-red parallel back-to-front markings toward the center (most fiddler crabs have what appear to be creases in the carapace that roughly form the letter H…it is the vertical arms of the H that are red in this species) and a solid red band at the front of the carapace. Most of the rest of the crab is usually a bold and bright scarlet-orange, with the tips of the large claw (particularly on the movable finger) trending toward white. Somewhat unusually, females are colored more-or-less identically to males (in many species where the males have bold and bright colors, the females are more dull and cryptic), although sometimes with the red extending even further onto the carapace.

Photo provided by Pat Backwell.

A slightly unusual aspect of this species is that the juveniles are a very different color, usually almost a uniform gray-blue with darker-blue eyestalks and a pale yellow large claw in males. The complete lack of red and the fact that young males will sometimes wave could mistakenly lead one to think they might be a separate species (von Hagen and Jones, 1989).

The Flame-backed fiddler is found in northern and northwestern Australia, as well as on the western half of New Guinea. It is a large species, with a narrow front (eyes close together), and a robust looking major claw, usually with very obvious bumps (tubercles) and grooves when examined closely.

Photo provided by Pat Backwell.

It’s waving display is fairly vertical (mostly up and down without much movement of the large claw out to the side), usually starting with an initial strong wave, followed by a series of diminishing smallerĀ ones (Crane 1975). In what I am now realizing is going to be a very common statement, the biology of Uca flammula is relatively unstudied, although some recent papers by Madeleine Nobbs has examined how its distribution on shorelines is related to vegetation patterns, so perhaps more information about this species is on the near horizon.