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 New Guinea and neighboring small islands, which indludes the country of Papua New Guinea as well as part of Indonesia. This area has at least 11 species:
The presence of a few of these species on New Guinea is slightly unusual. Four of them are primarily only found in Australia: Austruca mjoebergi, Gelasimus vomeris, Tubuca flammula, and Tubuca seismella. In addition to eastern Australia, Gelasimus vomeris is also known from New Caledonia, so its presence in New Guinea is not particularly surprising, but the other three are otherwise only known from Australia. There have been a few scattered, questionable records of Austruca mjoebergi from other islands in southern Indoneisa, but its presence both in those areas and in New Guinea is the most highly suspect of these occurences. It seems likely that the Tubuca flammula and Tubuca seismella observations are correct, although both species might be very rare in New Guinea.
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”). Four of these species (Austruca mjoebergi, Austruca perplexa, Austruca triangularis, and Paraleptuca crassipes) are broad front species, while the other seven (Gelasimus jocelynae, Gelasimus tetragonon, Gelasimus vomeris, Tubuca coarctata, Tubuca dussumieri, Tubuca flammula, and Tubuca seismella) are narrow front species. Note that the carapace between the eyestalks appears pinched together in the narrow front species, but more trapezoidal in the broad front species.
Austruca perplexa is likely one of the more common species on New Guinea, but can be highly variable across its range. It has a carapace that is generally black and white, frequently with moderately distinct striping; its carapace usually leans more toward black than white, but more solid colros, including almost solid white are possible. Its large claw is tends to be a mix of yellow and white, but can also be a solid color of either. Its legs are usually white or brown or yellow; very rarely red.
Although not universal, male Austruca perplexa frequently have a small but distinct tooth near the tip of the lower finger on the large claw.
As mentioned above, the presence of Austruca mjoebergi in New Guinea is somewhat questionable. As it looks quite a bit like Austruca perplexa, it is easily possible for the two species to be confused.
Austruca mjoebergi has a bright yellow claw, with almost no white on it. Its legs are frequently red. Its carapace tends to appear more-or-less solid colored, often a gray or off white, although black and white stripes or speckles are not unheard of.
Distinguishing between these two species in the field is likely to occur more be general tendencies than absolute differences. The claw of Austruca mjoebergi is almost always a bright, solid yellow; that of Austruca perplexa tends to be a duller yellow and more often tha not has white on at least some of the fingers. The carapace of Austruca mjoebergi tends to be mostly solid, paler colors, while that of Austruca perplexa tends to be more striped with distinct black and white patches. The legs of Austruca mjoebergi are frequently red, while those of Austruca perplexa are rarely red. Unfotunately, none of these color characteristics appears to be absolute for either species. The distinct tooth on the lower finger of the large claw that is sometimes (but not always) found in Austruca perplexa appears to be entirely absent from Austruca mjoebergi.
Austruca triangularis is superficially similar in color to both Austruca perplexa and Austruca mjoebergi, generally having a black/brow and white/cream carapace (occasionally yellow), frequently will broad stripes, and a claw which tends to be off white, but may include yellow. Despite this general similarity, the species should be relatively easy to tell apart.
There are at least three features which can be used to distinguish them. First, the carapace shape is noticably different, with the front corners of the carapace of Austruca triangularis extremely pointy and tending to point more toward the sides than the front, with the sides of the carapace converging much more strongly in Austruca triangularis than in either Austruca mjoebergi or Austruca perplexa. Second, the hand of the large claw in Austruca triangularis is generally covered with brown spots, giving it a polka-dotted appearance (these spots are sometimes foundon the carapace as well). Third, but more subtle, on average Austruca triangularis tends to be smaller than the other two species, although there is overlap in size.
The final broad front species in this region, Paraleptuca crassipes is easily distinguishable from the others by color. The colors of Paraleptuca crassipes do overlap with one of the narrow front species, Tubuca coarctata, described below.
Paraleptuca crassipes is a highly variable species with a carapace that is generally some combiantion of black, pale blue, and bright red—occasionally with a hint of yellow or green—with solid red and all-but-solid black carapaces not uncommon (it seems never to be solid blue). It's eyestalks are usually yellow-green, but sometimes red, particularly in individuals with heavily red carapaces. The large claw is bright red-to-reddish pink, with paler pink or white fingers.
Gelasimus tetragonon is a wide-spread species that is something of an oceanic island specialist, although it is occasionally found on continental shores. It usually has a predominantly blue and black carapace with bright red legs (which occasionally may be darker) and gray eyestalks. The large claw of Gelasimus tetragonon is usually orange, with a noticeably darker red spot near the base of the pollex, and a white dactyl. Gelasimus tetragonon frequently has brown spots on the top part of the hand of the claw.
Gelasimus tetragonon has a lot of additional variability. 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.
Tubuca coarctata is a highly variable narrow front species that is most likely to be confused in this region with Tubuca dussumieri or the broad front species Paraleptuca crassipes. Differences between Tubuca coarctata and Tubuca dussumieri will be described below, so this section will focus more on Tubuca coarctata vs Paraleptuca crassipes. On average, front breadth might be the easiest way to distinguish these as the high variaiblity of both species creates a lot of general overlap in color pattern.
The most common carapace pattern in Tubuca coarctata is a mostly black carapace with some spots of pale blue, yellow, red, or white. In many cases, however, individuals may have carapaces that appear nearly solid of any of these colors or consist more of two-toned stripes of two of these colors (the latter pattern might be more common in females than males).
The legs of Tubuca coarctata generally follow this same color variation as the carapace, although the two rear legs generally have a large distinct, pale blue (sometimes almost white) blotch on the rear of the the top segment. This color will sometimes spread beyond this segment.
The large claw usually has a red or orange hand, with most of the upper finger and the outer half of the lower finger white. Sometimes the white will bleed further onto the upper part of the hand. There is also usually a darker red patch at the base of the lower finger (as is common in many other species). The prototypical male Tubuca coarctata has a distinct an obvious flat edged projection along the lower portion of the tip of the upper finger of the large claw. In some individuals this feature is more subtle, while other species may have a claw shape that is at least somewhat similar, although within this region this shape is more-or-less unique to this species.
Other than front breadth, the most likely methods of distinguishing Tubuca coarctata from Paraleptuca crassipes are the shape of the tip of the large claw and the presence of the blue spots on the backs of the rear legs. Paraleptuca crassipes also leans much more heavily toward bright reds than Tubuca coarctata, whose redder forms are more orange.
Tubuca dussumieri lacks the extreme variability of Tubuca coarctata but overlaps in color. Tubuca dussumieri generally has a dark blue or black carapace, frequently solid colored although some spotting is possible, which can be similar to that found in Tubuca coarctata, including occasional pale spots on the rear legs.
The large claw of male Tubuca dussumieri is generally two-toned, orange on the bottom and white on the top, with a darker orange/red patch at the base of the bottom finger. The fingers of the large claw tend to be fairly straight, and the upper finger lacks the distinct flattened edge that tends to be found in Tubuca coarctata. There may be some flattening of that finger tip in Tubuca dussumieri, but it is more subtle than that of Tubuca coarctata.
A final feature that distinguishes the two species may be too subtle to see in most cases, but is worth mentioning. Tubuca dussumieri has two long grooves running across most of the length of the upper finger of the large claw; Tubuca coarctata only has one. Unfortunately, these grooves are not always readily visible, particularly as the pale upper fingers of the claw frequently appear slightly overexposed in photographs.
Although likely rare in New Guinea, Tubuca flammula should be fairly easy-to-identify as it generally has a solid black carapace with a bit of bright red on the front edge (a pair of parallel grooves that run front-to-back down the middle of the carapace may also be reddish), while the rest of the crab is more-or-less entirely bright scarlet. The upper finger of the large claw may be a bit paler and trend toward white.
Another presumably rare species on New Guinea, Tubuca seismella should still be fairly distinct if observed. It lacks the brighter colors many of the other species. Its carapace is a light marbled brown or gray, frequently with lighter speckles, and its large claw tends to have a pale peach/pink hand with white fingers. The fingers have a flattened and smooth appearance which is not found on any of the other local species.
The two species from the subgenus Gelasimus (Gelasimus) are very similar to each other and may be difficult to tell apart. They can be distinguished from most other species by both color and the structure of the large claw in males. The claw has a distinct shape only found in the species of this subgenus. Specifically, the upper finger tends to be moderately broad, tapering smoothly to a point, while the lower finger will usually curve upwards with a characteristic wave on the inner surface highlighting one to two large teeth. Individuals with regenerating claws will often lack much of this shape, but the claw still looks readily different from other species in the region.
The large claw of Gelasimus vomeris is strikingly two-toned, generally orange on the bottom half and white or pink on the upper half; sometimes there is a darker orange spot around the base of the lower finger. There is a large triangular depression around the base of this finger as well, that is frequently outlined in mud. Its carapace is usually a mix of pale blue or white and black, although other colors, including green and brown are possible. Its legs tend to be brown or gray. The lower finger of the large claw generally has a large tooth near the tip, although regenerated claws may appear smoother and toothless.
The small claws, on both males and females, is frequently orange.
Gelasimus jocelynae is the same basic colors as Gelasimus vomeris, with the primary difference being a mildly subtle difference in claw shape. While the general shape and toothiness of the claw is the same, the difference is found in the thickness of the dactyl (upper finger) of the large claw. In Gelasimus jocelynae this finger is thicker than that of the pollex (lower finger) (particularly around the first 1/3-1/2 of the finger), while in Gelasimus vomeris the dactyl is a bit thinner. The small claw of Gelasimus jocelynae appears to rarely be orange, while it is usually orange in Gelasimus vomeris.
