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 Pacific coast of Mexico, which contains in total nine species:
These nine species are not distributed evely across the coast. The coast can be divided into three rough subregions: the Pacific coast of Baja California (with two species present); the northern Gulf of California (with five species present, including two endemics); and the rest of the coastline, including Baja California Sur, the southern Gulf of California, and the entire coastline south to Guatemala (with seven species present). Specific distributions are summarized below:
Pacific coast of Baja California: Leptuca crenulata and Uca princeps
Northern Gulf of California: Leptuca coloradensis, Leptuca crenulata, Leptuca musica, Uca monilifera, and Uca princeps
Baja California Sur and the southern Gulf of California to the Guatemala border: Leptuca crenulata, Leptuca latimanus, Leptuca musica, Minuca brevifrons, Minuca ecuadoriensis, Minuca zacae, and Uca princeps
Note: This guide is not complete. While most are described, a few of the species are obscure enough that I do not currently have enough information to reliably describe their appearance or how they differ from some of the other overlapping species.
A number of features can be used to distinguish among these species, but a good place to start is to look at the distance between the base of the eyestalks. Fiddler crabs tend to split into two groups, those with the eyestalks very close together (“narrow front”) and those with the eyestalks separated a bit more (“broad front”). The two species in the genus Uca are narrow front species, while the other seven are broad front species.
Uca monilifera is easy to identify due to the generally powder-blue color of the large claw and carapace; no other species has this coloration. It is only found in the northern part of the Gulf of California.
Uca princeps is the most common narrow-front species on the Pacific coast of the Americas, has the longest range (southern California through Chile), and is the most variable in appearance (there is some suggestion it might be a mix of multiple, yet unidentified species).
The most typical colors in this species are pale yellow and orange, with some white. The carapace is typically yellow or fading-to-white, the legs are generally more orange-yellow, and the claw is often a bit brighter orange, with a white movable finger (dactyl).
Unfortunately, there is a lot of variation beyond this typical pattern. Carapace and legs may darken to a dark orange or even brown; sometimes the orange shifts to more red. In rare cases the carapace can be dark blue, almost black. Despite this variation, it should still be easy to identify within Mexico due to the narrow front and lack of powder blue colors unique to Uca monilifera.
As a general rule, species in the genus Minuca tend to have very broad fronts while those in the genus Leptuca tend to have narrower broad-fronts (medium fronts?) but there is overlap among species in the two genera so front breadth by itself is not an absolute indicator of the genus. In addition, the Minuca species tend to be larger than the Leptuca species, although there is overlap once again. As a rule of thumb, though, larger species with particularly broad fronts are probably Minuca while very small species with narrower (but not very narrow and pinched) fronts are probably Leptuca. Within Mexico it generally should be fairly easy to identify fiddler crabs to the correct genus and species, except perhaps for the two Minuca species with the broadest fronts where there is some uncertainty.
Leptuca latimanus is quite distinct, but could superficially be confused with Minuca zacae. Its carapace is predominantly white, with dark legs, and its large claw is a dark brownish-red with pale tips to the fingers. The claw is particularly heavy looking with a very broad palm and extremely short stubby fingers. When waving, its inner arm may show shades of blue. It is found along the Mexican coast as far north as the southern Gulf of California.
Leptuca musica should be identifiable due to a shocking mauve pink color that can cover the entire front of the crab, including limbs and the large claw. The carapace is generally a marbled gray and pink. It is found along the entire Pacific Mexican coast except for the Pacific side of Baja California.
A more subtle character is found in the small claw; the width of the opening between the fingers (the gape) is particularly large and wide in this species compared to most others. This is most clearly seen in the first of the above photos.
There is one additional character unique to only this species and one other (found south of Guatemala), but it is almost impossible to see in the field or in photos without capturing a crab and looking for it. On the lower edge of the palm of the large claw, these two species uniquely have a series of parallel striae/ridges; behaviorally they rub their legs against these ridges to make sounds. Unfortunately, these are extremely subtle and unlikely to be seen without actively handling a crab and knowing where to look for them. In the photo below, taken with a microscope from a captured animal, you can see see a series of small parallel lines (circled in red) running from the bottom edge of the inside of the large claw almost straight horizontally across the photo right into the joint where the rest of the limb attaches. These “stridulating ridges” are the unique character for these two species. Unfortunately, these are nearly impossible to see in a photo of a live animal in the field.
Leptuca coloradensis is only found in the Gulf of California, which limits the species it can be confused with. Although generally similar in shape and form to Leptuca crenulata and Leptuca musica, it is a different color than both of those species. Leptuca coloradensis is a dark reddish species with an orange claw with yellowish fingers. Leptuca crenulata is not predominantly red and Leptuca musica is more vividly pink. Also, the minor claw of Leptuca coloradensis does not show the extra broad gape that is found in Leptuca musica.
Leptuca crenulata is likely the most common species along the coast of Mexico, reaching northward into southern California in the USA. It is often described as a fairly dull and boring species. It's colors are predominantly tan and beige, with some gray or white, and occasionally a touch of green or pink.
Minuca zacae is the odd-ball species among the Minuca, being particularly small (~ 1 cm) and having a thick handed-claw with relatively shorter fingers. It has a dark carapace with gold and black marbling, a red-brown to orange-pink claw, with white fingers. Its claw shape is somewhat similar to that of Leptuca latimanus, but the two species are very different colors. Although the size and shape of the claw should be enough to identify this species, the challenge is that it can resemble juveniles of other species.
Minuca ecuadoriensis can be a confusing species and seems to come in a variety of different color forms, which may or may not be due to multiple species being mixed together (in addition to just mistaken IDs, there was a short scientific paper many years ago that suggested that Minuca ecuadoriensis was actually a complex of three different species; unfortunately, there has never been a follow-up to that paper to actually describe them so we are left with some uncertainty).
The one color morph of this species that is easy to identify is a broad-front species which is all or mostly a dark red color, with white fingers.
Other crabs identified as this species are often brown, lacking the distinct red color although usually with the whitish fingers. I am not positive if these IDs are correct or not.
The last species is another obscure one without established photos. Written descriptions based on only two to three individuals, described the (only observed in the wild) female as a bright coral red (orange red) color and the males as having a carapace that was dark brown/red with black marbling and brown-to-orange pink claws and limbs, with white fingers.
