The Perplexing Fiddler Crab, Uca perplexa, is found throughout most of the islands of the western Pacific, from Japan down to Malaysia across Indonesia, the Philippines and Papua New Guinea, the eastern coast of Australia and out toward remote islands such as Fiji, Samoa, and French Polynesia. It’s name comes from the confusion that arose in trying to separate this species from closely related forms, particularly Uca annulipes and Uca lactea with which its range overlaps in many areas. In the field these species can be told apart somewhat by color: the major claw in Uca perplexa tends to be more yellow while that of Uca annulipes is more red, although there are other similar species which may also have red and/or yellow claws so its not an absolute indicator.
Because of its spread over so many remote Pacific Islands, in terms of raw area (as well as longitudinal range), it is one of the most widespread fiddler crab species, likely second only to Uca tetragonon (which resides in much of the same area, but is also common throughout the Indian Ocean as well). Given that most of its range area is open ocean, it’s an open question as to whether it actually occupies more shoreline than a widespread continental species such as Uca tangeri.
The Gulf Marsh Fiddler Crab, Uca longisignalis, is found along the Gulf of Mexico coast of the United States, from northwestern Florida through Texas. It’s a medium-sized species, sometimes mixed up with close relatives such as Uca minax or Uca rapax. It has a medium to pale turquoise/blue carapace, brown legs, and a white/yellow major claw.
The name of this species (essentially “long signalling”) comes from its extended waving and acoustic display relative to similar species from which it was originally distinguished.
The Gulf Marsh Fiddler has not been the primary focus of many studies, although it has often been examined in comparative cross-species studies, particularly by Carl Thurman and his students and collaborators.
The Bonin Islands Fiddler Crab, Uca boninensis is fairly unique in that it is both endemic to the Ogasawara Islands of Japan (“Bonin Islands” is a historical name) and is not sympatric with any other species, being the only fiddler crab found on these islands. The only other fiddler crab with no sympatric fiddler associates is Uca tangeri, which has an immense range across most of the west coat of Africa and even the southern edge of Europe. There are a few other species endemic to small islands, but each of these overlaps with one or more other fiddler crabs.
Until its recent description, this species was thought to be Uca crassipes. Shih et al. (2013) identified the new species through clear genetic differentiation between it and other populations of U. crassipes; they also found some minor morphological differences between these two very similar species, although nothing so striking as to allow easy identification. Little is known about the ecology of Uca boninensis since almost all previous work simply noted its presence on these islands. It is a medium sized fiddler crab, found in the upper intertidal zone of the estuaries of small rivers, in fragmented rock and muddy-sand substrates.
Because this species is known from only a few remote islands in the Pacific Ocean, its existence is likely threatened by climate change and is a candidate for conservation evaluation and protection. The Ogasawara Islands contain many endemic, unique, and endangered species across all forms of life and were added to the UNESCO World Heritage List in 2011.
The Bowed Fiddler Crab, Uca arcuata, is one of the more widespread and better known species from eastern Asia, found along the coasts of China, South Korea, Taiwan, and Japan. It is a large species, with a dark brown or maroon carapace (often with cream edges and highlights), cream eyestalks, black-to-red legs, and a robust major claw with a red hand and cream fingers.
In parts of its range, both males and females of this species build chimneys around the mouths of their burrows, extending the entrance upward from the muddy surface. They are often found on open mud flats and banks near the mouths of rivers. This species has been the focus of many studies by marine ecologists in both China and Japan.
Jocelyn Crane reported that this species could sometimes be found canned in local seafood markets in Japan, making it one of only two species eaten by humans.
Note: the fiddler crab of the week series was temporarily derailed due to some unexpected health issues, but will hopefully be back on track going forward.
This week we find another species from the Pacific coast of the Americas, Uca beebei, named after the famous naturalist, explorer and author William Beebe. This small species is found from El Salvador through Peru on open mudflats. In the Pacific entrance of the Panama canal it tends to be one of the more numerous species, often found mixed with larger species such as Uca stylifera or Uca heteropleura or more similarly sized species such as Uca deichmanni. Relative to many other species, it is also more of a generalist when it comes to substrate preference, ranging across both sandier and muddier mudflats. Not the most colorful species, it tends to be a grayish-brown with some teal on the back of the carapace; the long, slender fingers of the large claw are white, while the rest of the major limb often has dark purple highlights. The eyestalks are thick and yellow.
Unlike many of the other species in the series so far, Uca beebei is actually fairly well studied. John Christy and colleagues spent many years studying the behavior of this species in Panama. It has a fairly classic, basic wave, quickly moving laterally its claw to the side, then forward in an overhead circle back to the rest position. The small claw often goes up and down as well. The waving can be done either standing in place or sometimes shuffling toward or away from another crab.
Beebe’s Fiddler Crab is one of a number of species which build structures around the mouth of their burrow. As seen in the photo below, the males build a sort of wall or pillar or partial hood on one side of the burrow opening. The exact purpose of these structures is unknown, but in part it seems to be used as a beacon to allow themselves to find their burrow more quickly if they’ve wandered away from it.
The Slender-legged Fiddler Crab (Uca tenuipedis), is a very small species from the eastern Pacific Ocean, ranging from about El Salvador to northern Peru. It is similar in size to some of its sympatric associates, such as Uca batuenta, Uca intermedia, and Uca saltitanta, but is readily distinguishable from the first of these by the somewhat triangular shape of the pollex and manus, and the latter two by its color: a fairly cryptic-colored brown and gray body, with a white pollex and pinkish-salmon dactyl and top part of the major claw.
The waving display of this species involves bringing both the large and small claws straight out to the side, pausing for a moment, then bringing them up and forward in a circle back to the starting open-wide position. The whole body rocks backwards slightly with this motion and sometimes the first few legs may come off the ground as well.
Unlike some of the other local species, this one seems to stay closer to the shoreline, found among the muddy stream banks or the fringes of mangroves rather than out on the open mudflat.
The Pygmy Fiddler Crab (Uca pygmaea) is an obscure, very small fiddler found on the Pacific coast of the Americas, from southern Costa Rica through northwestern Colombia. It’s name refers to it’s tiny size. Almost nothing is known about this species; there appear to be no observational studies of it in nature beyond its original collection along the muddy bank of a stream in Costa Rica.
The large claw of this species is interesting in that the hand is very thick and the fingers are relatively short and stubby. Many species have juveniles with claws that are thick and stubby; as they get older the proportions shift into those that we tend to associate with most fiddler crab claws today. This change in shape with size is known as allometry. It has been proposed that the Pygmy fiddlers’ claws stop developing at a more “juvenile” shape, a pattern known by the technical term paedomorphosis. It is certainly not a requirement that a species this size have a claw of this shape; while thicker, stubbier claws are not uncommon among some of the other very small species (e.g., U. saltitanta and U. inaequalis), one of the very smallest, Uca batuenta, has a perfectly “normally” proportioned claw.
This week we move to the Americas for the first time and to one of my favorite species, the Styled Fiddler Crab, Uca stylifera. The Styled Fiddler is arguably the most striking species found in the western hemisphere and one of the easiest to identify. It is a medium (trending toward large) species where males have a white body (sometimes more dull yellow if inactive); white and purple legs; a robust large claw which is reddish-purple at the base, with an orange pollex (the immovable finger) and white dactyl (the movable finger); and yellow eyestalks. A single long style (as long or longer than the eyestalk) projects from the top of the eye found on the same size as the large claw.
If it looks familiar, that might be because it is the species featured in the logo of this site.
Because they tend to be found on open, dark sandy mud flats, males of this species are among the most conspicuous fiddlers you can find. In contrast, except for the yellow eyestalks, females are a more-or-less solid muddy brown which allows them to readily blend into the background.
The Styled Fiddler is found on the Pacific coast of Central America and northern South America, from El Salvador to northern Peru. It’s found on open mudflats (rather than in mangroves) in what might be considered the mid-intertidal zone. It seems to prefer a muddy substrate which is a bit firmer, with somewhat higher sand content than many other species; it’s not found on sandy beaches or pure sand bars, but neither is it found in the thicker, stickier, softer mud. More than a dozen other fiddler crab species can be found at the same locations as this species, but since they tend to stick to different parts of the intertidal region, only a few are actually likely to directly intermix with Uca stylifera, with Uca beebei and Uca heteropleura being fairly common.
The waving display consists primarily of the male holding his claw out to the side and then making moderately slow, fairly tight back-and-forth motions with it while simultaneously taking a “stride” (if that’s what you call the combination of all eight legs taking one step) to one side or the other. The small claw is often also held up in the air during the wave as well. The video below is old and low quality, but shows a fairly typical display. The smaller crabs seen in the background are Uca beebei.
The obvious question about this species (or at least, the one I get most often) is “What’s up with the style?” To a large extent, we really don’t know.
This is not the only species which can have a style; they are sometimes found in adults of the closely relates species Uca heteropleura, as well as in juveniles of other similar species. However, Uca stylifera is the only species in which (a) every male has a style, and (b) the style is as long as the entire eyestalk. In the other species which have them, the style tends to be very short (less than half, or even a third of the length of the stalk) and is only found in a small proportion of males. Some non-fiddler crab relatives also have styles (e.g., some species of ghost crab), so the character is not entirely unique to fiddlers.
Very little is known about the function or purpose of the style; it plays no role in vision and serves no obvious function in courtship or display. Superficially it seems like a character that could be driven by sexual selection and female choice, although there is no evidence females choose males on the basis of the style (to be fair, experiments on this are lacking). The only study that has ever looked into elongated eyes (and secondarily, the styles) in Ocypodid crabs in any serious way (von Hagen 1970) suggested styles were likely a developmental artifact and likely served no primary function. More work may be necessary if we really want to understand this odd feature.
One aspect of the style actually highlights an interesting, often missed feature of fiddler crabs. The most striking characteristic of male fiddlers is the asymmetry of the claws: this is the character best known and recognized by most people. What is underappreciated is that the asymmetry is not restricted to the claw; it just happens to be the structure in which it is most obvious. In Uca stylifera, the presence of the style is asymmetric: it is only found on one eye and always on the eye on the same side as the large claw. In fact, in most male fiddler crabs, the entire side of the body with the large claw is slightly larger than the side with the small claw. The legs tend to be slightly longer and heavier, the carapace can be a little asymmetric and heavier, the internal muscles on that side of the body tend to be larger (likely necessary to support the asymmetric weight), etc. Even the eyes are different lengths. It’s not always obvious in smaller species, but look at the photos at the top of this post: even ignoring the style, you’ll see that the eye on the side of the large claw is longer than the eye on the side of the small claw.
Whole body asymmetry is pretty much true for males of all species, it is just that beyond the claws themselves, the other asymmetries tend to be substantially more subtle and pale in comparison to the differences in claw size. But whatever the developmental mechanism is that controls the asymmetry, it applies to the whole body, not just the claw. This is more obvious in Uca stylifera that most other species because the asymmetric presence of the style draws attention to a non-claw asymmetry.
The Flame-backed Fiddler Crab, Uca flammula, is a particularly striking species whose name derives from its color. In adults, the carapace is usually more-or-less solid black, except for a pair of whitish-red parallel back-to-front markings toward the center (most fiddler crabs have what appear to be creases in the carapace that roughly form the letter H…it is the vertical arms of the H that are red in this species) and a solid red band at the front of the carapace. Most of the rest of the crab is usually a bold and bright scarlet-orange, with the tips of the large claw (particularly on the movable finger) trending toward white. Somewhat unusually, females are colored more-or-less identically to males (in many species where the males have bold and bright colors, the females are more dull and cryptic), although sometimes with the red extending even further onto the carapace.
A slightly unusual aspect of this species is that the juveniles are a very different color, usually almost a uniform gray-blue with darker-blue eyestalks and a pale yellow large claw in males. The complete lack of red and the fact that young males will sometimes wave could mistakenly lead one to think they might be a separate species (von Hagen and Jones, 1989).
The Flame-backed fiddler is found in northern and northwestern Australia, as well as on the western half of New Guinea. It is a large species, with a narrow front (eyes close together), and a robust looking major claw, usually with very obvious bumps (tubercles) and grooves when examined closely.
It’s waving display is fairly vertical (mostly up and down without much movement of the large claw out to the side), usually starting with an initial strong wave, followed by a series of diminishing smaller ones (Crane 1975). In what I am now realizing is going to be a very common statement, the biology of Uca flammula is relatively unstudied, although some recent papers by Madeleine Nobbs has examined how its distribution on shorelines is related to vegetation patterns, so perhaps more information about this species is on the near horizon.
This week we move to the northern half of Australia to find the Shaking Fiddler Crab, Uca seismella, so named because males shake and vibrate their entire bodies as part of their waving display (video unavailable, unfortunately).
The Shaking Fiddler Crab is a small species, fairly cryptic when not waving (when waving it is apparently hard to miss) and less colorful than most of the other fiddler crabs of northern Australia. It has a carapace that is more or less pale brown and gray, with gray sides and legs. The large claw contains a mix of white and pale salmon-pink, with bright white fingers. The fingers of the claw appear particularly smooth and flattened. It has yellow eyestalks that are close together. Females are more uniformly gray.
Another species which hasn’t been heavily studied, Uca seismella is predominantly mentioned in the literature relative to other better studied species that it lives near or is somewhat similar to. It’s range in Australia coincides with at least half-a-dozen other species and it is often found in the same areas, if only intermixing on the fringes of their territories.
One of the more interesting tidbits about this species is it is one of the few in which female waving has been observed. Fiddler crabs are quite famous for the male waving displays, but it turns out that in a number of species females also occasionally wave their claws and limbs, or otherwise perform distinct behavioral displays. It is not clear how common female waving is in this species, but von Hagen (1993) filmed three females use waving and bobbing displays to fend off other females encroaching on their territories. Female displays are fairly understudied in fiddler crabs, but have been reported from a variety of species spread across the entire genus and may be more common than we realize.