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 southeastern Gulf of Mexico, including the states of Tabasco, Campeche, and Yucatan (Quintana Roo is covered in the guide for the Atlantic Coast of Central America). The southeastern Gulf of Mexico has a complex mix of species as it sits at the transition between the Gulf of Mexico to the north and west and the Caribbean to the east. Species found in this region include:
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, although Leptuca thayeri has one of the narrowest fronts of an otherwise 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 thayeri can be distinguished from the other species by a variety of characteristics. The more subtle difference which may not always be readily visible is the distance between the eyestalks. Other than Uca major whose eyes are extremely close together and pinched, Leptuca thayeri has eyestalks that are much closer together than any of the other broad front species. Compare the amount of carapace—as well as the shape of that part of the carapace—that is found between the eyestalks in the following photos, as well as those further down.
In Leptuca thayeri the bit of carapace that comes between the eyestalks has parallel edges, making the extrusion relatively rectangular in shape. In the other broad front species, the edges of the carapace between the eyes slope together, making the extrusion appear more trapezoidal. In Uca major the extrusion is not only very narrow, but appears pinched together.
Leptuca thayeri is generally a distinct color from the other species. Its body (both carapace and legs) tends to be relatively solid colored, either a dull brown or dark red. The large claw also tends to be almost entirely a solid color, usually dull orange or fading to a salmon pink, with the tips of the claw sometimes a paler white.
The final charactersistic to help distinguish Leptuca thayeri are some subtle differences in the shape of the large claw. Larger male Leptuca thayeri have fingers on the large claw that are relatively longer, compared to the size of the “hand”, than those of the other species. In addition, in the other species, the lower finger of the large claw (the pollex) is generally fairly straight, with a bit of upward curvature toward the end. In many male Leptuca thayeri (although not all), this finger will curve back downward toward the end. The extra-long fingers with the downward curve is quite distinct once you learn to recognize it, although smaller male Leptuca thayeri may not show this feature. Compare the shape of the lower finger of the large claw in the photos below to those of the other species (as well as the other Leptuca thayeri above).
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 speciosa tends to have a predominantly white, very smooth looking claw. In some cases the claw is entirely gleaming white. The body and limbs range a creamy pale yellow to medium green.
A subtlety of carapace shape can help identify this species as well. 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 speciosa is superficially similar to the paler forms of Leptuca panacea, but generally different enough to not be confused. In general form, the species that it is most similar to is Leptuca spinicarpa.
Leptuca spinicarpa is not paticularly well known, but is similar in shape and form to Leptuca speciosa, including the subtle carapace shape mentioned above and seen clearly in the first photo below.
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.
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 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.
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 the 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.
