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 smaller 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.

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.

The Indus Fiddler Crab, Uca sindensis (Fiddler Crab of the Week)

For our very first Fiddler Crab of the Week post, our random generator choose the Indus Fiddler Crab, Uca sindensis. Not the easiest species to start with.

The Indus Fiddler crab is a relatively unknown species found on the coasts of Pakistan, Iran, Iraq, and Kuwait. It has also been reported from the United Arab Emirates, near the Oman border (Ismail & Achmed, 1993), so it is feasible that it could be found on the southern shores of the Persian Gulf as well. It is arguably the most abundant species in Pakistan and most of what is known about its biology comes from the recent work and lab of Dr. Noor Us Saher at the Center of Excellence for Marine Biology at the University of Karachi.

It is a smallish fiddler crab (adult male carapace between 9 and 17mm wide, females a little smaller), lacking some of the more conspicuous colors of other species. It is generally grey in color; the carapace has grey and white bands, with the legs a bit darker grey-brown. The large claw of the male is described as generally pink, with a bit darker-purple toward the arm, shifting toward white toward the top and along the dactyl (the upper/movable finger), and more red toward the pollex (the lower/fixed finger). Unfortunately, I cannot find a photo of a living specimen, but some of the basic color can still be seen in this collected specimen.

A specimen of Uca sindensis from Pakistan. Photograph by Sahir Odhano.

This crab is usually found on open mudflats or muddy-sand in the upper intertidal zone, away from the waterline during low tide. In Pakistan it is often found intermixed with another species, Uca iranica; other fiddler crab species found on the same beaches are less likely to directly intermix.

Very little has been reported in the literature about its behavior, although like many species its waving display appears to involve much more than just the large claw. From minimal observations, Jocelyn Crane (1975, p. 108) stated:

“Wave obliquely vertical to lateral straight, the major cheliped not fully unflexed. Minor makes corresponding motion. Several ambulatories on minor side raised in turn during display.”

Phylogenetically, the Indus Fiddler Crab falls into the general category of “broad-front” species from the Indo-West Pacific (IWP) region. It was originally thought to be closely related to (and a subspecies of) Uca inversa, but now not only is recognized as a separate species, but thought to belong to an entirely different group of broad-front crabs (the subgenus Austruca).

So that’s the first Fiddler Crab of the Week entry. It would figure that my first one would involve one of the least-well known species on the planet (although not the very least).

Fiddler Crab of the Week

Starting next week, I’m going to pick one fiddler crab species at random every week to write a post about (no duplicates). With more than 100 species, it’ll take a bit more than two years to do them all, which if nothing else should give me some motivation to keep posting. It’ll be interesting to see which ones are easy to write about and which ones take a little more effort to find interesting tidbits.