Fiddler Crab Field Guide: Northern Gulf of Mexico (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 northern Gulf of Mexico, including Louisiana, Mississippi, Alabama and the northwest coast of Florida, from Citrus County through the panhandle. There are seven species within this region:
The southern part of the region in Florida contains just four species, with additional species gradually appearing as one moves north and west along the coastline. As you reach Mississippi and Louisiana, Leptuca pugilator and Leptuca speciosa likely vanish. Broken down by subarea, the expected species are:
Florida counties of Citrus, Levy, and Dixie: Leptuca pugilator, Leptuca speciosa, Minuca longisignalis, and Minuca minax
Florida counties of Taylor: Leptuca pugilator, Leptuca speciosa, Minuca longisignalis, Minuca minax, and Minuca virens
Florida counties of Jefferson and Wakulla: Leptuca panacea, Leptuca pugilator, Leptuca speciosa, Minuca longisignalis, Minuca minax, and Minuca virens
Mississippi, Alabama, and the Florida counties of Franklin, Gulf, Bay, Walton, Okaloosa, Santa Rosa, and Escambia: Leptuca panacea, Leptuca pugilator, Leptuca speciosa, Leptuca spinicarpa, Minuca longisignalis, Minuca minax, and Minuca virens
Leptuca pugilator is probably not found in Louisiana and may not be found in Mississippi, or at most, only the eastern part.
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.
Leptuca panacea looks very much like the paler versions of Leptuca pugilator; for a long time they were considered the same species. It's carapace usually ranges from pale gray to beige white, and the large claw has some pale red coloration with whiter fingers. There are two primary differences that should allow one to distinguish these species in the field.
First, as far as I can determine, Leptuca panacea lacks the purple patch on its carapace that is almost ubiquitous in Leptuca pugilator. I found one photo of Leptuca panacea where it had a very slight pink patch in roughly the same spot, otherwise this species seems to lack any semblance of this purple patch. Almost all Leptuca pugilator have some purple on their back, although it can be absent in the palest individuals (who are most similar looking to Leptuca panacea).
Second, 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 all of the above photos; in Leptuca pugilator this same depression (when it is distinct from the rest of the carapace) is medium gray (seen most clearly on the first four photos of Leptuca pugilator above).
Leptuca speciosa probably is not found in Louisiana, but is likely found in the rest of the region.
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 pugilator and 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.
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 often be used to distinguish different species.
Minuca minax is the largest species in this area, reaching about 37 mm (1.5 inches) in width, with only Minuca longisignalis approaching a similar size. The other species tend to be half this size or smaller. 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. 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 longisignalis is very similar to Minuca minax, including size, but instead of oliver-silver, its front part of its carapace generally ranges from aquamarine to pale green (individuals on the bluer end of the spectrum look very much like Minuca pugnax, except that species is not found in the Gulf of Mexico). Minuca longisignalis also lacks the reddish markings that are common around the joints of many Minuca minax.
Minuca virens is extremely similar to Minuca longisignalis, if a bit smaller (perhaps ¾ to ½ the size). The front part of its carapace is green, which can be readily confused with the paler green/blue that is often seen on Minuca longisignalis.
Superficially, the green coloration could lead to confusion between Minuca virens and Leptuca speciosa or Leptuca spinicarpa, but the latter two species lack the more distinct two-tone color of the carapace common to the Minuca.
