19 de agosto de 2022

Fiddler Crab Guide: East Africa

Fiddler Crabs of East Africa, 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 East Africa, from South Africa in the south through southern Somalia in the north, including Madagascar, the Seychelles, Comoros, Mayotte, Mauritius, and Réunion. Six species are found within this region:
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”). Three of these species (Austruca occidentalis, Cranuca inversa, and Paraleptuca chlorophthalmus) are broad front species, while the other three (the two Gelasimus and Tubuca urvillei) are narrow front species. The eyestalks of Paraleptuca chlorophthalmus are fairly close together and barely qualify as “broad front”.

“Narrow front” / eyestalks are close together

“Broad front” / eyestalks are separated

Broad front species

Cranuca inversa / Inversed Fiddler Crab

Male Cranuca inversa should be the easiest of any of the species to identify so long as one can get a good look at the large claw. In this species, the tip of the dactyl (the upper finger) on the large claw has a forked shape which is completely unique to this species. Compare the tip of the upper finger of the claw in the “broad front” photo above to those in the four photos below. While the colors of the crabs are very similar, the shape of the tip of the dactyl is distinctly different.

https://www.inaturalist.org/observations/73491591 https://www.inaturalist.org/observations/18730511
https://www.inaturalist.org/observations/18258981 https://www.inaturalist.org/observations/49297008
Beyond the shape of the dactyl tip, the “hand” and arm of the large claw tend to be pale pink, while the fingers are usually white. The carapace is generally a mix of black and white, usually although not always with more black than white. Unfortunately, this color scheme is very similar to the next species, so without a good view of the claw, it can be very hard to distinguish between them in the field. To my knowledge, there is no reliable way to tell females of the two species apart in the field.

Austruca occidentalis / East African Fiddler Crab

Austruca occidentalis looks a lot like Cranuca inversa. The carapace is predominantly black and white, with more black than white and the claw is often pink. The key feature is that the tip of the dactyl in Austruca occidentalis comes to a more “normal” point rather than the unique fork found in Cranuca inversa. Austruca occidentalis appears to have more color variation, however. In some males the claw is more red or orange rather than pink and the dactyl may be less white and closer to the same color as the rest of the claw. The color pattern of the carapace is subtly different between the two species as well: Austruca occidentalis tends to have smaller, more spotted markings while Cranuca inversa tends to have elongated, more stripe-like markings; I wouldn’t feel comfortable making identification solely on this pattern, however.

https://www.inaturalist.org/observations/38981491 https://www.inaturalist.org/observations/9262381
https://www.inaturalist.org/observations/578653 https://www.inaturalist.org/observations/9431179

Paraleptuca chlorophthalmus / Green-eyed Fiddler Crab

Paraleptuca chlorophthalmus should be readily distinguishable from the other two broad front species because its carapace is predominantly blue with black markings, although sometimes it will be nearly solid one or the other color. Its legs are frequently bright red (although occasionally darkening to almost black), and the large arm and claw tend to be bright red, with only partial whitening on the fingers. The eyestalks are usually yellow-green.

Compared to the previous two species, the large claw tends to appear a bit thicker and heavier, particularly with regard to the shape of the fingers.
For a broad front species, Paraleptuca chlorophthalmus has a relatively narrow front, so much so that the species it is most likely to be confused with is one of the narrow front species, Gelasimus tetragonon, described next.
https://www.inaturalist.org/observations/105022204 https://www.inaturalist.org/observations/58819159
https://www.inaturalist.org/observations/58819157 https://www.inaturalist.org/observations/67095501
https://www.inaturalist.org/observations/18258684 https://www.inaturalist.org/observations/106091778

Narrow front species

Gelasimus tetragonon (Tetragonal Fiddler Crab)

Narrow front Gelasimus tetragonon

Broad front Paraleptuca chlorophthalmus

Gelasimus tetragonon is a wide-spread species that can be confused with Paraleptuca chlorophthalmus, despite the latter being a broad front species and the former being a narrow front species, as Paraleptuca chlorophthalmus is a relatively narrow fronted broad front species. Both species have predominantly blue and black carapace and tend to have bright red legs (which occasionally may be darker). Gelasimus tetragonon tends to have gray eyestalks, as opposed to the more yellow-green of Paraleptuca chlorophthalmus.
For males, the large claws of these species tend to be different. While the large claw of Paraleptuca chlorophthalmus tends to be dominated by red, that of Gelasimus tetragonon is more likely to be orange, with a noticeably darker red spot near the base of the pollex. The dactyl of Gelasimus tetragonon is more likely to be mostly white, while in Paraleptuca chlorophthalmus it tends to be more red. Gelasimus tetragonon frequently has brown spots on the top part of the hand of the claw; these spots are never present in Paraleptuca chlorophthalmus.

Gelasimus tetragonon has a lot of additional variability that does not overlap with other local species. In some places the carapace can lighten so that there is almost no blue, just a cream or pale orange with black markings. The pattern of the colors on the carapace can vary from stripes or blotches to tiny spots.
https://www.inaturalist.org/observations/64160983\ https://www.inaturalist.org/observations/66985053\
https://www.inaturalist.org/observations/67192392 https://www.inaturalist.org/observations/19029852
https://www.inaturalist.org/observations/40014105 https://www.inaturalist.org/observations/97981509
https://www.inaturalist.org/observations/67599172 https://www.inaturalist.org/observations/65173309

Tubuca urvillei (d’Urville’s Fiddler Crab)

Superficially, Tubuca urvillei may seem similar to the previous two species as it is also a narrow front species with a predominantly dark blue carapace with black markings and a large claw with orange-red and white colors. This superficial description fails to convey some rather striking differences that usually make it easy to pick out from the previous two.

While the carapace of Tubuca urvillei is blue and black, it tends to have less complex patterning and more solid colors relative to the previous two species; in some cases it may appear almost solid dark blue. The shade of blue of Tubuca urvillei tends to be darker, with the other two a bit brighter or aqua, although this is not universal. Tubuca urvillei tends to have black or blue legs, and even if pale, they are never red.
https://www.inaturalist.org/observations/11109457 https://www.inaturalist.org/observations/5073575
https://www.inaturalist.org/observations/5073575 https://www.inaturalist.org/observations/25370185
https://www.inaturalist.org/observations/35123891 https://www.inaturalist.org/observations/22895957
https://www.inaturalist.org/observations/111128228 https://www.inaturalist.org/observations/106091776
Color-wise the claw of Tubuca urvillei is similar to that of Gelasimus tetragonon with an orange or brown-orange hand, frequently with a darker orange-red patch at the base of the pollex, with the entire dactyl and the latter half of the pollex predominantly white. However, it is structurally somewhat different. Tubuca urvillei has a more robust appearing claw, with clear, large tubercles (bumps) covering most of the outer part of the hand. The fingers are noticeably thicker and tend to be less pointed than the previous species, often having a noticeable flat edge along the lower tip of the dactyl. It may be hard to see through binoculars in the field, but Tubuca urvillei also has a clear deep groove running along much of the length of the pollex. You can see this in the photos in the first column above (particularly, the first three).

Gelasimus hesperiae (Western Calling Fiddler Crab)

Gelasimus hesperiae is a narrow front species with a white or pale greenish-brown carapace and a distinctively shaped large claw. It never has any blue and should be readily distinguishable from all of the other species. The dactyl on the large claw is usually white or pink, while the rest of the claw usually ranges from orange to pale yellow. The dactyl is relatively thick and straight for about half its length before clearly curving to a thick point. The pollex will frequently have two very clear large teeth, one about midway along the length and one near the tip, although one or both can be absent. Like Tubuca urvillei, it has large bumps (tubercles) on the hand of the claw and will often have a noticeable groove along the base of the pollex.

https://www.inaturalist.org/observations/67192391 https://www.inaturalist.org/observations/47636654
https://www.inaturalist.org/observations/9890177 https://www.inaturalist.org/observations/56720520
Anotado en 19 de agosto de 2022 a las 09:37 PM por msr msr | 0 comentarios | Deja un comentario

17 de agosto de 2022

Fiddler Crab Guide: Southern California, USA

Fiddler Crabs of Southern California, 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 southern California in the United States. Prior to 2018, only a single species of fiddler crab was found in California. Now there are two known species:
These two species are extremely easy to tell apart and within California they should always be readily identifiable.
The two primary features you should focus on are size and how close the eyestalks are to each other. Uca princeps is substantially larger than Leptuca crenulata, as shown in the following photos:
https://www.inaturalist.org/observations/32684153 https://www.inaturalist.org/observations/13824586
The larger crab in both photos is Uca princeps, while the smaller surrounding crabs are Leptuca crenulata. Leptuca crenulata generally only grows between about 10-15 mm (approx ½ inch) in width (that is the width of the carapace at the front of the crab, from corner to corner), while Uca princeps can reach 35-50 mm (approx. 1 ½ to 2 inches).
Beyond size, another key difference is the relative distance between the eyestalks. Uca princeps is what is known as a “narrow front” fiddler crab, meaning the bases of the eyestalks are close together with only a tiny bit of carapace squeezed between them. In contrast, Leptuca crenulata is what is known as a “broad front” fiddler crab, meaning the bases of the eyestalks are farther apart and there is substantial carapace between them. This difference is very striking as shown in the examples below.

Uca princeps
“Narrow front” / eyestalks are close together

Leptuca crenulata
“Broad front” / eyestalks are separated

While there are other features that can be used to distinguish the two species (e.g., notice the differences in claw shape in the above photos), both size and eyestalk configuration are individually adequate to identify these two species within California in the USA.

Leptuca crenulata is the more common of the two species in California and ranges as far north as Santa Barbara. Uca princeps was only first discovered in southern California in 2018, and while it has been found there every year since, it is substantially rarer and as of now has only been found as far north as Huntington Beach.
Anotado en 17 de agosto de 2022 a las 03:08 AM por msr msr | 0 comentarios | Deja un comentario

Fiddler Crab Guide: West Africa and Europe

This is part 4 of a series of planned posts about identifying fiddler crabs. Previous entries include:
  1. How to Identify Fiddler Crabs from Photos (or in the field)
  2. Is It a Fiddler Crab?
  3. Fiddler Crabs of the Atlantic Coast of the USA

Fiddler Crabs of West Africa and Europe

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 West Africa and Europe. There is only one species, so identification is guaranteed.

Afruca tangeri is the only fiddler crab found in the eastern Atlantic Ocean, including the West coast of Africa, from Angola in the south to Morocco and the Strait of Gibraltar in the north, major islands such as Cape Verde and the Canary Islands, and southwestern Europe, including the southwestern coast of Spain and the southern and southwestern coasts of Portugal. It is not found past the Strait of Gibraltar into the Mediterranean.
Anotado en 17 de agosto de 2022 a las 02:53 AM por msr msr | 0 comentarios | Deja un comentario

16 de agosto de 2022

Fiddler Crab Guide: Atlantic Coast of the USA

This is part 3 of a series of planned posts about identifying fiddler crabs. Previous entries include:
  1. How to Identify Fiddler Crabs from Photos (or in the field)
  2. Is It a Fiddler Crab?

Fiddler Crabs of the Atlantic Coast of the 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 Atlantic coast of the United States north of Florida (from New Hampshire in the north through Georgia in the south). Three species of fiddler crab are found in this region:
In general, these three species can readily be told apart by color, although some individuals can be ambiguous, particularly the female fiddler crabs (those with two small claws, rather than one large and one small).

Leptuca pugilator / Atlantic Sand Fiddler Crab

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 the three 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. 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. Also, if the carapace is at all white it is almost certainly this species (ignoring the presence of dried mud which might make the carapace appear to be white).
The following photos were chosen to demonstrate the range of variation in carapace colors.
https://www.inaturalist.org/observations/112474878 https://www.inaturalist.org/observations/109172261
https://www.inaturalist.org/observations/111374875 https://www.inaturalist.org/observations/108237300
https://www.inaturalist.org/observations/84516743 https://www.inaturalist.org/observations/97400063
https://www.inaturalist.org/observations/57290165 https://www.inaturalist.org/observations/80726667
https://www.inaturalist.org/observations/79595789 https://www.inaturalist.org/observations/43940001
https://www.inaturalist.org/observations/81505656 https://www.inaturalist.org/observations/80677001
In contrast, the two Minuca species tend to have carapaces that are 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.

Minuca pugnax / Atlantic Mud Fiddler Crab

Like Leptuca pugilator, Minuca pugnax is generally found on ocean or near-ocean shorelines, but tends to be in muddier areas. Although they will often subdivide a shoreline based on the mud/sand component (i.e., you might find one species in the sandier areas and the other in the muddier areas), it is not uncommon for the two species to intermix and be found together. They are more-or-less the same size, with full-sized adults usually less than 20 mm (¾ inch) wide (that is the width of the carapace at the front of the crab, from corner to corner), so can only be readily distinguished by color (in the absence of capturing them).

Minuca pugnax generally has a carapace that is a dark brown (often with paler speckles), with a very noticeable cobalt blue stripe across the front just behind the eyes. The width of this stripe can vary, but is usually (although not always) present; in rare cases the entire carapace will be blue. The eyestalks will often have some blue tint to them as well. Females are more likely to be solid or nearly solid brown, without the obvious blue tint.
https://www.inaturalist.org/observations/112330333 https://www.inaturalist.org/observations/105988247
https://www.inaturalist.org/observations/110164204 https://www.inaturalist.org/observations/96972001
https://www.inaturalist.org/observations/109337025 https://www.inaturalist.org/observations/93912141
https://www.inaturalist.org/observations/98540443 https://www.inaturalist.org/observations/61031876
The large claw of male Minuca pugnax tends to be a yellowish-cream color, with the “hand” of the claw often a bit darker brown. Occasionally the claw will be a bit paler, more gray than yellow, but the yellow-cream color is more typical. The small claw tends to be the same pale yellow. While Leptuca pugilator will often have pale large claws, they lack the yellow tint that is common in Minuca pugnax (compare the colors of the claws of the two species in the previous photos).

Minuca minax / Red-jointed Fiddler Crab

Unlike the other two species, Minuca minax tends to be found farther from the ocean shoreline, generally preferring slightly more brackish (fresh) water and is therefore more likely to be found farther up rivers, as well as farther from the water’s edge. In areas where all three species might coexist, if you consider the layout of the intertidal zone (the area between the water’s edges at high and low tides), Minuca minax will be found in the upper intertidal zone (closer to the high tide edge) while the other two species will be found in the lower intertidal zone (closer to the low tide edge). Although all three species can be found on the same shoreline, it is much less common to find Minuca minax intermingling with the other two. Minuca minax is also notably larger than the other two species, almost double the size on average, with full sized adults reaching 37 mm (1.5 inches). Until you have experience with all three species, however, size can be hard to judge in a vacuum.

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. This color is distinct from that of the other two species.
https://www.inaturalist.org/observations/98207108 https://www.inaturalist.org/observations/78085489
https://www.inaturalist.org/observations/90903317 https://www.inaturalist.org/observations/80096373
https://www.inaturalist.org/observations/87317267 https://www.inaturalist.org/observations/78790507
https://www.inaturalist.org/observations/10907753 https://www.inaturalist.org/observations/82034976
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.
https://www.inaturalist.org/observations/106018263 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.
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.
By and large, Minuca pugnax and Minuca minax tend to be less variable than Leptuca pugilator, so within this Atlantic coast region individuals that are not good matches for any of the three are more likely to be Leptuca pugilator than the other two.
Anotado en 16 de agosto de 2022 a las 06:42 PM por msr msr | 1 comentario | Deja un comentario

21 de julio de 2022

Fiddler Crab Guide: Is it a fiddler crab?

This part 2 of a series of planned posts about identifying fiddler crabs. A prequel part 1 was previously posted, titled How to Identify Fiddler Crabs from Photos (or in the field). Subsequent entries will be about identifying fiddlers in specific geographic areas.

Is it a fiddler crab?

Generally speaking, it is usually easy to identify whether an unknown crab is a fiddler crab or not. That being said, people often do make mistakes for a variety of reasons. Do an internet search for “fiddler crab” and you’ll find lots of results labeled as such which, in fact, are not.

The key feature of fiddler crabs is that in male fiddler crabs, one of the claws is very large (the major claw) and the other is very small (the minor claw). Females have two small claws resembling those of the minor in the male.
https://www.inaturalist.org/observations/49829863 https://www.inaturalist.org/observations/61031876
https://www.inaturalist.org/observations/44867063 https://www.inaturalist.org/observations/97960408

Male (top) and female (bottom) of fiddlers from two different species (Afruca tangeri on left, Minuca pugnax on right). Note claw sizes.

Unfortunately, many assume that any claw asymmetry means that a crab is a fiddler crab, but many crustaceans have claws of different sizes. In male fiddler crabs it is simply much more extreme than in the others. For example, the crabs most closely related to fiddlers and most frequently confused with them by people online are the ghost crabs (genus Ocypode). Here is a typical ghost crab:

While the claws are clearly different in both size and shape, they are not nearly as different from each other as those in a male fiddler crab. The smaller claw in the ghost crab is still quite a bit closer in size to the larger claw than you find in a fiddler. Another difference to note are the eyes. They are quite a bit thicker and bigger on the ghost crab than the fiddler crabs above. While eye size and length varies among fiddler crabs, they are never as thick and large as on ghost crabs.

Another feature of fiddler crabs is that the carapace tends to be roughly trapezoidal when seen from above, with the longer edge behind the eyes and the shorter edge at their back.

While it varies among fiddler species (some are more extremely trapezoidal while others are almost square), the carapace is never rounded nor does it have scalloped edges. Ghost crabs have fairly square carapaces.

A final character (less useful in photos) is that fiddler crabs are generally quite small. The very largest species only reach about 5-6 cm (2-2.5 inches) wide (measured across the carapace, the large claw can be a longer), and most species are half of that or smaller (the smallest species are less than 1 cm in width). Most of the other crabs you tend to "see" wandering around beaches are in pools are actually larger than this.

Some other species commonly mistaken for fiddlers include:

Ucides cordatus (Atlantic Mangrove Ghost Crab)

The next closest relative to fiddlers after typical ghost crabs, the carapace is too round, eyestalks are too short and thick, and claws (not seen well in this photo) are too large. Species is also much larger than any fiddler.

Callinectes sapidus (Atlantic Blue Crab)

So many differences that I’m not sure how anyone would confuse them, but it does happen. For this particular comparison the carapace is completely the wrong shape and has scalloping along the front edge. No fiddlers have scalloping.

Sesarma reticulatum (Purple Marsh Crab)

Claws are too large, eyestalks are short and thick and on the outer corners of the carapace, rather than closer to the center.

One group of crabs that can appear very fiddler-like are those in the family Macrophthalimidae, which are found in the Indian Ocean and Western Pacific Oceans. These are more closely related to fiddlers than most of the others mentioned so far (other than the ghost crabs). Not all species in this family are readily confused with fiddlers, but some have similar looking eyestalks and two smallish claws that superficially can be confused for a female fiddler. A closer look shows that the claws are usually a bit too large, the carapace is a somewhat different shape, the body tends to be flatter and thinner than a fiddler, and the legs are usually a bit thicker. It is easy to see how this group causes confusion though as some species are superficially similar.

Finally, I’ll mention that it is not uncommon for people to mix up fiddler crabs with hermit crabs: those crabs famous for living in the shells of other animals (or cans, jars, and other similar litter). Fiddler crabs and hermit crabs look nothing alike, but some people get confused over the common names.
Anotado en 21 de julio de 2022 a las 07:04 PM por msr msr | 0 comentarios | Deja un comentario

18 de abril de 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 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.


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.

Anotado en 18 de abril de 2022 a las 09:56 PM por msr msr | 1 comentario | Deja un comentario

17 de noviembre de 2019

Splitting Fiddlers

Nearly 1000 observations later, I've more or less finished splitting all of the fiddlers labeled only to subfamily Gelasiminae into two tribes (or better). Also took a "break" of sorts in the middle to go through every observation from Uca/Afruca and confirm/update/etc.

Next step will be to compile key indicators for genus/species and do region by region checks for accuracy and specificity. Only 4300 or so to review/re-review. Yeah, think that's not going to happen soon.

Anotado en 17 de noviembre de 2019 a las 02:37 PM por msr msr | 0 comentarios | Deja un comentario