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 Brazil. There are ten species found in Brazil:
Leptuca uruguayensis is only found in the southern part of Brazil, starting around Rio de Janeiro. Minuca victoriana is found in the central eastern coast of Brazil, from about Ceará to São Paulo. The other species are more or less found across the entire coastline of Brazil south to about São Paulo or Santa Catarina, although there has been some suggestion that Leptuca cumulanta has a disjunct distribution, being found in both the northern and southern parts of the country, but missing from the center. Leptuca uruguayensis is the only fiddler crab species found in the very southern part of the country.
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 maracoani, is a narrow front species, while the others are broad front species.
Uca maracoani, is about as different in form as one can imagine and completely distinct from all of the other fiddler crabs in the Atlantic Ocean. Beyond being a narrow front species, it is very large, among the largest and heaviest in the world, dwarfing all of the other species in the region. It has an unusual bolt-cutter shaped large claw (the only other two species with a similar-shaped claw are found on the Pacific coast of the Americas). It is generally a deep, dark purplish red color, with the fingers of the large claw orange, although the colors may be masked as the species is frequently covered in heavy mud.
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 the narrow front species Uca maracoani, Leptuca thayeri has eyestalks that are much closer together than 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.
In general, species in the genus Minuca have eyes farther apart than those in the genus Leptuca, but even compared to Leptuca leptodactyla and Leptuca cumulanta, Leptuca thayeri has eyes that are particularly close together. 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 three species, the edges of the carapace between the eyes slope together, making the extrusion appear more trapezoidal. In Uca maracoani that narrow part of the front is 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 leptodactyla prefers sandier beaches and is generally distinguishable by color and the particularly long, thin fingers on its large claw. It has a carapace that ranges from dark gray to almost pure white, occasionally somewhat yellow. The limbs and claw are generally a mix of yellow and white.
It will sometimes build structures next to its burrows (see fourth photo above), much like its Pacific coast relatives.
Leptuca cumulanta is a small species, about the same size as Leptuca leptodactyla, but smaller than all of the others in the area with a carapce about 1 cm in width. Its carapace is frequently a shining turquoise blue or green, often with marbled dark and light sections. The large claw is generally a yellow-brown with white-yellow fingers, occasionally with a touch of pink at the tips or base.
Leptuca cumulanta does not have overly long, thin fingers of Leptuca leptodactyla and a narrower front than the various Minuca, making it moderately easy to identify in the area.
Leptuca uruguayensis is the southernmost fiddler crab in the world and the only American species that extends south of Brazil into Uruguay and northern Argentina. Although generally similar in form to Leptuca cumulanta, it has a distinct color scheme. It usually has a marbled aqua blue and black carapace (sometimes the blue will fade to white), red legs, and a red claw with whiter 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 (probably) Minuca victoriana have relatively narrower fronts, while Minuca panema, Minuca mordax, 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.
Although in some sense the least well known of any of the Brazilian Minuca, male Minuca victoriana may be among the easiest to identify if one has a good view of the front of the large claw. Minuca victoriana is unique in having a small groove at the base of the lower finger, frequently filled with small hairs.
Its carapace is generally marbled browns with paler specks; the large claw is primarily brown, with a white lower finger. The upper finger may be brown or white, but the upper edge is generally darker rather than paler.
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 panema and Minuca mordax 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.
In Minuca panema, the front half of the carapace is gray with white spots, while the back half is a cherry red-brown; the H-shaped depression on the carapace is generally bright red with thin lines. The large claw is yellow-orange with paler fingers. The legs range from red to a light brown or tan.
Note: Minuca burgersi was recently split into two species, with the second species named Minuca panema. At this time there is no evidence of geographic overlap between the species, but there is some uncertainty as to which of the two species is present in different parts of northern South America. I am currently operating under the assumption that the species in Venezuela and Colmbia is Minuca burgersi while that further east and south is Minuca panema, but will update the guides accordingly as more information is discovered, including possible overlap of their ranges.
Minuca rapax appears to be one of the most common, and most variable, of the western Atlantic species. It is quite similar to Minuca panema, 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 mordax tends to have a solid colored carapace, generally a shade of gray, sometimes more yellowish gray, sometimes more whitish gray. The large claw is generally yellow, with white fingers. Some individuals will have a bit of a blue tint, particularly on the walking legs.
The very broad front, solid color in shades of gray, and lack of patches of pile (hairs) on the carapace should allow this species to be distinguished from the others in this region.
