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 southwestern Gulf of Mexico, including the Mexican states of Tamaulipas and Veracruz. Species found in this region include:
Leptuca subcylindrica and Minuca virens are predominantly only found in Tamaulipas or the northern part of Veracruz, while Uca major is only found in the souther part of the area in Veracruz. The other species are more widely found in both states.
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”). One of these species, Uca major, is a narrow front species, while the others are broad front species.
Uca major is readily recognizable in the region as it is both the only narrow front and much larger than other species in the area. Its carapace is generally white or dark brown/red, it has long yellow eyestalks, the upper finger on the claw is white, and the lower finger has a deep wine red patch on it.
Leptuca panacea has a carapace which usually ranges from pale gray to beige white, and the large claw has some pale red coloration with whiter fingers. Fiddler crabs have an H-shaped depression on the back of their carapace. In some species this depression blends into the rest of the carapace, but in others it may have a sharp, contrasting color. In Leptuca panacea this H-shaped depression is a rust-red color, very noticable in the photos.
Leptuca spinicarpa is not paticularly well known. In the few reasonably definitive photos of this species, its carapace appears to be a medium gray/green with brow spotted markings, and with a dull orange claw with white fingers. A subtlety of carapace shape can help identify this species. Compared to some other species, the sides of the carapace coming back from the front corners tend to be appear particularly straight and squared off, before sharply converging.
Leptuca subcylindrica is a very unusual species, able to tolerate much drier conditions than any other fiddler crab and generally found much farther from the shoreline, in fresher bodies of water.
It tends to be very dull, pale colors, mostly beiges and tans and grays. Its most striking characteristic is the shape of the carapace. Leptuca subcylindrica has a substantially more rounded carapace than other species. Specifically, the curvature is more dramatic when moving from front to back, particularly in the back half of the carapace (like other fiddler crabs, the basic shape when seen from above is still essentially trapezoidal). Most fiddler crabs have flatter carapaces with less curve to them.
Its somewhat difficult to find good photos of this because the curvature is predominantly noticable when viewed from the side, which is an angle that most people tend not to deliberately photograph.
The front breadth of Leptuca subcylindrica is generally around ⅓ of the carapace breadth, placing it directly in between the particularly broader front species Minuca minax and Minuca vocator and the narrower of the broader front species such as Minuca virens, Minuca longisignalis, and the other Leptuca.
Generally, the Atlantic Minuca species can be very difficult to tell apart as they largely have similar shapes and structures and at best only differ by subtle color differences. Many of the Atlantic Minuca have predominantly two-toned carapaces, tending to have one color (or pattern) over the front half and a different, darker color over the back half. While the back half is frequently a dark brown, the color in the front half can sometimes be used to distinguish different species.
One very subtle difference between some of the species in this genus is the breadth of the front. While Minuca generally have the broadest fronts of any fiddler crabs, within the genus the species split into roughly two groups, those whose front is less than ⅓ of the width of the total carapace and those whose front is ⅓ or more of the width of the total carapace. Of the species in this region, Minuca virens, Minuca rapax, and Minuca marguerita have relatively narrower fronts, while Minuca burgersi and Minuca vocator have relatively broader fronts. Front width can be difficult to estimate with any accuracy (particularly in the field), as it is formally measured as the distance between the margins of the eyebrows, but there are individuals where the front is particularly broader or narrower. An additional complication is that within a species, the relative width of the front increases as crabs get larger (smaller individuals have relatively narrower fronts than larger individuals). Despite all of this, it is still a character that may sometimes be useful in distinguishing these species.
Minuca virens is extremely similar to Minuca longisignalis, if a bit smaller (perhaps ¾ to ½ the size). The front part of its carapace is green, which can be readily confused with the paler green/blue that is often seen on Minuca longisignalis.
Superficially, the green coloration could lead to confusion between Minuca virens and Leptuca speciosa or Leptuca spinicarpa, but the latter two species lack the more distinct two-tone color of the carapace common to the Minuca.
Color descriptions of Minuca vocator are sparse, but generally it is described as having a solid colored carapace (rather than the two toned ones found in many other Minuca), ranging from dull brown or gray to pale yellow or dull red or orange. The claw is generally described as a pale yellow or red, with white fingers. Minuca vocator has a particularly wide front, generally consisting of more than ⅓ of the total carapace breadth, with only Minuca burgersi similarly broad within this region. Other species in the region generally have a front breadth that is less than ⅓ of the total carapace breadth.
Minuca vocator has many small clumps of hairs scatterd around its carapace; these attract mud, giving it a patchy, muddy look that is different from other species. There are simliar hairs covering much of their walking legs, giving their legs a particularly muddy appearance as well.
For Minuca burgersi, the front of the carrapace is generally gray, with some purple marbling, with the back half more solid chocolate-brown or purple-brown. The H-shaped depression in the carapace is frequently a purplish red-brown. The large claw is generally a medium orange with paler fingers. Like Minuca vocator, the front of Minuca burgersi is on the wider side, generally consisting of more than ⅓ of the total carapace breadth.
Minuca rapax appears to be one of the most common, and most variable, of the western Atlantic species. It is quite similar to Minuca burgersi, although trends toward more grays and less purple/reds. One of the challenges with a site like iNaturalist is that Minuca rapax seems to be the default "suggested species" for a large part of the western Atlantic, so a lot of the photographs identified as this species are likely other similar species from the same genus, which makes identifying the actual degree and range of color variation in this species difficult.
The two-toned coloration of the carapace of Minuca rapax appears to primarily be a lighter gray in the front half and a darker gray in the back half, but there may be quite a bit more variation beyond this. The claw is generally a medium to pale yellow-orange, with the lower finger trending toward white. There is frequently some purple speckling at the front of the carapace behind the eyes. It is less usual for the H-shaped depresion to be distinctly colored.
Minuca marguerita is an obscure species only found in the southern half of the Gulf of Mexico. It is generally described as fairly similar in appearance to Minuca virens and Minuca rapax, with a front on the narrower side as opposed to the wider fronts of Minuca vocator or Minuca burgersi. The shape of the front of Minuca marguerita is described as “distinctly spatulate”, rather than the “angular and truncated” front of Minuca rapax. There is apparently a difference in the angle of the eyebrows relative to the carapace (those of Minuca marguerita are much more horizontal than those of Minuca rapax or Minuca virens), but this may be all but impossible to estimate without definitive examples or photos.
There are some differences in the tubercle (bump) pattern on the inside of the large claw, but these rarely serve as useful field characters as these features are almost never visible in standard photos.
I am not aware of any color photos of the species, and the only description of its color is “brown, dark green, or gray” which is not particularly informative or useful in distinguishing it from almost any species it might overlap with.
