Fiddler Crab Field Guide: Pacific Coasts of Colombia, Ecuador, and Northern Peru

This guide is designed for identification “in the field” where you might be looking at live crabs by eye or through binoculars or from photographs. I will generally try to avoid characters that will require you to physically catch the crab, although I may mention a few for secondary verification. It does not include the more strict taxonomist-style characters that may only be visible under a microscope or via dissection. It is also assumed that the individuals are living, as death (and even capture) can cause dramatic color change.

This is a guide to the fiddler crabs of the Pacific coastlines of Colombia, Ecuador, and the very northern part of Peru (to about Sechura). This coastline has one of the richest species diversity of fiddler crabs in the world, with 24 species present:

• Leptuca batuenta (Beating Fiddler Crab)
• Leptuca beebei (Beebe's Fiddler Crab)
• Leptuca dorotheae (Dorothy's Fiddler Crab)
• Leptuca festae (Festa's Fiddler Crab)
• Leptuca inaequalis (Uneven Fiddler Crab)
• Leptuca latimanus (Lateral-handed Fiddler Crab)
• Leptuca pygmaea (Pygmy Fiddler Crab)
• Leptuca saltitanta (Energetic Fiddler Crab)
• Leptuca stenodactylus (Narrow-fingered Fiddler Crab)
• Leptuca tallanica (Peruvian Fiddler Crab)
• Leptuca tenuipedis (Slender-legged Fiddler Crab)
• Leptuca terpsichores (Dancing Fiddler Crab)
• Leptuca tomentosa (Matted Fiddler Crab)
• Leptuca umbratila (Pacific Mangrove Fiddler Crab)
• Minuca argillicola (Clay Fiddler Crab)
• Minuca ecuadoriensis (Pacific Hairback Fiddler Crab)
• Minuca galapagensis (Galapagos Fiddler Crab)
• Petruca panamensis (Rock Fiddler Crab)
• Uca heteropleura (American Red Fiddler Crab)
• Uca insignis (Distinguished Fiddler Crab)
• Uca intermedia (Intermediate Fiddler Crab)
• Uca ornata (Ornate Fiddler Crab)
• Uca princeps (Greater Mexican Fiddler Crab)
• Uca stylifera (Styled Fiddler Crab)

Note: This guide is not complete. While most are described, a few of the species are obscure enough that I do not currently have enough information to reliably describe their appearance or how they differ from some of the other overlapping species.

A number of features can be used to distinguish among these species, but a good place to start is to look at the distance between the base of the eyestalks. Fiddler crabs tend to split into two groups, those with the eyestalks very close together (“narrow front”) and those with the eyestalks separated a bit more (“broad front”). The six species in the genus Uca are narrow front species, while the other 18 are broad front species.

Narrow Front Species

Of the six narrow front species along the coast, all in the genus Uca, at this time two should be easy to identify, two are moderately easy, and two are very difficult. We'll order these from easy to hard.

Uca stylifera (Styled Fiddler Crab)

Uca stylifera is one of the easiest species to identify. Its primary distinguishing feature in male crabs is a long style coming out of the eye on the same side as the large claw; the style is approximately the same length as the rest of the eyestalk. While some other species will occasionally have styles, they are substantially shorter than those found in Uca stylifera. In addition, the colors of Uca stylifera are distinct: males have a white carapace (in rare instances more yellow than white), yellow eyes, orange-red-to-purple legs, and a large claw with an orange fixed-finger (pollex) and white movable finger (dactyl). Some Uca princeps may be similarly colored, but the two species should be easy to tell apart. Female Uca stylifera are a bit more tricky as they lack the style and tend to be a dull muddy-brown color (you can see a female partially hidden by the male in the first photo below). They're most easily identified by association with the males.

Uca intermedia (Intermediate Fiddler Crab)

Uca intermedia is only known from the coasts of Panama and Colombia and appears to be very rare. If actually found, however, it should be easy to identify as it is uniquely an essentially solid black fiddler crab with a red large claw.

Uca heteropleura (American Red Fiddler Crab)

Uca heteropleura should be easy to tell apart from the other species, but in practice it can be confused with some of the color variants of Uca princeps. Uca heteropleura has a predominantly red-to-black carapace and legs (sometimes lightening to white or slightly more purple), with a red lower part of the claw and white movable finger. Its eyestalks are generally black. The large claw tends to have relatively short and stout fingers and is usually noticeably bumpy on the outside of the hand.

While most Uca princeps are very different colored and have claws with relatively longer fingers, some are similarly enough to make them more difficult to distinguish. The primary features to keep an eye on when encountering a red narrow-front crab that might be either species are eyestalk color—I believe Uca princeps has yellow eyestalks while Uca heteropleura has black—and the length and shape of the movable finger (the dactyl). In Uca heteropleura the dactyl tends to be shorter relative to the length of the claw, much more curved, and if there is a small tooth/bump on it, the tooth tends to be closer to the tip of the claw. In Uca princeps the dactyl is relatively longer and there is usually small tooth/bump right around the center of the dactyl. The outside of the large claw in Uca heteropleura is almost always noticably rough and bumpy, while in Uca princeps it may be a lot smoother.

Behaviorally, the waving display of Uca heteropleura is extremely different from that of Uca princeps and can be used to identify them in the field, and even sometimes in a still photo. Male Uca heteropleura wave by holding the large claw in front of them then raising their entire body vertically onto the tips of their legs, with the claw held up above them. In contrast, Uca princeps prances back and forth with the the large claw held laterally out to the side. The following photos are a good illustration of the vertical hold position of Uca heteropleura vs. the more typical side waving of Uca princeps. Note that at its extreme position, Uca heteropleura raises a number of its walking legs off of the ground, while all of Uca princeps walking legs remain on the ground while it waves.

Vertical waving of Uca heteropleura

Lateral waving of Uca princeps

Uca princeps (Greater Mexican Fiddler Crab)

Uca princeps is the most common narrow-front species on the Pacific coast of the Americas, has the longest range (southern California through Chile), and is the most variable in appearance (there is some suggestion it might be a mix of multiple, yet unidentified species).

The most typical colors in this species are pale yellow and orange, with some white. The carapace is typically yellow or fading-to-white, the legs are generally more orange-yellow, and the claw is often a bit brighter orange, with a white movable finger (dactyl).

Unfortunately, there is a lot of variation beyond this typical pattern. Carapace and legs may darken to a dark orange or even brown; sometimes the orange shifts to more red, which is when it can become confused with Uca heteropleura. In rare cases the carapace can be dark blue, almost black.

As mentioned above, I believe the eyestalks tend to be pale, much more yellow, as opposed to the dark black eyestalks of Uca heteropleura. The other species that it can be superficially confused with is Uca stylifera as the colors can sometimes converge, but the presence of the long style in males on the eye on the same side as the large claw is diagnostic.

Another character to look for is that the large claw of Uca princeps generally has fingers which are longer than the palm, while in the other species the fingers tend to be the same length or shorter than the palm.

Uca ornata (Ornate Fiddler Crab)

Uca ornata and Uca insignis are very easy to identify relative to the other fiddlers in the region, but very difficult to tell apart. They are among the largest of all fiddler crabs and the shape of the large claw is very distinct (only matched worldwide by the Atlantic coast species Uca maracoani). The claw almost resembles bolt cutters, with large flattened fingers which meet at a very straight inner edge, and with the upper edge of the movable finger distinctly curved.

The carapace of Uca ornata can be a pale white or beige or a darker orange or reddish-purple (particularly on females). When not covered in mud (more on this below), the claw is usually orange or dull yellow.

Unfortunately, Uca ornata and Uca insignis can be difficult to tell apart, with some of the more telling characters fairly subtle. One surprising feature that may be used to distinguish males of the two species is the pattern of mud on the crab, particularly on the large claw! It is not unusual for Uca ornata to be completely covered with mud, but even if most of the crab is clean, the lower finger (pollex) of the large claw is almost never so. The reason for this is the outside of the pollex of the large claw in Uca ornata has pits filled with small hairs (generally referred to as “pile”). Mud sticks to the pile and does not come off readily, more or less permanently giving males a layer of mud over the lower part of the large claw. Uca insignis has little pile on the outside of its pollex, so does not tend to have mud covering it.

Another trait that distinguishes the species, but is usually difficult to see, is the presence of a series of large tubercles (bumps) on the outside upper edges of the rear part of the carapace (the dorsal-lateral margin). Uca ornata has large, distinct tubercles on this edge; Uca insignis does not, having maybe a single large tubercles at the rear end of the carapace. You can see this row of tubercles, indicated by arrows, in the the following photos.

Female Uca ornata generally have dark orange, mud covered carapaces, and dark purple legs.

Uca insignis (Distinguished Fiddler Crab)

As mentioned above, Uca insignis is extremely similar to Uca ornata in appearance and we have no solid color description of the species to definitively discuss differences in color. Two photos on iNaturalist that are fairly certain to be this species are not particulary outside the general color range of Uca ornata. Beyond some subtle differences mentioned above, one character which is unique to Uca insignis—but very difficult to see—is the presense of “vein-like” lines on the sides of the carapace as illustrated in the following photo.

Another extremely subtle character is that Uca insignis generally has small bumps on the surface of the upper finger (dactyl), at least proximally near where it connects to the hand, while the dactyl of Uca ornata is generally smooth. This may be too subtle for general field use; I could not find any photos clearly illustrating the difference.

Broad Front Species

Many of the 18 broad front species along the coast are difficult to distinguish, but I'll describe key characters for those where we have good data. One of the species is in its own genus, Petruca, which will be described first. The rest are split between two other genera. As a general rule, species in the genus Minuca tend to have very broad fronts while those in the genus Leptuca tend to have narrower broad-fronts (medium fronts?) but there is overlap among species in the two genera so front breadth by itself is not an absolute indicator of the genus. In addition, the Minuca species tend to be larger than the Leptuca species, although there is overlap once again. As a rule of thumb, though, larger species with particularly broad fronts are probably Minuca while very small species with narrower (but not very narrow and pinched) fronts are probably Leptuca.

Petruca panamensis (Rock Fiddler Crab)

Petruca panamensis is distinguishable primarily by its environment, rather than a physical character: it is the only species of fiddler crab in the world that lives predominantly in and among large rocks, rather than the more typical mud or sand substrate that all other fiddlers live upon. If you find a species waving on top of a large rock, it is probably this species.

The species is not particularly visually distinct, although it tends to have a flatter carapace than other species, and the large claw seems particularly smooth and relatively featureless. It tends to be relatively solid colors, usually pale or dark gray. The large claw tends toward beige, with the lower finger white. In many cases the top edge of the upper finger (pollex) will be noticably darker than the rest of the finger.

“Distinct” Leptuca

The first set of Leptuca have distinct physical characteristics, beyond color, that makes them readily identifiable.

Leptuca batuenta (Beating Fiddler Crab)

The smallest of all fiddler crab species, often only about 5-6 mm in breadth, Leptuca batuenta is easy to overlook due to its size. If seen, however, it is surprisingly easy to identify due to the unique shape of the large claw. Specifically, the lower/fixed finger (pollex) has a distinct tooth pattern where the pollex curves upward to this tooth about 3/4 of the way along the pollex, then curves back down with a concave edge to a point. The claw is generally white, while the limbs tend to be brown-red.

Leptuca saltitanta (Energetic Fiddler Crab)

Another very small fiddler crab (~6 mm), this species is also easily distinguishable from others due to a combination of color and claw shape. It is one of the few species that is essentially solid white (in fact, its claw and limbs are translucent), with yellow eyestalks. The lower finger (pollex) of the large claw is strikingly thick and triangular, while the upper finger (dactyl) is very thin. While a few other species have a claw of similar shape, they are not solid white.

Behaviorally the species raises the claw up into the air then sharply brings it down to very rapidly drum on the mud surface (it looks like a rapid vibration).

Leptuca inaequalis (Uneven Fiddler Crab)

Another very small fiddler crab (~7 mm), Leptuca inaequalis is somewhat similar in shape to Leptuca saltitanta, but entirely different colors. The large claw is gray-brown with a distinct dark red/brown patch along the lower edge by the base of the pollex, the fingers are generally white, and there is frequently an orange tint running along the upper edge of the movable finger (dactyl). The gape of the claw is almost always filled with brown mud, because there are small hairs on the upper edge of the first half of the pollex that the mud clings to. The shape of the polllex is not as strikingly triangular as that of Leptuca saltitanta, but does have a very heavy palm with short thick fingers.

Leptuca latimanus (Lateral-handed Fiddler Crab)

Leptuca latimanus sort of looks like a cross between the previous two species, Leptuca saltitanta and Leptuca inaequalis, although it is a bit larger than either being 1 cm or more in width. Its carapace is predominantly white, with dark legs, and its large claw is a dark brownish-red with pale tips to the fingers. The claw is particularly heavy looking with a very broad palm and very short stubby fingers. The lower finger (pollex) is not triangular as seen in the previous two species. When waving, its inner arm may show shades of blue.

Leptuca latimanus is one of the few fiddler crab species that build hoods next to their burrow entrances, as seen in the first two photos above.

Leptuca tenuipedis (Slender-legged Fiddler Crab)

Leptuca tenuipedis is another very small (5-7 mm) fiddler crab which is shaped somewhat like Leptuca saltitanta and Leptuca inaequalis, but with a different color pattern and a somewhat less triangular pollex. Its body is dull brown/gray, while the claw is orange/red, including the top finger (dactyl) while the lower finger (pollex) is white. The colors of the fingers are reversed from many fiddlers, where—if only a single finger is white—it is usually the upper and not the lower.

Leptuca umbratila (Pacific Mangrove Fiddler Crab)

Leptuca umbratila is not a particularly well known species, but should be relatively easy to identify. It is generally a nondescript species, mostly dark brown with dark brown/red eyestalks and limbs which are either muddy brown or dull orange. The claw is dull orange with paler fingers.

One of its notable features are particularly thick walking legs: the primary leg segment which one sees coming from the body (the merus, which is technically the second limb segment, but the first is too small to generally observe) is very high and rounded, where this same segment is much thinner in most other species. This is most clear in the second photo.

Leptuca tallanica (Peruvian Fiddler Crab)

Leptuca tallanica is a small (1 cm), poorly known species, although still reasonably distinct. Its coloration is very similar to that of the next species Leptuca tomentosa, but it has a somewhat different claw shape. The end of the lower finger of its large claw flattens into a straight edge leading to the tip (slightly similar to some of the patterns seen in a few of the previous species). It has a marbled dark and light green carapace, with yellow speckles; green eyestalks; and a pale claw with some light brown on the margins.

“Typical” Leptuca

The next set of species are all very similar in size, shape, and structure, so much so that if your removed all color and behavior, they would be very difficult to distinguish in the field. Thankfully, there are distinct color differences for many of them. They are all about 1 cm in width and have a “typical” large claw which is slender with long fingers.

Leptuca tomentosa (Matted Fiddler Crab)

Leptuca tomentosa is very similar to Leptuca tallanica, if slightly larger (1–1.5 cm), with a very similar marbled yellow-green carapace (in limited observations, the green shades of Leptuca tomentosa lean more toward yellows, while those of Leptuca tallanica lean more toward blues.). Unlike Leptuca tallanica, the claw of Leptuca tomentosa follows a more typical shape with slender fingers which come to unadorned points.

Leptuca terpsichores (Dancing Fiddler Crab)

Leptuca terpsichores is another fiddler crab that is usually easy to identify. When males first come out of their burrows they are generally an innocuous marbled brown-red color.

However, within 15 minutes or so their color shifts to almost entirely white, except for some pink/purple on the base of the large claw. This is the color form that will be seen the vast majority of the time.

A more subtle character is found in the small claw; the width of the opening between the fingers (the gape) is particularly large and wide in this species compared to most others. In the following photos, focus on how far apart the fingers are on the small claw.

Leptuca terpsichores lives on sandier beaches and is famous for building hoods next to their burrow openings, which they wave in front of, as seen in many of the above photos.

There is one additional character unique to only this species and one other (found only in Mexico), but it is almost impossible to see in the field or in photos without capturing a crab and looking for it. On the lower edge of the palm of the large claw, these two species uniquely have a series of parallel striae/ridges; behaviorally they rub their legs against these ridges to make sounds. Unfortunately, these are extremely subtle and unlikely to be seen without actively handling a crab and knowing where to look for them. In the photo below, taken with a microscope from a captured animal, you can see see a series of small parallel lines (circled in red) running from the bottom edge of the inside of the large claw almost straight horizontally across the photo right into the joint where the rest of the limb attaches. These “stridulating ridges” are the unique character for these two species. Unfortunately, these are nearly impossible to see in a photo of a live animal in the field.

Leptuca stenodactylus (Narrow-fingered Fiddler Crab)

Like Leptuca terpsichores, Leptuca stenodactylus tends to be found on sandier substrates, and the two species frequently overlap and intermix. Although about the same size and shape, their colors are quite different, with Leptuca stenodactylus one of the more colorful species along the coast, having a blue and white carapace, bright red legs, and a claw with pink or white.

The gape of the small claw of Leptuca stenodactylus is wider than in most other species, although less so than that found in Leptuca terpsichores.

Leptuca beebei (Beebe's Fiddler Crab)

Leptuca beebei is one of the more common species along the coast, but is less colorful and striking than those already described. It looks somewhat like a slightly dull Leptuca stenodactylus but without the red legs. Its carapace is generally a mix of dull green or blue or brown, the large claw is often pale gray or white but with a dark purple/burgundy patch at the base of the fixed finger, and its eyestalks tend to be yellow. It is more of a mud/sand generalist than many of the other species described so far and will often overlap in space with a lot of them as it seems to be less picky about the substrate it lives on.

Similar to Leptuca terpsichores, Leptuca beebei frequently builds structures next to its burrows, but instead of large overhanging hoods it builds smaller pillars.

Behaviorally, Leptuca beebei has a very classic wave where it moves the claw out to its side then brings it up and back to its front in a come-hither sort of gesture. This is particularly useful when distinguishing it from the next species, Leptuca deichmanni.

Leptuca festae (Festa's Fiddler Crab)

Leptuca festae can be a bit larger than the other species combined under “typical” Leptuca, ranging up to 1.5 cm. It is generally a dull brown or brown-gray color, sometimes almost black, with yellow eyestalks, and only slight whitening of the claw. For its size, it tends to have substantially longer fingers on the major claw than the other species.

Poorly Known Leptuca

The following species are poorly known or obscure, with no known photos. Basic descriptions are given but identification requires caution.

Leptuca dorotheae (Dorothy's Fiddler Crab)

I am not aware of any living or color photos of Leptuca dorotheae. It is described as extremely similar to Leptuca beebei in shape and form, and even builds similar pillars by its burrows. Its carapace is supposed to generally be olive-green, speckled with yellow; eyestalks are olive green; and large claw is brownish-red to wine-red on the lower margins, except for the fingers which are generally white. Beyond this loose color description, there is supposed to be a difference in the way the carapace between the eyes looks in Leptuca dorotheae and Leptuca beebei; Leptuca dorotheae also lacks noticable serration on the fingers of the minor claw that is present in Leptuca beebei. It's not clear if either of these would be detectable in the field or in most photos.

Leptuca pygmaea (Pygmy Fiddler Crab)

Another small (< 1 cm), poorly known species, Leptuca pygmaea has a thick and heavy claw, somewhat similar to Minuca zacae below. In both species, the fingers are about the same length as the hand; another species with short fingers, Leptuca latimanus, has fingers noticably shorter than the hand. There are no written descriptions of living Leptuca pygmaea, so nothing is known about its color.

Minuca

Unlike many of the Leptuca which come in a variety of shapes, particularly with respect to the large claw, most of the Minuca are shaped more-or-less similarly, leaving only color or behavior as possible field characters. This makes most of them generally harder to distinguish from each other.

Minuca ecuadoriensis (Pacific Hairback Fiddler Crab)

Minuca ecuadoriensis can be a confusing species and seems to come in a variety of different color forms, which may or may not be due to multiple species being mixed together (in addition to just mistaken IDs, there was a short scientific paper many years ago that suggested that Minuca ecuadoriensis was actually a complex of three different species; unfortunately, there has never been a follow-up to that paper to actually describe them so we're left with some confusion).

The one color morph of this species that is easy to identify is a broad-front species which is all or mostly a dark red color, with white fingers.

Other crabs identified as this species are often brown, lacking the distinct red color although usually with the whitish fingers. I am not positive if these IDs are correct or not.

Minuca argillicola (Clay Fiddler Crab)

Minuca argillicola is another obscure species. It is fairly small (~ 1 cm), predominantly beige with a slightly darker large claw, known for being very lethargic and inactive. It is thought to primarily live on river banks with heavy clay concentrations, which is probably the easiest way to identify it, as it is otherwise a fairly generic looking fiddler crab, somewhat similar in color to Minuca galapagensis, although Minuca galapagensis grows about twice as large.

I am not aware of any particularly good photos of this species.

Minuca galapagensis (Galapagos Fiddler Crab)

Potentially the most distinct feature in identifying Minuca galapagensis is it appears to be one of the few Minuca which is predominantly white. The “canonical” coloration of this species appears to be a mostly white (or pale beige) body with a more orange claw (sometimes more yellow-orange, sometimes more red-orange). Unfortunately, other individuals may be more dull brown, orange, or a mix, confusing identification as this starts to look a lot more like other Minuca species. While Minuca argillicola may be predominantly beige/white as well, it lacks the orange of the claw and is thought to be restricted to clay embankments.