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 southern coast of Florida, including the counties of Palm Beach, Broward, Miami-Dade, and Monroe. Florida has a complex mix of 12 species as it sits at the transition between three separate faunal assemblages: the Atlantic coast of the USA (north of Florida), the Gulf of Mexico (west of Florida), and the Caribbean (south of Florida). There are six species within this subregion of Florida:
Leptuca leptodactyla and Minuca burgersi are relatively rare occurences within the state (although perhaps more common in the southern keys), while the others are more common through much of Florida.
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. All of the Floridian species are considered part of the American “broad front” group, but Leptuca thayeri has eyestalks that are much closer together than any other species within the group, so much so that it has occasionaly been considered to be a “narrow 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 pugilator, 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.
Another characteristic that can help distinguish the species is size. Leptuca thayeri is much larger than all of the other species in this region reaching about 37 mm (1.5 inches) wide (that is the width of the carapace at the front of the crab, from corner to corner), while the other species are generally about half that size or smaller.
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 pugilator is generally found on ocean or near-ocean shorelines up and down the coast, particularly in sandier areas. It is the most variable colored of this region's species, with a carapace (that is the “shell” covering its back) that can range from appearing almost a solid, very dark blue to almost pure white. More often than not it will be in between these extremes, often with a blotchy/marbled appearance. In some individuals, the outer edges of the carapace may be red or orange.
The key indicator, however, is that there is almost always a patch of purple coloration in the center of the upper half of the carapace (in rare cases it may be missing when an individual appears bleached to almost white). This patch tends to be shaped roughly like a V with the bottom of the letter pointing between the eyes, although the shape of the patch is less important than any evidence of purple. You can see it in every single one of the following photos, even in the ones that appear mostly blue, if you look carefully. This purple coloration is a key indicator; if it is present, then it is this species.
The following photos were chosen to demonstrate the range of variation in carapace colors.
In contrast, the other species in this area tend to have carapaces that are somewhat more uniform in color, without the wide variation found in Leptuca pugilator.
Like the carapace, both the large and small claws of Leptuca pugilator can vary quite a bit in color, from a dark purple/red to mostly white.
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. 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.
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.
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 speciosa is the most similar species in southern Florida to Leptuca leptodactyla, but with a claw with relatively shorter and thicker fingers (not particularly short or thick, just more so than the extreme that is found in Leptuca leptodactyla). It sits somewhat between the extreme delicateness of the claw of Leptuca leptodactyla and the slightly heavier looking claws of Minuca rapax and Minuca burgersi. It also appears somewhat like the paler forms of Leptuca pugilator but the dominant color patterns of these species tend to be quite different.
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.