Fiddler Crab Field Guide: East Coast of Florida, USA
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 east coast of Florida, essentially the Atlantic shoreline north of Palm Beach County. 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:
All six species are found in the middle of this area; Minuca pugnax is absent from the southern part and Leptuca speciosa is absent from the northern part. Broken down by county, the expected species are:
Nassau, Duval, and St. Johns: Leptuca pugilator, Leptuca thayeri, Minuca minax, Minuca pugnax, and Minuca rapax
Flagler, Volusia, and Brevard: Leptuca pugilator, Leptuca speciosa, Leptuca thayeri, Minuca minax, Minuca pugnax, and Minuca rapax
Indian River, St. Lucie, and Martin: Leptuca pugilator, Leptuca speciosa, Leptuca thayeri, Minuca minax, and Minuca rapax
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 most of the other species, although it is similar in size to to Minuca minax. These two larger species reach 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.
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.
Minuca pugnax is only found in the northern two-thirds of this region, from Nassau through Brevard counties. 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.
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 species, although somewhat similar to Minuca rapax. Although somewhat subtle, the front of Minuca minax (the part of the carapace that comes between the eyestalks) is relatively wider than the other species (much wider than Leptuca thayeri), placing the eyestalks farther apart and frequently consisting of more than ⅓ of the entire carapace breadth (the other species generally have a front less than ⅓ of the carapace breadth).
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.
Minuca rapax appears to be one of the most common, and most variable, of the western Atlantic species. 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. This speckling is very different than the purple patch that is common in Leptuca pugilator. Minuca rapax lacks the olive-tones of Minuca minax and the blues of Minuca pugnax.
Leptuca speciosa is only found in the southern two-thirds of this region from Flagler through Matin counties. 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.
Within this region, the species most similar to Leptuca speciosa is probably Leptuca pugilator, as these are the two species most likely to trend to whites, but otherwise they are not particularly similar in appearance.
