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 Georgia, USA. Until relatively recently, the fiddler crabs of this state would have been considered identical to those further north. However, a fourth species which has never been reported in the scientific literature from Georgia—although has sporadically been found in the state for a number of years—appears to be increasing in frequency (likely as sea tempeartures rise) to an extent that it seems worthwhile to have a separate guide for the state which includes this species. The four species found in Georgia are:
In general, these four species can readily be told apart by color, although some individuals can be ambiguous, particularly the female fiddler crabs (those with two small claws, rather than one large and one small).
Leptuca thayeri is a predominantly tropical fiddler crab which appears to have recently settled into Georgia, likely as water temperature has increased. It can be distinguished from the other three species by a variety of characteristics. The more subtle difference which may not always be readily visible is the distance between the eyestalks. All four 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 both Leptuca pugilator and Minuca pugnax, although similar in size to Minuca minax, with the two larger species 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 two smaller species only reach about 20 mm (¾ inch) wide.
Leptuca thayeri is generally a distinct color from the other three 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. This color scheme is completely different from the other three species, particularly Leptuca pugilator and Minuca pugnax.
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 three species. In addition, in the other three 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 the four 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. 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. Also, if the carapace is at all white it is almost certainly this species (ignoring the presence of dried mud which might make the carapace appear to be white).
The following photos were chosen to demonstrate the range of variation in carapace colors.
Like Leptuca pugilator, Minuca pugnax is generally found on ocean or near-ocean shorelines, but tends to be in muddier areas. Although they will often subdivide a shoreline based on the mud/sand component (i.e., you might find one species in the sandier areas and the other in the muddier areas), it is not uncommon for the two species to intermix and be found together. They are more-or-less the same size, with full-sized adults usually less than 20 mm (¾ inch) wide (that is the width of the carapace at the front of the crab, from corner to corner), so can only be readily distinguished by color (in the absence of capturing them).
Minuca pugnax generally has a carapace that is a dark brown (often with paler speckles), with a very noticeable cobalt blue stripe across the front just behind the eyes. The width of this stripe can vary, but is usually (although not always) present; in rare cases the entire carapace will be blue. The eyestalks will often have some blue tint to them as well. Females are more likely to be solid or nearly solid brown, without the obvious blue tint.
The large claw of male Minuca pugnax tends to be a yellowish-cream color, with the “hand” of the claw often a bit darker brown. Occasionally the claw will be a bit paler, more gray than yellow, but the yellow-cream color is more typical. The small claw tends to be the same pale yellow. While Leptuca pugilator will often have pale large claws, they lack the yellow tint that is common in Minuca pugnax (compare the colors of the claws of the two species in the previous photos).
Unlike the previous two species, Minuca minax tends to be found farther from the ocean shoreline, generally preferring slightly more brackish (fresh) water and is therefore more likely to be found farther up rivers, as well as farther from the water’s edge. In areas where all four species might coexist, if you consider the layout of the intertidal zone (the area between the water’s edges at high and low tides), Minuca minax and Leptuca thayeri will be found in the upper intertidal zone (closer to the high tide edge) while the other two species will be found in the lower intertidal zone (closer to the low tide edge). Although all four species can be found on the same shoreline, it is much less common to find Minuca minax intermingling with the others. Minuca minax and Leptuca thayeri are also notably larger than the other two species, almost double the size on average, with full sized adults reaching 37 mm (1.5 inches). Until you have experience with all four species, however, size can be hard to judge in a vacuum.
Minuca minax has a carapace that tends to be a sort of olive-gray or brown, generally lighter toward the front of the crab and darker toward the back. This color is distinct from that of the other three species.
Beyond the carapace color, many (although not all) individuals will have one or more distinct red markings along the joints of the legs and the claw. You can see these in many of the previous photos, particularly along the edge of the major dactyl (the movable finger on the large claw) and the outside edge of where the large claw attaches to the male crab’s arm. These markings can appear on other joints as well, including the small claw and the walking legs.
Note that the red markings are essentially highlights on the joints, they are not red limb segments. Some Leptuca pugilator can have reddish limbs, particularly on the large claw; which are very different from the red markings along the joints of Minuca minax.
The Leptuca pugilator in this photo has an unusual amount of red along the joints. This color appears to be part of the joint itself, which is quite different than the red markings in the earlier photos of Minuca minax where the red along the various joints are highlights on the fringe of the harder shell covering the limbs, rather than actually within the joints. https://www.inaturalist.org/observations/106018263
Not all Minuca minax will have these red marks along the joints; while the presence of these markings is essentially diagnostic, the absence cannot be used to automatically eliminate the species.
By and large, Minuca pugnax, Minuca minax, and Leptuca thayeri tend to be less variable than Leptuca pugilator, so within Georgia, individuals that are not good matches for any of the four are more likely to be Leptuca pugilator than the other three.
