The Styled Fiddler Crab, Uca stylifera (Fiddler Crab of the Week)

This week we move to the Americas for the first time and to one of my favorite species, the Styled Fiddler Crab, Uca stylifera. The Styled Fiddler is arguably the most striking species found in the western hemisphere and one of the easiest to identify. It is a medium (trending toward large) species where males have a white body (sometimes more dull yellow if inactive); white and purple legs; a robust large claw which is reddish-purple at the base, with an orange pollex (the immovable finger) and white dactyl (the movable finger); and yellow eyestalks. A single long style (as long or longer than the eyestalk) projects from the top of the eye found on the same size as the large claw.

Photo provided by Pat Backwell.

If it looks familiar, that might be because it is the species featured in the logo of this site.

Because they tend to be found on open, dark sandy mud flats, males of this species are among the most conspicuous fiddlers you can find. In contrast, except for the yellow eyestalks, females are a more-or-less solid muddy brown which allows them to readily blend into the background.

Photo by M. Rosenberg, Panama, 1997. Note the brown female to the left and behind the male.

The Styled Fiddler is found on the Pacific coast of Central America and northern South America, from El Salvador to northern Peru. It’s found on open mudflats (rather than in mangroves) in what might be considered the mid-intertidal zone. It seems to prefer a muddy substrate which is a bit firmer, with somewhat higher sand content than many other species; it’s not found on sandy beaches or pure sand bars, but neither is it found in the thicker, stickier, softer mud. More than a dozen other fiddler crab species can be found at the same locations as this species, but since they tend to stick to different parts of the intertidal region, only a few are actually likely to directly intermix with Uca stylifera, with Uca beebei and Uca heteropleura being fairly common.

The waving display consists primarily of the male holding his claw out to the side and then making moderately slow, fairly tight back-and-forth motions with it while simultaneously taking a “stride” (if that’s what you call the combination of all eight legs taking one step) to one side or the other. The small claw is often also held up in the air during the wave as well. The video below is old and low quality, but shows a fairly typical display. The smaller crabs seen in the background are Uca beebei.

The obvious question about this species (or at least, the one I get most often) is “What’s up with the style?” To a large extent, we really don’t know.

This is not the only species which can have a style; they are sometimes found in adults of the closely relates species Uca heteropleura, as well as in juveniles of other similar species. However, Uca stylifera is the only species in which (a) every male has a style, and (b) the style is as long as the entire eyestalk. In the other species which have them, the style tends to be very short (less than half, or even a third of the length of the stalk) and is only found in a small proportion of males. Some non-fiddler crab relatives also have styles (e.g., some species of ghost crab), so the character is not entirely unique to fiddlers.

Very little is known about the function or purpose of the style; it plays no role in vision and serves no obvious function in courtship or display. Superficially it seems like a character that could be driven by sexual selection and female choice, although there is no evidence females choose males on the basis of the style (to be fair, experiments on this are lacking). The only study that has ever looked into elongated eyes (and secondarily, the styles) in Ocypodid crabs in any serious way (von Hagen 1970) suggested styles were likely a developmental artifact and likely served no primary function. More work may be necessary if we really want to understand this odd feature.

One aspect of the style actually highlights an interesting, often missed feature of fiddler crabs. The most striking characteristic of male fiddlers is the asymmetry of the claws: this is the character best known and recognized by most people. What is underappreciated is that the asymmetry is not restricted to the claw; it just happens to be the structure in which it is most obvious. In Uca stylifera, the presence of the style is asymmetric: it is only found on one eye and always on the eye on the same side as the large claw. In fact, in most male fiddler crabs, the entire side of the body with the large claw is slightly larger than the side with the small claw. The legs tend to be slightly longer and heavier, the carapace can be a little asymmetric and heavier, the internal muscles on that side of the body tend to be larger (likely necessary to support the asymmetric weight), etc. Even the eyes are different lengths. It’s not always obvious in smaller species, but look at the photos at the top of this post: even ignoring the style, you’ll see that the eye on the side of the large claw is longer than the eye on the side of the small claw.

Whole body asymmetry is pretty much true for males of all species, it is just that beyond the claws themselves, the other asymmetries tend to be substantially more subtle and pale in comparison to the differences in claw size. But whatever the developmental mechanism is that controls the asymmetry, it applies to the whole body, not just the claw. This is more obvious in Uca stylifera that most other species because the asymmetric presence of the style draws attention to a non-claw asymmetry.