Fiddler Crab Guide: Pacific Coast of the Americas

Fiddler Crabs of the Pacific Coast of the Americas (partial)

The previous guides were all designed to be "complete", thus the decisions on what areas were chosen. This guide will not be complete as I don't currently have enough info on every species in the region to definitively describe how to distinguish each of them. I'm writing this now, however, because some guidelines will be more useful than none and it could be a long time before we have enough knowledge to create a complete guide.
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 the Americas, from Mexico through Chile (a link to a simpler guide for southern California of the USA was previously published). Thirty-six species are found along this coast! We can subdivide the area into a number of regions:
  1. the Pacific coast of Mexico (8 species), which can be further subdivided

    • Pacific Baja and southern California, USA (2 species, see guide mentioned above)
    • Northern Gulf of California (4 species)
    • Southern Gulf of California through SW Mexico (6 species)
  2. the Pacific coast of Central America from Guatemala through northern Peru (31 species), which can be further subdivided

    • northern coastline: Guatemala through Nicaragua (24 species)
    • central coastline: Costa Rica and Panama (30 species)
    • southern coastline: Colombia through northern Peru (24 species)
  3. central Peru through northern Chile (3 species)
  4. Galapagos Islands (2 species)
With more data we would plan on providing separate guides for each region, and possibly each subregion, but at this incomplete stage we'll stick with all as a single block, particularly considering the amount of species overlap among many of these areas.
The full list of species found here (not all of which will be in included in the guide at this time) is:
  1. Leptuca batuenta (Beating Fiddler Crab)
  2. Leptuca beebei (Beebe's Fiddler Crab)
  3. Leptuca coloradensis (Painted Fiddler Crab)
  4. Leptuca crenulata (Mexican Fiddler Crab)
  5. Leptuca deichmanni (Deichmann's Fiddler Crab)
  6. Leptuca dorotheae (Dorothy's Fiddler Crab)
  7. Leptuca festae (Festa's Fiddler Crab)
  8. Leptuca helleri (Heller's Fiddler Crab)
  9. Leptuca inaequalis (Uneven Fiddler Crab)
  10. Leptuca latimanus (Lateral-handed Fiddler Crab)
  11. Leptuca limicola (Pacific Mud Fiddler Crab)
  12. Leptuca musica (Musical Fiddler Crab)
  13. Leptuca oerstedi (Aqua Fiddler Crab)
  14. Leptuca pygmaea (Pygmy Fiddler Crab)
  15. Leptuca saltitanta (Energetic Fiddler Crab)
  16. Leptuca stenodactylus (Narrow-fingered Fiddler Crab)
  17. Leptuca tallanica (Peruvian Fiddler Crab)
  18. Leptuca tenuipedis (Slender-legged Fiddler Crab)
  19. Leptuca terpsichores (Dancing Fiddler Crab)
  20. Leptuca tomentosa (Matted Fiddler Crab)
  21. Leptuca umbratila (Pacific Mangrove Fiddler Crab)
  22. Minuca argillicola (Clay Fiddler Crab)
  23. Minuca brevifrons (Narrow-fronted Fiddler Crab)
  24. Minuca ecuadoriensis (Pacific Hairback Fiddler Crab)
  25. Minuca galapagensis (Galápagos Fiddler Crab)
  26. Minuca herradurensis (La Herradura Fiddler Crab)
  27. Minuca osa (Osa Fiddler Crab)
  28. Minuca zacae (Lesser Mexican Fiddler Crab)
  29. Petruca panamensis (Rock Fiddler Crab)
  30. Uca heteropleura (American Red Fiddler Crab)
  31. Uca insignis (Distinguished Fiddler Crab)
  32. Uca intermedia (Intermediate Fiddler Crab)
  33. Uca monilifera (Necklaced Fiddler Crab)
  34. Uca ornata (Ornate Fiddler Crab)
  35. Uca princeps (Large Mexican Fiddler Crab)
  36. Uca stylifera (Styled Fiddler Crab)
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”). In the Americas, all of the narrow-front species are in the genus Uca, while the broad front species are in three other genera. The narrow front species also tend to be much larger than the broad front species. In most cases, whether a species is narrow or broad fronted should be unambiguous.


https://www.inaturalist.org/observations/65186639
“Narrow front” / eyestalks are close together


https://www.inaturalist.org/observations/36689104
“Broad front” / eyestalks are separated

Narrow front species

Of the seven narrow front species along the coast, all in the genus Uca, at this time three should be easy to identify, two are moderately easy, and two are very difficult. We'll go from easy to hard.


Uca monilifera / Necklaced Fiddler Crab

This species 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 also only found in the northern part of the Gulf of California, reducing the number of species it could be confused with (the only narrow-front species it overlaps with is Uca princeps).
https://www.inaturalist.org/observations/13589748 https://www.inaturalist.org/observations/13589747


Uca stylifera / Styled Fiddler Crab


Another easy species to identify, the primary distinguishing feature in male crabs is a long style coming out of the eye on the same side as the large claw; the style is approximately the same length as the rest of the eyestalk. While some other species will occasionally have styles, they are substantially shorter than those found in Uca stylifera. In addition, the colors of Uca stylifera are distinct: males have a white carapace (in rare instances more yellow than white), yellow eyes, orange-red-to-purple legs, and a large claw with an orange fixed-finger (pollex) and white movable finger (dactyl). Some Uca princeps may be similarly colored, but the two species should be easy to tell apart. Female Uca stylifera are a bit more tricky as they lack the style and tend to be a dull muddy-brown color (you can see a female partially hidden by the male in the first photo below). They're most easily identified by association with the males.

This species ranges from approximately El Salvador through northern Peru.
https://www.inaturalist.org/observations/10074 https://www.inaturalist.org/observations/140789885


Uca intermedia / Intermediate Fiddler Crab


This species is only known from the coasts of Panama and Colombia and appears to be very rare; there are only two observations on iNaturalist (one is my own from 1997). If actually found, however, it should be easy to identify as it is uniquely an essentially solid black fiddler crab with a red large claw.

https://www.inaturalist.org/observations/10075

https://www.inaturalist.org/observations/183936765


Uca heteropleura / American Red Fiddler Crab



Uca heteropleura should be easy to tell apart from the other species, but in practice it can be confused with some of the color variants of Uca princeps. Uca heteropleura has a predominantly red-to-black carapace and legs (sometimes lightening to white or slightly more purple), with a red lower part of the claw and white movable finger. Its eyestalks are generally black. The large claw tends to have relatively short and stout fingers and is usually noticeably bumpy on the outside of the hand.
https://www.inaturalist.org/observations/108681451 https://www.inaturalist.org/observations/108681448
https://www.inaturalist.org/observations/10073 https://www.inaturalist.org/observations/148447479
While most Uca princeps are very different colored and have claws with relatively longer fingers, some are similarly enough to make them more difficult to distinguish. The primary features to keep an eye on when encountering a red narrow-front crab that might be either species are eyestalk color (I think Uca princeps has yellow eyestalks while Uca heteropleura has black) and the length and shape of the movable finger (the dactyl). In Uca heteropleura the dactyl tends to be shorter relative to the length of the claw, much more curved, and if there is a small tooth/bump on it, the tooth tends to be closer to the tip of the claw. In Uca princeps the dactyl is relatively longer and there is usually small tooth/bump right around the center of the dactyl. The outside of the large claw in Uca heteropleura is almost always noticably rough and bumpy, while in Uca princeps it may be a lot smoother.
Behaviorally, the waving display of Uca heteropleura is extremely different from that of Uca princeps and can be used to identify them in the field, and even sometimes in a still photo. Male Uca heteropleura wave by holding the large claw in front of them then raising their entire body vertically onto the tips of their legs, with the claw held up above them. In contrast, Uca princeps prances back and forth with the the large claw held laterally out to the side. The following photos are a good illustration of the vertical hold position of Uca heteropleura (first row) vs. the more typical side waving of Uca princeps (second row).
Vertical waving of Uca heteropleura
https://www.inaturalist.org/observations/21387973 https://www.inaturalist.org/observations/10073
Lateral waving of Uca princeps
https://www.inaturalist.org/observations/177912890 https://www.inaturalist.org/observations/177267896

This species ranges from approximately El Salvador through northern Peru.


Uca princeps / Large Mexican Fiddler Crab



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).
https://www.inaturalist.org/observations/177912899 https://www.inaturalist.org/observations/120221155
https://www.inaturalist.org/observations/108538460 https://www.inaturalist.org/observations/77227400

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, which is when it can become confused with Uca heteropleura. In rare cases the carapace can be dark blue, almost black.
https://www.inaturalist.org/observations/176868524 https://www.inaturalist.org/observations/152290026
https://www.inaturalist.org/observations/144049877 https://www.inaturalist.org/observations/10067
As mentioned above, I believe the eyestalks tend to be pale, much more yellow, as opposed to the dark black eyestalks of Uca heteropleura. The other species that it can be superficially confused with is Uca stylifera as the colors can sometimes converge, but the presence of the long style in males on the eye on the same side as the large claw is diagnostic.

Another character to look for is that the large claw of Uca princeps generally has fingers which are longer than the palm, while in the other species the fingers tend to be the same length or shorter than the palm.


Uca ornata / Ornate Fiddler Crab & Uca insignis / Distinguished Fiddler Crab


It is very easy to identify a male fiddler from one of these two species, among the largest of all fiddler crabs, because the shape of the large claw is very distinct (only matched worldwide by the Atlantic coast species Uca maracoani). The claw almost resembles pruning shears, with large flattened fingers which meet at a very straight inner edge, and with the upper edge of the movable finger distinctly curved.

Unfortunately it is not clear how to tell them apart from each other. Most of the formal diagnostic characters are subtle and require examination of specimens in a lab setting. There are probably color differences, but written descriptions are inadequate and we lack definitive photos of both species to confidently identify field characters at this time. All of the species shown below are listed on iNaturalist as Uca ornata but without more color information Uca insignis looks more-or-less the same with respect to size and shape.
https://www.inaturalist.org/observations/10065 https://www.inaturalist.org/observations/67921229
https://www.inaturalist.org/observations/167149248 https://www.inaturalist.org/observations/175547390

Broad front species


Many of the twenty-nine broad front species along the coast are difficult to distinguish, but I'll describe key characters for those where we have good data. One of the species is in its own genus, Petruca which will be described first. The rest are split between two other genera. 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.


Petruca panamensis / Rock Fiddler Crab



Petruca panamensis is distinguishable primarily by its environment, rather than a physical character: it is the only species of fiddler crab in the world that lives predominantly in and among large rocks, rather than the more typical mud or sand substrate that all other fiddlers live upon. If you find a species waving on top of a large rock, it's probably this species, which is found from approximately El Salvador through northern Peru.

https://www.inaturalist.org/observations/10058

The species is not particularly visually distinct, although it tends to have a flatter carapace than other species, and the large claw seems particularly smooth and relatively featureless.


Leptuca batuenta / Beating Fiddler Crab


Found from approximately El Salvador to northern Peru, the smallest of all fiddler crab species, often only about 5-6mm in breadth, this species is easy to overlook due to its size. If seen, however, it is surprisingly easy to identify due to the unique shape of the large claw. Specifically, the lower/fixed finger (pollex) has a distinct tooth pattern where the pollex curves upward to this tooth about 3/4 of the way along the pollex, then curves back down with a concave edge to a point. The claw is generally white, while the limbs tend to be brown-red.

https://www.inaturalist.org/observations/10076


Leptuca saltitanta / Energetic Fiddler Crab


Another very small fiddler crab (~6mm), found from approximately El Salvador to Colombia, this species is also easily distinguishable from other due to a combination of color and claw shape. It is one of the few species that is essentially solid white (in fact, it's claw and limbs are translucent), with yellow eyestalks. The lower finger (pollex) of the large claw is strikingly thick and triangular, while the upper finger (dactyl) is very thin. While a few other species have a claw of similar shape, they are not solid white.

https://www.inaturalist.org/observations/10068

Behaviorally the species raises the claw up into the air then sharply brings it down to very rapidly drum on the mud surface (it looks like a rapid vibration).


Leptuca inaequalis / Uneven Fiddler Crab


Another very small fiddler crab (~7mm), found from approximately El Salvador to northern Peru, this species is somewhat similar in shape to Leptuca saltitanta, but entirely different colors. The large claw is gray-brown with a dark red/brown patch along the lower edge by the base of the pollex, the fingers are generally white, and there is frequently an orange tint running along the upper edge of the movable finger (dactyl). The gape of the claw is almost always filled with brown mud, because there are small hairs on the upper edge of the first half of the pollex that the mud clings to. The shape of the polllex is not as strikingly triangular as that of Leptuca saltitanta, but does have a very heavy palm with short thick fingers.

https://www.inaturalist.org/observations/10072


Leptuca latimanus / Lateral-handed Fiddler Crab



Leptuca latimanus sort of looks like a cross between the previous two species, Leptuca saltitanta and Leptuca inaequalis, although it's a bit larger than either being 1 cm or larger. It's 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 very short stubby fingers. The lower finger (pollex) is not triangular as seen in the previous two species. When waving, it's inner arm may show shades of blue. Leptuca latimanus has a broad range, found from Mexico through Ecuador.
https://www.inaturalist.org/observations/168093656 https://www.inaturalist.org/observations/120735011
https://www.inaturalist.org/observations/141419032 https://www.inaturalist.org/observations/141419032


Leptuca tenuipedis / Slender-legged Fiddler Crab



Leptuca tenuipedis is another very small (5-7 mm) fiddler crab which is shaped somewhat like Leptuca saltitanta and Leptuca inaequalis, but with a different color pattern and a somewhat less triangular pollex. It's body is dull brown/gray, while the claw is orange/red, including the top finger (dactyl) while the lower finger (pollex) is white. The colors of the fingers are reversed from most fiddlers, where--if only a single finger is white--it is usually the upper and not the lower. The only photo on iNaturalist is not particularly diagnostic.

https://www.inaturalist.org/observations/10070


Leptuca oerstedi / Aqua Fiddler Crab


Another small fiddler crab (~1 cm), found from approximately El Salvador to Panama, this species is distinct due to the unique aqua blue coloration that is frequently seen across the entire front of the crab, although not always on the carapace or from the back. The claw also has a distinctive character with a shallow notch at the base of the lower finger (pollex) followed by an almost straight edge to the end of the finger (unfortunately, neither of the photos on iNaturalist shows the claw shape clearly).
https://www.inaturalist.org/observations/10071 https://www.inaturalist.org/observations/10071

"Typical" Leptuca


The next set of species are all very similar in size, shape, and structure, so much so that if your removed all color and behavior, they would be very difficult to distinguish in the field. Thankfully, there are distinct color differences for many of them. They are all about 1 cm in width and have a "typical" large claw which is slender with long fingers.




Leptuca terpsichores / Dancing Fiddler Crab



Leptuca terpsichores is another fiddler crab that is usually distinct and easy to identify. When males first come out of their burrows they are generally an innocuous brown-red color, but within 15 minutes or so their color shifts to almost entirely white, except for some pink/purple on the base of the large claw.
https://www.inaturalist.org/observations/36689104 https://www.inaturalist.org/observations/21641124

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.
https://www.inaturalist.org/observations/97336215 https://www.inaturalist.org/observations/39803361
This species lives on sandier beaches, from El Salvador to northern Peru, and is famous for building hoods next to their burrow openings.
https://www.inaturalist.org/observations/10066 https://www.inaturalist.org/observations/101431145

There is one additional character unique to only this species and the next one, Leptuca musica, 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 very 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 a large series of tubercles (bumps) that run between the bottom and top edges of the claw like a mountain chain, going straight through the middle of the palm for about 3/4 of the way, before curving to the left for the last 1/4. This series of bumps is NOT what I am referring to. About half way between the bottom of this bumpy ridge and the back of the claw, you can see a much more subtle series of small parallel lines 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.

https://www.inaturalist.org/observations/141419035


Leptuca musica / Musical Fiddler Crab


Tentatively, Leptuca musica has the same shape and structure as Leptuca terpsichores, including in particular the large gape in the small claw and the all-but-impossible-to-see ridges on the lower edge of the large claw, but instead of being white the species is red and pink. It's range is restricted to Mexico and northern Guatemala.
https://www.inaturalist.org/observations/8606515 https://www.inaturalist.org/observations/7926356

One confounding factor is that there is at least one other species in part of its range, Leptuca coloradensis, whose colors are unknown to me at this time and it is possible the colors of the two species might overlap enough to be confused.


Leptuca stenodactylus / Narrow-fingered Fiddler Crab



Leptuca stenodactylus ranges from El Salvador to Chile, and like Leptuca terpsichores, Leptuca stenodactylus tends to be found on sandier substrates. The two species frequently overlap and intermix. Although about the same size and shape, their colors are quite different, with Leptuca stenodactylus one of the more colorful species along the coast, having a blue carapace, bright red legs, and a claw with pink or white.
https://www.inaturalist.org/observations/6148920 https://www.inaturalist.org/observations/20767845
https://www.inaturalist.org/observations/81601111 https://www.inaturalist.org/observations/153522170

The gape of the small claw of Leptuca stenodactylus is wider than in most other species, although less so than that found in Leptuca stenodactylus.


Leptuca beebei / Beebe's Fiddler Crab



Leptuca beebei is one of the more common species along the coast, ranging from El Salvador to northern Peru, but is less colorful and striking than those already described. It's carapace is generally a mix of dull green or blue or brown, the large claw is often pale gray or white but with a dark purple/burgundy patch at the base of the fixed finger, and its eyestalks tend to be yellow. It is more of a mud/sand generalist than many of the other species described so far and will often overlap in space with a lot of them as it seems to be less picky about the substrate it lives on.
https://www.inaturalist.org/observations/111703393 https://www.inaturalist.org/observations/92607057
Similar to Leptuca terpsichores, Leptuca beebei frequently builds structures next to its burrows, but instead of large overhanging hoods it builds smaller pillars.

https://www.inaturalist.org/observations/10064
Behaviorally, Leptuca beebei has a very classic wave where it moves the claw out to its side then brings it up and back to its front in a come-hither sort of gesture. This is particularly useful when distinguishing it from the next species, Leptuca deichmanni.


Leptuca deichmanni / Deichmann's Fiddler Crab

Superficially, Leptuca deichmanni looks very similar to Leptuca beebei, although it's claw tends to be more solid white and lacks the purple/burgundy patch. It prefers sandier substrates (much like Leptuca terpsichores and Leptuca stenodactylus) but can intermix with Leptuca beebei in space. Behaviorally, however, the wave of Leptuca deichmanni is completely different from that of Leptuca beebei. While Leptuca beebei has a circular come-hither sort of wave, that of Leptuca deichmanni has more of a vertical up-and-down motion, with a distinct pause in the up position. Seen side-by-side they are noticeably different.

https://www.inaturalist.org/observations/10059
Unfortunately, there are no good photos of this species on iNaturalist.


Leptuca dorotheae / Dorothy's Fiddler Crab

There are no observations of this species on iNaturalist and I have never seen a living individual from this species, in person or photo. It is described as extremely similar to Leptuca beebei. It's carapace is supposed to generally be olive-green, speckled with yellow, with a brownish-red to wine-red major claw, except for a generally white upper finger (Note; I have often found these written descriptions of species which I know fairly well to not be particularly representative). It is found from Costa Rica to northern Peru.


Leptuca festae / Festa's Fiddler Crab

Leptuca festae can be a bit larger than the other species combined under "typical" Leptuca, ranging up to 1.5 cm. It is generally a dull brown or brown-gray color, with yellow eyestalks, and only slight whitening of the claw. For it's size, it tends to have substantially longer fingers on the major claw than the other species. It is found from El Salvador to Ecuador.


https://www.inaturalist.org/observations/10061


Leptuca coloradensis / Painted Fiddler Crab


Leptuca coloradensis is only found in the Gulf of California, which limits to a certain extent the species it can be confused with. All or most of the observations on iNaturalist have a bit of a "must be this by default" feel to them, i.e., it's coloration doesn't really match that of two of the other similarly shaped species in the region, Leptuca crenulata or Leptuca musica. Assuming these identifications are correct, Leptuca coloradensis is a 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.

https://www.inaturalist.org/observations/13589761


Leptuca crenulata / Mexican Fiddler Crab


Leptuca crenulata is a common species along the coast of Mexico, reaching northward into southern California in the USA. In some sense, iNaturalist seems to view it as the default species for fiddlers along the Mexican coast. 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. Of the >1100 observations of this species on iNaturalist, >900 are from southern California, which says a lot more about user usage on iNaturalist than the actual distribution of this species.
https://www.inaturalist.org/observations/177769749 https://www.inaturalist.org/observations/177267886
https://www.inaturalist.org/observations/171342399 https://www.inaturalist.org/observations/117969451


Leptuca umbratila / Pacific Mangrove Fiddler Crab

There are no photos of this species on iNaturalist, but it should be distinguishable by a particularly narrow front for an otherwise broad-fronted fiddler. Basically it has a front that is only moderately broader than the narrow-fronted Uca described earlier, but without the pinched look between the eyes that those species have. A similar species on the Atlantic coast of the Americas is Leptuca thayeri, although I do not know if the colors are remotely similar.


Leptuca helleri / Heller's Fiddler Crab

As one of only two species found in the Galapagos and the only species restricted to those islands (ironically, Minuca galapagensis is also found on the mainland), one would think this species would be easy to identify. However, there are no color descriptions of this species, which makes most identification based on "not galapagensis". The basic physical difference between the two species is that Leptuca helleri should have a narrower-front than Minuca galapagensis, but this is often difficult to use in a vacuum. Leptucah helleri is also a lot smaller, around 1 cm in width while Minuca galapagensis may be almost double that size, but while useful in the field, size is frequently hard to determine in a photo.
Both species can be white, although Minuca galapagensis will frequently have an orange claw, while that of Leptuca helleri would be more white. Another frequent color in Leptuca helleri is a pale green. Darker individuals, particularly those that lack orange, are probably Leptuca helleri.
https://www.inaturalist.org/observations/59721274 https://www.inaturalist.org/observations/2847360
https://www.inaturalist.org/observations/166131733 https://www.inaturalist.org/observations/4212927
Supposedly the two species never intermix on the islands; at any given location you will find one or the other, but apparently not both. In fact, there is some suggestion they rarely are found even on the same island. If this is true, based on iNaturalist observations one might expect to find the following pattern among the islands:
  • Isla Santa Cruz: both species present
  • Isla Isabella: most photos on this island appear to be M. galapagensis but a few are almost certainly L. helleri
  • Isla de San Cristobal: predominantly L. helleri, but one observation might be M. galapagensis
  • Isla Genovesa: L. helleri
  • Isla Floreana: M. galapagensis
  • Isla Santiago: M. galapagensis

Other Leptuca

There are no known observations of any of the following species on iNaturalist and I do not know enough about them to feel comfortable attempting to describe how to identify them at this time:
  • Leptuca limicola / Pacific Mud Fiddler Crab
  • Leptuca pygmaea / Pygmy Fiddler Crab
  • Leptuca tallanica / Peruvian Fiddler Crab
  • Leptuca tomentosa / Matted Fiddler Crab

Minuca

Unlike many of the Leptuca which come in a variety of shapes, particularly with respect to the large claw, most of the Minuca are shaped more-or-less similarly, leaving only color or behavior as possible field characters. This makes most of them generally harder to distinguish from each other.


Minuca zacae / Lesser Mexican Fiddler Crab


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 is described as having a dark carapace with gold and black marbling, a red-brown to orange-pink claw, with white fingers. Although the size and shape of the claw should be enough to identify it, the challenge is that it can resemble juveniles of other species. It is found from Mexico to Costa Rica.
https://www.inaturalist.org/observations/135310455 https://www.inaturalist.org/observations/135310455
https://www.inaturalist.org/observations/101428524 https://www.inaturalist.org/observations/135310453


Minuca ecuadoriensis / Pacific Hairback Fiddler Crab


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're left with some confusion). The species is found from Mexico through northern Peru.
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.
https://www.inaturalist.org/observations/143019540 https://www.inaturalist.org/observations/143018769
https://www.inaturalist.org/observations/128202760 https://www.inaturalist.org/observations/2577751
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.


Minuca argillicola / Clay Fiddler Crab

This is an obscure species only known from Costa Rica and Panama. It is predominantly beige and known for being very lethargic and inactive. It is thought to primarily live on river banks with heavy clay concentrations, which is probably the easiest way to identify it, as it is otherwise a fairly generic looking fiddler crab. The only observation on iNaturalist (my own) has a very poor photo (crab at the top, left side).

https://www.inaturalist.org/observations/10063


Minuca osa / Osa Fiddler Crab

Another obscure species, currently known from only two bays, one in Costa Rica and one in Panama, the limited living photos of this species show a crab with a mottled black and white back and orange limbs, including the large claw. The mottled black and white carapace is very common among species along the Indian Ocean and western Pacific, but may be unique to this species along the eastern Pacific coastline. There are no observations currently on iNaturalist.


Minuca galapagensis / Galápagos Fiddler Crab

Despite it's name, the Galápagos Fiddler Crab is found in both the Galápagos and along the coast of the Americas from southern Nicaragua to Chile (unlike Leptuca helleri which is only found in the Galápagos). Thus, identifying this species in the Galápagos should be easier, while identifying it along the coast may be a lot harder.
Potentially the most distinct feature in identifying this species is it appears to be one of the few Minuca which is predominantly white. The "canonical" coloration of this species appears to be a mostly white (or pale beige) body with a more orange claw (sometimes more yellow-orange, sometimes more red-orange). Unfortunately, other individuals may be more dull brown, orange, or a mix, confusing identification as this starts to look a lot more like other Minuca species. While Minuca argillicola may be predominantly white as well, it lacks the orange of the claw and is thought to be restricted to clay embankments.
https://www.inaturalist.org/observations/155006637 https://www.inaturalist.org/observations/151262446
https://www.inaturalist.org/observations/143951339 https://www.inaturalist.org/observations/66145831


Minuca herradurensis / La Herradura Fiddler Crab


Minuca herradurensis is found from El Salvador through Panama and is not particularly well known, other than it is supposed to be very similar to Minuca galapagensis, but possibly without the striking white color (more "pale buff") and without an orange phase. This is not particularly distinctive and the two species overlap in their ranges. A limited number of solid observations on iNaturalist are not enough to draw conclusions about color patterns.
https://www.inaturalist.org/observations/117527273 https://www.inaturalist.org/observations/117525450


Minuca brevifrons / Narrow-fronted Fiddler Crab

The last Minuca species, although found from Mexico to Panama, seems to be obscure and I have never seen a definitive photo of this species nor do I have a feel for what it looks like relative to others it may overlap with. 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.
Julkaistu elokuu 28, 2023 09:45 IP. käyttäjältä msr msr

Kommentit

Wow this is fantastic! I'll find this very useful. FYI @austinsmith

Lähettänyt loarie 10 kuukautta sitten

Brilliant work msr :)

Lähettänyt jarronevsbaru 10 kuukautta sitten

Awesome! This will definitely be helpful. Thanks for tagging me @loarie !

Lähettänyt austinsmith 9 kuukautta sitten

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