Introducing the Fiddler Crab Paper of the Month

This endeavor will be similar to my Fiddler Crab of the Week series, except less often and without the random component. The goal will be to try to highlight one fiddler crab research study every month. Around the start of each month I will look at all of the papers about fiddlers published in the previous month (more or less…I’ll also include older papers I happened to only discover in the previous month) and choose one to discuss. The basic rules about choosing a paper will be:

  1. One or more fiddler crab species have to play a prominent role in the paper, preferably as the focus, but optionally as a major component of the study.
  2. Any sort of publication is eligible: journal articles, books, book chapters, dissertations, theses, etc.
  3. I need to be able to get a hold of a copy of the publication to discuss it.
  4. My own publications are ineligible (since I’ll likely discuss and/or highlight them on the blog anyway).

If nothing meets the criteria from those published and/or discovered in the previous month, I may just choose an older publication that I find interesting. I will also tentatively try to avoid repeating papers by the same author(s) if they happen to be particularly productive (over a reasonable time span), and will also try to choose papers on a wide variety of topics rather than sticking to one particular subarea of research.

Nothing precludes me from highlighting multiple papers in a single month if the mood (and time) strikes me, but the goal will be to get to at least one. The precise publication schedule is not fixed, but my aim will be to post within the first week of the month. Keep an eye out for the first of these in the next week or so.

The Pygmy Fiddler Crab, Uca pygmaea (Fiddler Crab of the Week)

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.

The Styled Fiddler Crab, Uca stylifera (Fiddler Crab of the Week)

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.

Photo provided by Pat Backwell.

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.

Photo by M. Rosenberg, Panama, 1997. Note the brown female to the left and behind the male.

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.

Constructing Fiddler Range Maps, part 3: A Better Way?

This is the third of four planned posts about how I construct the range maps for fiddler crabs. The first part gave the history and background of how these maps were drawn in the first place. The second part discussed where the maps become problematic when we want to use them as input data for analysis. This part will present a possible solution to the problem detailed in the second part. The fourth and final part will step back and ask if we’re actually thinking about range maps the wrong way entirely.

As highlighted in part two, even beyond general questions of their accuracy there are some potential problems with using the range maps as data, particularly if we are thinking about estimating the sizes of their ranges. Three parts of the solution are completely obvious. First, for any question involving coastline length (whether to measure available space or estimate a fiddler range), one must be very specific about the map scale and background map used to make such measures since changing these could change the results. Second, when comparing species and/or regions, one must use the same base maps (optimally) or maps constructed from identical scales (suboptimal, but tolerable if necessary) to calculated values for each species or region. Third, species ranges and coastline lengths have to be determined from the identical maps if they are to be at all compared.

This last part is where the most work suddenly looms. As detailed in the previous post, currently our species ranges are generated from one map set while our coastlines are generated from a different one. To make them match, we would likely need to recreate the range data…again…on our new coastlines. Doing so highlights the fragility of the current process…but also leads to the realization that there is likely a better way to store the base information about fiddler crab ranges.

I’ve mentioned a few times that fiddler crab ranges should likely be viewed as one-dimensional lines rather than two-dimensional areas. But we can take advantage of areas and polygons to define a fiddler crab range in an easy and flexible manner. The idea here (which one can view as theoretical since I have not implemented it yet) is that for a given species we only need to define the general boundaries of the range—define a loose polygon which includes all of the within-range coastline and none of the outside-range coastline. Whenever we need to draw the actual range or estimate a distance, we then just need an algorithm which compares that polygon to a coastline map and extracts just the coastline inside the polygon.

There are a number of clear advantages to this approach.

  1. The range data is not fixed to a specific background coastline map, allowing any coastline map to be used to generate the actual mapped or measured range. Changing maps would not require redoing the range data.
  2. Updating the range data for a species simply requires updating the enclosing polygon, likely a vaster simpler operation in most cases than the current system.
  3. The polygon that describes the range data does not require tremendous precision over most of its boundaries. A simple rectangle might be adequate for many species (some will require more complicated shapes, unfortunately).

In theory, for some species with extreme simple ranges (a single contiguous coastline without any outlying islands), one could define the entire range by just noting the end points. In reality, we’ll likely define these by a simple shape that intersects the coast at those two points.

For example, below is a range map for Uca maracoani which I used in the first post of this series, with the addition of a blue rectangle to serve as the polygon denoting its “range.”

This rectangle adequately represents the range of the species, as long as we recognize that the range is the coastline within the rectangle and not the area of the rectangle itself. We need not concern ourselves with how far into the Atlantic the rectangle extends (as long as it isn’t so wide as to clip Africa), nor that it includes two landlocked countries without any shoreline. This rectangle would serve as the masking template for the species, to be applied to any coastline map. An algorithm would simply need to extract the coastline in the rectangle (marked in red) as needed for display or analytical purposes. If we need to change the range of the species we make the rectangle bigger or smaller or use a more complicated polygon as necessary (for example, we could not extend the rectangle west to encompass the Atlantic coast of Colombia and Panama without it intersecting the Pacific coast of Peru…in that case we’d have to use a slightly more complicated polygon that avoided intersecting the Pacific coast).

The biggest question mark about this approach is how complicated the computation will need to be to extract the correct coastline from a complex polygon of an arbitrary shape (simple polygons would be pretty easy), but my presumption is not too complicated. If nothing else, this problem is not unique and  has been solved in many other applications (e.g., masking or clipping figures using complex shapes in vector drawing programs such as Illustrator or Inkscape) so likely a workable solution already exists.

While not solving every problem, using this simpler bounding concept with algorithmic coastline extraction seems like a much more flexible manner of storing the range data. Of course, maybe the way we are thinking about range maps is completely wrong to begin with. Stay tuned for final thoughts…

The Flame-backed Fiddler Crab, Uca flammula (Fiddler Crab of the Week)

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.

Photo provided by Pat Backwell.

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.

Photo provided by Pat Backwell.

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.

Pool Mosaic

In the early days of the site I would collect any images of fiddler crabs I could find. One I stumbled across was this small photo of a stylized painting of a fiddler crab from Fiji (at this point I don’t recall if the painting, the crab, or both were from Fiji…the original site is long since gone).

A little over a decade ago, we had to have our swimming pool completely rebuilt. As part of the rebuild, my wife wanted to put some mosaics into the surface of the pool to add some color and style. These included some large swimming dolphins (complete with shadows to give them a 3D effect) and some small turtles on a bench. Obviously, I felt we should have a fiddler crab as well. Of course, you cannot find fiddler crab mosaics for pools (or anything else, for that matter). You can find other crabs (particularly blue crabs), but not fiddlers. It turned out, however, that one of the local tile suppliers could custom build mosaics for you, if you provided them with a design. The above painting seemed like a good starting model, so a little simplification and tile color matching, then…

Our pool mosaic. The figure is maybe 9-12 inches wide.

We are likely the only people in the world with a fiddler crab mosaic in our swimming pool.

Constructing Fiddler Range Maps, part 2: The Problem of Ranges as Data

This is the second of four planned posts about how I constructed the range maps for fiddler crabs. The first part gave the history and background of how these maps were drawn in the first place. This part will discuss where the maps become problematic when we want to use them as input data for analysis. The third  part will present a possible solution to the problem detailed in the second part. The fourth and final part will step back and ask if we’re actually thinking about range maps the wrong way entirely.

In the first post I discussed how the range maps were originally created and have evolved over time. As general tools for the display of information, they work perfectly fine (there are some limitations that will be raised in part 4).

But what if we want to go beyond the display itself and think about the ranges as input data for other analyses. What type of analysis? As an example, a few years ago, Jeff Levinton (my PhD advisor) published a study on the Latitudinal diversity relationships of fiddler crabs. In general, fiddler crab diversity declines as latitude increases (as it does for many other groups of species), and this paper explored potential factors that explain this pattern. In this paper, the species ranges themselves made up a key piece of the raw data.

For the purposes of that sort of study, there’s nothing wrong with the ranges as data. You mostly only need to know the upper and lower latitudes between which a species is found, and while there may be some uncertainty on the precise boundaries (I’ll come back to this in part four), small errors are not likely to make a huge difference in the results. Other aspects of the ranges may become more problematic if looked at too closely, however.

For example, one of the results that can be found in the above paper is a slight northern bias to fiddler crab species diversity: globally, peak diversity is not found at the equator, but rather about 10° north (regionally, the northern bias is strongest in the Americas, but largely absent from the Indo-West Pacific). There are any number of reasons why there may be a slight northern bias, but one hypothesis that could be suggested is that there is more land in the northern hemisphere than the southern and diversity is partially tracking habitat availability. Since land area as a whole is fairly meaningless to fiddler crabs, this hypothesis can only make sense if the increased land mass in the northern hemisphere corresponds to an increased coastline. There are a number of reasons I suspect this hypothesis about fiddler diversity is likely incorrect (the simplest of which is that the greatest species diversity is often found in very small areas), but what if we wanted to test it? We would need a way of measuring the amount of of coastline available. Because fiddlers only live on coastlines, this also leads to the idea of measuring the “size” of a fiddler crab range by the total length of coastline it inhabits. Unlike most other species, fiddler crab ranges can be thought of as one-dimensional lengths measured in km, rather than two-dimensional areas measured in km2 (this 1D argument might fall apart for some of the species which range over large parts of the western Pacific islands, but that is a discussion for another time).

So, whether we are interested in the range of a species of the potential habitat it inhabits, we are looking at measuring the length of the coastline. How do we do this? The coastlines and species ranges on our maps are recorded as a series of connected coordinates, so it’s simple to imagine simply calculating the distances between connected pairs and adding them together. Voilà, species range and/or coastline length! Except, now we need to go back and look at our map data more closely.

The coastlines and countries boundaries in our new cartoon maps (see previous post) came from the Natural Earth data sets. These maps come in three different scales: 1:10 m, 1:50 m, and 1:110 m. Essentially, fine scale to rough scale. For simple display purposes, most of the fiddler crab ranges could use the medium or roughest scale; the finer scale maps are only really needed if we need to zoom in to fairly small regions. For example,  Uca osa is a recently described species of fiddler crab known only from the Gulf of Dulce, in Costa Rica. Below is a zoomed-in look at the Gulf drawn from two of these data sets.

Maps of the Gulf of Dulce, Costa Rica. The top map is from the 1:50 m scale, the bottom map 1:10 m scale.

At this level, the two maps are strikingly different. Because the gulf is so small, it does not even show up in the 1:110 m map data (not shown)! But it is more than just a visual difference. The measurement of coastline will be different in each of these. The finer the scale of the map, the longer the coastline.

This issue has been known for a long time; in fact, coastline length is considered to be a fractal mathematics problem called the coastline paradox. Theoretically (if not practically), if you could keep measuring a coastline at greater and greater accuracy, it’s length would continue to increase…all the way to infinity. One of my favorite oddities of fractal mathematics is the proof that a finite area can contain an infinitely long line (e.g., see the Koch Snowflake). If we want to measure the length of the coastline, we need to be concerned with the scale at which we measure it.

Since our species ranges are also based on coastlines, they have the same issue. But here, a secondary problem arises. The ranges are currently based on yet a different map set (this one extracted from Google Earth). If we draw the coastline data for Uca osa on top of one of our Gulf maps…

we immediately find that it is at yet a different scale than any of our background maps.

This all just highlights how we need to be careful about thinking of these species ranges as data. They’re perfectly good for generally asking about where species are and questions of overlap, but if we want to translate these ranges into measures of distance or area, more thought is needed. Some of those more thoughts in part three…

The Shaking Fiddler Crab, Uca seismella (Fiddler Crab of the Week)

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).

Check me out! Photo from Pat Backwell.

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.

Photo from Pat Backwell.

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.

nanoblock Fiddler Crab

And now for something a little less sciencey.

I’m always on the lookout for good fiddler crab arts or crafts…there pretty much isn’t any: most of what you find is reproductions of old figures and/or poor quality, while a lot of the rest that is labeled as fiddler crabs are actually other types of crabs entirely. Recently, however, I heard that there was a fiddler crab LEGO set; I found that rather surprising and went looking for it. Turns out the information was partly correct: nanoblock, a LEGO competitor from Japan, does make a fiddler crab mini-set. Naturally, I had to get it.

If you’re not familiar with them, nanoblocks are very similar to LEGOs, except much smaller. Way way smaller. I honestly wasn’t prepared for how small the pieces were. This makes building the set quite a bit more challenging, if for no other reason than my thick fingers had trouble dealing with tiny pieces at times.

The set comes in a small, resealable envelope. More than 150 pieces seems a bit non-specific, but that may be in part because it comes with a decent number of extras beyond those necessary to build the model. Ages 8+ (whew…I cleared that bar), mostly because younger would likely eat the small pieces. Actually, an 8 year old can probably handle the tiny pieces way better than I can. Interestingly, it rates 4 out 5 on the difficulty scale.

I’ll skip describing the build process beyond a few general comments.

  1. The instructions are not as clear as they could be and there were a few places I had to backtrack to discover that I’d missed something.
  2. Beyond the difficulties generated by their size, nanoblocks don’t stick together as tightly as I remember LEGOs doing so. This seems to primarily be due to the lack of tubes in the underside of the brick providing extra support. There are obvious pluses and minuses to this: the plus is it allows you to combine bricks off center in a way you cannot with most LEGOs; the minus is a lack of stability and tendency for pieces to come apart when you don’t want them to.
The finished model. This is the ad image, not my own personal model, although mine looks pretty identical to this, just without the pure white background.

The end result is pretty nice. It definitely looks like a fiddler crab, other than the eyes which strike me as a bit more ghost-crabby. It’s life-size for some of the larger fiddler crab species (the carapace of the model is slightly more than 3 cm wide, which puts it in my large category of fiddler crabs). Because the company is from Japan (although the toy itself is made in China), the color-scheme makes me think it might be Uca arcuata, but I’m sure I’m reading a bit too much into that. The large and small claws (and to a very tiny extent, the legs) can actually be rotated and adjusted in or out, so we can pose him with the claws in as if he were feeding or out as if he were a horizontal waver (to move the claw vertically would require redesigning the build and the pieces are too fragile and small for me to bother).

The built model is sitting in a carefully chosen spot on my desk at home where cats and/or cleaning implements are unlikely to bump into it and cause it to fall apart.

How Big are Fiddler Crabs?

The simple answer to the question posed in the title of this post is: small.

Fiddler crabs are very small compared to most of the crabs people are familiar with. The largest species is relatively small (barely reaching 5 cm wide in the largest individuals) and the smallest species is quite tiny (under 1 cm wide). However, within fiddler crabs there is quite a lot of range and when tasked with describing a particular species (e.g., in my Fiddler Crab of the Week series), one of the first things I generally want to comment on is its size (compared to other fiddlers).

Size differences among species. Both crabs in this photo are adult males. The large, red and black crab is Uca forcipata, while the small, white one is Uca triangularis. Photo taken in Bali.

For species toward the ends of the size range this is fairly simple, but for a lot of those in the middle we don’t really have good guidelines to what a small or medium or large species is. For example, the following figure from my dissertation was used to illustrate the relative sizes of 20 species of fiddlers observed on the Pacific coast of Panama.

The largest of these is among the largest in the genus and the smallest among the smallest in the genus, so it more-or-less encompasses the entire size range, but as it only represents about 20% of the genus it may not be representative of the overall distribution of sizes within that range. Based on this figure, it would be difficult to set distinct barriers barriers between size classes (not that I would believe there necessarily should be any), making the designation of large, medium, or small more strikingly arbitrary.

The question of species size distribution may be more than an esoteric one; many fiddler crab species live in the same places (sympatry), but ecological interactions among them tend to be restricted to those that are roughly the same size. Males will defend territory and burrows against males of other species if they are of similar size (even directly fighting on occasion) and sometimes even attempt to court females of similar-sized species. Generally, they mostly seem to ignore individuals of species very dissimilar in size.

For the purposes of this discussion, I will restrict my measurement of size to carapace width (sometimes called carapace breadth). Sometimes this is measured as the widest part of the carapace, other times as the the distance between the tips (antero-lateral angles) of the front of the carapace (in many species, the distance between these tips is the widest part of the carapace, but as in the figure below, not always). The difference between these two measures is very small relative to the total width of the carapace.

There are a few practical reasons for choosing carapace width/breadth as our measure of crab size. (1) This width is easier to make and subject to less measurement error than the length or depth of the carapace. (2) The mass of the crab is dependent on hydration and mass measurements of dead specimens may be very different than live specimens, let alone live specimens in different hydration states. Carapace width is a stable measurement for both living and dead specimens. (3) The measure applies to both males and females, unlike, for example, claw length. (4) This measure has been used as a standard in many studies and allometrically scales with most other measures one might use instead.


In her monograph, Jocelyn Crane lists sizes for most of the species under their descriptions, but usually only a measurement for 2-3 males and 2-3 females (often categorized as large, medium, or small) rather than a more thorough population sampling. She usually tried to include the largest individual she could find, and then one or two other representative sizes. For females, she usually tried to include a small ovigerous female, in addition to the largest female. In total, she provides carapace breadth for 213 individuals from 88 different taxa. Here is a summary of all of her measurements:

Carapace breadth in mm for 88 species from Crane (1975). Each point is an individual, each column of points a single species. Sorted by largest male of the species.

A few things immediately stand out. First, across all species, the largest male is almost always larger than the largest female. Otherwise, male and female sizes do not appear strikingly different from this figure. Second, there is more than a five-fold difference in size among species; the largest species top out just over 50 mm, the smallest under 8 mm. Third, there are relatively fewer large species overall (as illustrated by the change in slope of the largest male after the first dozen or so species), as opposed to medium and small species. Finally, for the most part, there are no distinct divisions that would allow one to set clear size classes; for most of the size range the transition to smaller and smaller crabs is fairly smooth. The possible exception is seen at the very top of the distribution, where a few slight gaps representing an 8–10% difference in size between adjacently-ranked species stand out. However, it is worth noting that a 5–10% difference can be found in many of the smaller contrasts as well, they just do not stand out in the figure when plotted on this scale. More importantly, we don’t know how representative these measurements are.

To get at this, let’s compare this to a different data set. As part of my dissertation, I measured the width of over 850 adult male crabs from about the same number of species. Samples per species ranged from as low as a single individual to as many as 50. There were a couple of species from Crane’s data set which I did not have comparison data for (although I left her measurements on the figure below); six additional species for which I do have data, but which were not part of her data set are not shown (they generally fall in the middle of the overall distribution).

Male carapace breadth for 88 species. Blue points are from Crane (1975) as above; red points are from Rosenberg (2000, 2002). Species order is unchanged from the previous figure.

Again, a few things stand out. First, there were a small number of species for which I measured a larger (sometimes quite a bit so) individual than the largest recorded by Crane. The size of one species in particular (Uca pugnax) was clearly underestimated by Crane, as I recorded many individuals larger than what she reported and, in fact, her largest measure was only a little above the average of my sample (this is the stack of red points above the blue top line directly under the “a” in “Crane”).

For most of the species, however, her largest measures either correspond with or are larger than what I recorded. Overall, her ranges likely overestimate slightly the average size of most of the species, as there are frequently large numbers of individuals in my sample below her lowest measure. For most species, the average carapace breadth in my data set (not shown) tends to be below or close to the smallest breadth reported by Crane.

The literature certainly contains additional data on individual species which we could use to enhance these figures further, but the main message is clear. Fiddlers as a whole more-or-less uniformly occupy the size range between the smallest and largest species, and any boundary between small, medium, and large is functionally arbitrary. Based on the above figures, I’d likely just divide the size space into 10 mm intervals. Species that grow above 40 mm are “very large,” between 30–40 mm are “large,” 20–30 mm are “medium,” 10–20 mm are “small,” and below 10 mm are “very small.” Using Crane’s measures of largest size, we would find the following distribution:

Histogram of largest carapace breadth of male crabs as reported by Crane (1975).

The modal maximum size is 18–20 mm, right at the boundary of small and medium, with more than 2/3 of the genus within 10 mm of that size and classified in one of those two categories. That seems quite reasonable and puts slightly more than 10% of the genus in the large category and less than 10% in each of the very small and very large categories.

Obviously the divisions are arbitrary and a number of species will move up a category as we find larger individuals (e.g., three of the species would move up a size class just based on my own data from the previous figure) but this is a workable definition of fiddler crab size and we can always hedge species near the boundaries as small–medium or medium–large.

If this were the grade distribution for a large class, the students near the mode would all be complaining about +/- and begging me to change the curve. Thankfully, fiddler crabs don’t care.

So I guess I have my own answer now as to what I’ll mean if I say a species is small, medium, etc.

One final point: It’s useful to remember that even the measures for the species with better sample sizes likely do not entirely capture the species adult size range. Others have observed that there can be differences in size among populations within a species (likely driven by environmental factors, although genetic differences cannot be ruled out). It is also quite feasible that there may be seasonal differences in population size, particularly for temperate species which are dormant over the winter but may go through a number of growth molts over the summer.

To truly characterize the size of a species and the contributing factors would be quite a bit of work, let alone to compare across the genus as a whole.