The Slender-legged Fiddler Crab, Uca tenuipedis (Fiddler Crab of the Week)

The Slender-legged Fiddler Crab (Uca tenuipedis), is a very small species from the eastern Pacific Ocean, ranging from about El Salvador to northern Peru. It is similar in size to some of its sympatric associates, such as Uca batuenta, Uca intermedia, and Uca saltitanta, but is readily distinguishable from the first of these by the somewhat triangular shape of the pollex and manus, and the latter two by its color: a fairly cryptic-colored brown and gray body, with a white pollex and pinkish-salmon dactyl and top part of the major claw.

Photos by M. Rosenberg, Panama, 1997.

The waving display of this species involves bringing both the large and small claws straight out to the side, pausing for a moment, then bringing them up and forward in a circle back to the starting open-wide position. The whole body rocks backwards slightly with this motion and sometimes the first few legs may come off the ground as well.

Unlike some of the other local species, this one seems to stay closer to the shoreline, found among the muddy stream banks or the fringes of mangroves rather than out on the open mudflat.

The Pygmy Fiddler Crab, Uca pygmaea (Fiddler Crab of the Week)

The Pygmy Fiddler Crab (Uca pygmaea) is an obscure, very small fiddler found on the Pacific coast of the Americas, from southern Costa Rica through northwestern Colombia. It’s name refers to it’s tiny size. Almost nothing is known about this species; there appear to be no observational studies of it in nature beyond its original collection along the muddy bank of a stream in Costa Rica.

The large claw of this species is interesting in that the hand is very thick and the fingers are relatively short and stubby. Many species have juveniles with claws that are thick and stubby; as they get older the proportions shift into those that we tend to associate with most fiddler crab claws today. This change in shape with size is known as allometry. It has been proposed that the Pygmy fiddlers’ claws stop developing at a more “juvenile” shape, a pattern known by the technical term paedomorphosis. It is certainly not a requirement that a species this size have a claw of this shape; while thicker, stubbier claws are not uncommon among some of the other very small species (e.g., U. saltitanta and U. inaequalis), one of the very smallest, Uca batuenta, has a perfectly “normally” proportioned claw.

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.

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.