Fiddler Crab Field Guide: 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 Georgia (from Maine in the north through South Carolina in the south). Three species of fiddler crab are found in this region, although only Minuca pugnax is currently found in the northern end of this area in New Hampshire and Maine:
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 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.
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.
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).
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.
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.
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. https://www.inaturalist.org/observations/106018263
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.
