ארכיון יומן של אפריל 2022

אפריל 18, 2022

How to Identify Fiddler Crabs from Photos (or in the field)

With some regularity I get asked about how to identify fiddler crabs and decided maybe it's long past time for me to try to write up an explanation. As with many things, it's not necessarily easy, and takes some practice and experience. In many cases you're more-or-less out of luck when trying to get it to species level. But here is the basic idea.

Start with broad geography

The first thing you should do is take advantage of knowing where the observation was found. There's no point in trying to distinguish between species that can never overlap in nature. In the absolute broadest sense, fiddlers split into three major regions:

  • the Americas, including four genera: Uca, Minuca, Leptuca, Petruca
  • Western Africa and southern Europe: only a single species, Afruca tangeri, so you're already done!
  • Indo-West Pacific (IWP) (coastlines along the Indian Ocean, and the western and central Pacific Ocean), including six genera: Tubuca, Xeruca, Gelasimus, Cranuca, Paraleptuca, Austruca

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 doesn't sound too bad. Unfortunately, there are numerous places where it can be 10 or more, and at the very extreme there are about 29 all in one place! Still, geography helps.

Is it possible to identify a species from a photo with no knowledge of where it came from?

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're trying to narrow down species by a geographic area, I'd suggest using the location guide at fiddlercrab.info. It's by no means perfect, but it's the best guide to fiddler crab geography you'll find. 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's not known from a particular area as species ranges might be incomplete or change over time. But it's a good starting place to narrow down the species you need to think about.

Look at the distance between the eyestalks

This is one of features that most people aren't 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).

A good example of a narrow-front species

A good example of a broad-front species

Front-breadth is useful because it immediately helps narrow down the possible genus and it's 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 Minuca, Leptuca, and Petruca.

In the IWP, narrow-front species are in the genera Tubuca, Xeruca, and Gelasimus, while broad-front species are in the genera Cranuca, Paraleptuca, and Austruca.

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 species, the larger the crab, the (relatively) farther the eyestalks tend to be).

Front-breadth doesn't always help you (all fiddler crabs in the United States are broad-front) but it's an easy starting place.

Look at the shape of the large claw

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. Some examples are here, here, and here. Again, this character is essentially useless in most of the United States where pretty much all species have this same generic claw shape.

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:

  • Pruning-shears: I've never been sure how to describe this shape, but it is extremely distinct when you see it. Kind of like rounded pruning shears. Only a small number of species in the genus Uca have claws shaped like this.
  • the "classic" vocans: there is a group of closely related species that often have a fairly distinct shape to their large claw, a good example of which is here. The upper finger starts off slightly thicker, curves upwards along its lower edge, before going back down to more of a point. The lower finger has two distinct, large "teeth". This particular shape is really only seen in the genus Gelasimus. Not all species in the genus have claws that shape, but if a crab does have a claw with that shape, it's probably in that genus.
  • a group of species in the genus Tubuca mostly, although not entirely, restricted to Australia often have claws which appear to have particularly flat surfaces, often with curving fingers and with a very distinct extension on the last quarter or so of the lower finger, as shown here
  • some species have very distinctly shaped claws, e.g., with very short fingers relative to the rest of the claw, such as this one or a very triangular lower finger such as this one. These tend to be very tiny species that most people don't even see unless they're actively looking for them, but if you do see one the unique shape of the claw makes them a lot easier to identify.

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

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 don't 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're 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're looking not just at the basic color, but more often than not, also patterns. Is it solid, is it striped or blotchy, is the color 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's more of a gestalt that you get a feel for with experience.

Size

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 show up on iNaturalist 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 US, 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, can readily be distinguished 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. Two other examples of very different sized sympatric species are here and here (not on iNaturalist).

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.

Other Traits

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.

פורסם ב אפריל 18, 2022 09:56 אחה"צ על־ידי msr msr | תגובה 1 | הוספת תגובה

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