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 Texas. There are seven species within the state:
Minuca minax is only found in the nothern part of the state, perhaps reaching as far south as Corpus Christi, trading off with Leptuca subcylindrica and Minuca vocator which are only found from Corphus Christi and further south.
Leptuca panacea has a carapace which usually ranges from pale gray to beige white, and the large claw has some pale red coloration with whiter fingers. 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 the photos.
Leptuca spinicarpa is not paticularly well known. 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. A subtlety of carapace shape can help identify this species. 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 subcylindrica is a very unusual species, able to tolerate much drier conditions than any other fiddler crab and generally found much farther from the shoreline, in fresher bodies of water.
It tends to be very dull, pale colors, mostly beiges and tans and grays. Its most striking characteristic is the shape of the carapace. Leptuca subcylindrica has a substantially more rounded carapace than other species. Specifically, the curvature is more dramatic when moving from front to back, particularly in the back half of the carapace (like other fiddler crabs, the basic shape when seen from above is still essentially trapezoidal). Most fiddler crabs have flatter carapaces with less curve to them.
Its somewhat difficult to find good photos of this because the curvature is predominantly noticable when viewed from the side, which is an angle that most people tend not to deliberately photograph.
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.
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 3/4 to 1/2 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.
Minuca vocator is primarily a tropcal species found on the coasts of Central and South America, which extends as far north as southern Texas. It is generally described as similar in appearance to many of the other Minuca, so precise identification may be difficult, although in theory it should not be all that similar to the others found in Texas.
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 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.
