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.
With some regularity I get asked about how to identify fiddler crabs and decided it is long past time for me to try to write up an explanation. As with many things, it is not necessarily easy, and takes some practice and experience. In many cases you are more-or-less out of luck when trying to identify to the species level. But here is the basic idea.
The first thing you should do is take advantage of knowing where the observation was found. Outside of a museum collection, there is little point in trying to distinguish between species that can never overlap in nature. In the absolute broadest sense, fiddler crabs can be split into three major geographic regions:
Within each region there are still a lot of geographic subdivisions you can take advantage of, for example, within the Americas the species on the Atlantic coast are completely distinct from the species on the Pacific coast, so knowing which coast you are on immediately reduces the possible species set (three of the four genera are found on both coasts, though, so you cannot reduce it to genus quite that easily).
The median number of overlapping fiddler crab species in most of the world is five, so for a lot of places you only need to distinguish between five possibilities, which it not so bad. Unfortunately, there are numerous places where it can be 10 or more, and at the very extreme there are about 30 all in one place! Still, geography helps.
Sometimes, yes! Some species are very distinct in appearance and can be fairly well identified even without knowing where they were from. But geography can help narrow it down a lot for those which are less certain.
If you are trying to narrow down species by a geographic area, there are two methods to doing so using the guides on this site. First, the various guides are organized by geographic region so you should be able to choose one which is appropriate for your area. Second, if you are uncertain which of the broader area guides you should be looking at, look at the longer list of locations on this site. Each location includes a link to the various field guides that should be applicable to that particular area.
You need to be careful about being too restrictive with geography; the last thing you want to do is eliminate a potential species because it is not known from a particular area as species ranges might be incomplete or change over time. But it is a good starting place to narrow down the species you need to think about.
This is one of features that most people are not aware of, but fiddler crabs roughly fall into two groups: those with eyestalks very close together (narrow front) and those with eyestalks farther apart (broad front). When appropriate, every field guide on this site starts by illustrating that difference with two local species.
Front breadth is useful because it immediately helps narrow down the possible genus and is usually fairly unambiguous if you can see where the eyestalks attach to the carapace. In the Americas, all narrow front species are in the genus Uca, while broad front species are in the genera Leptuca, Minuca, and Petruca.
In the IWP, narrow front species are in the genera Gelasimus, Tubuca, and Xeruca, while broad front species are in the genera Austruca, Cranuca, and Paraleptuca.
While all narrow front species are basically, obviously narrow, broad front species can vary from extremely broad to only-kinda broad. Minuca tend to have the broadest fronts, but they do overlap with Leptuca and front breadth can change as crabs get bigger (within a broad front species, the larger the crab, the (relatively) farther the eyestalks tend to be).
Another character than can sometimes be useful is the shape of the large claw. Obviously this does not work on female fiddler crabs (which have two small claws). Also, a lot of species, particularly in the Americas have what I would call a fairly generic claw shape, so this character is essentially useless in most of the United States where almost all species have similar shaped claws.
In other parts of the world, however, claws can vary quite a bit and can help narrow it down to genus or a subset of species. Some examples (illustrated with photos in appropriate guides) are:
As with any other trait, the usefulness of claw shape depends a lot on where you are. For example, on the east coast of Africa there are two species from different genera that are superficially very similar looking (Austruca occidentalis and Cranuca inversa) as they can sometimes be very similar in color and size. However, if you can get a good look at the tip of the upper finger on the large claw, they are easily distinguishable as Cranuca inversa has a unique forked shape on the tip.
Color is the final big player, with a number of caveats. First, in many species, individuals can change their color over the course of an hour or two, often going from darker to lighter as they get more active and it gets hotter. Second, some species are extremely variable in color, while others seem to be more fixed. Some of this might be geographic variation, but there can be a lot of variation even within a single place. Third, younger crabs may be different colors than older crabs, and males and females may have very different color patterns. Finally, for a lot of species we just do not have good descriptions of color that allow for diagnosis. This is the biggest barrier to identifying most species from photos.
So what colors should you focus on? For the most part, colors of the large claw (in males) and colors on the back of the carapace. In a few instances you might be able to use leg or eyestalk color to distinguish species, but this is less usual.
For the claws, you are focusing on things like the color of the fingers and are there obvious patches of different color (e.g., frequently a darker purple or red patch near the base of the lower finger).
For the carapace you are looking not just at the basic color, but more often than not, also patterns. Is it solid or striped or blotchy or a gradient? All of these can help distinguish species. Occasionally a species will have a distinct color pattern that makes it stand out or readily distinguishable from others. Other times it is more of a gestalt that you get a feel for with experience.
All fiddler crabs are small, with the largest only being about 5 cm in width, and most species being between 2-3 cm wide. There is a lot of variation among species, though, with some tending to be small and others tending to be large. Size is generally not determinable from photos, but if you are in the field and/or can tell the size, it is another feature that might allow you to narrow down possible species.
The smallest fiddler species are under 1 cm in width and rarely observed because they are too small for most people to even notice and/or photograph. In other cases, though, size can be useful. For example, on the east coast of the United States, Minuca minax is noticeably larger than the other two species (which are roughly the same size as each other) and once you get a feel for their sizes, one can readily distinguish Minuca minax in the field by size alone (other traits such as color differ as well). In California there are now two species which are very different in size.
Generally speaking if two species are very different in size, they likely differ in a lot of other obvious traits, but it is another way of sorting possibilities for places where a lot of different sized species may overlap.
The behavioral trait that fiddler crabs are most famous for is the waving of the large claw by males. What is less appreciated is every species essentially has a unique waving pattern, with differences including the directions of movements of the claw; the timing and speed of the wave, including pauses or jerky motions; whether other limbs are involved in the display, including both the small claw and the walking legs; and whether the male waves in place or dances around as part of the display.
Waving display is usually not useful for identification from photos (although there are one or two exceptions where unique poses can be captured), but can be an excellent field character. Unfortunately, it is extremely hard to describe waving patterns, so these are generally not included in most of the field guides at this time. This may change in the future if enough quality video becomes available for comparative purposes.
Other traits that can be used to distinguish species fall into two categories: those that are possible in the field and those that are impossible. Possible traits include things like features of carapace shape or groove patterns on the large claw. These are usually very subtle: sometimes you might be able to make these out well enough to distinguish similar species, but they often won't show up in photos unless you go out of your way to try to photograph them. Even then, some of these traits are so small as to be nearly impossible to see unless you capture the crab and deliberately photograph it from just the right angle (e.g., there is a physical feature that easily distinguishes males from one of the east coast USA species from the other two, but it is nearly impossible to see in a natural photograph because it requires you to clearly see the inside of the palm of the large claw).
Other traits that are used to separate species by taxonomists require dissection or microscopy to see, and thus have no practical value for field identification.