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.

Constructing Fiddler Range Maps, part 1: History

This is the first of four planned posts about how I constructed the range maps for fiddler crabs. This first part (likely the longest) will give the history and background of how these maps were drawn in the first place. The second 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.

1. The Original Maps

In mid-2002 I decided to add maps depicting the distributions of each fiddler crab species to the website. I don’t recall why; possibly I just wanted updated information, possibly there was another motivation. This might be around when I expanded the site beyond its original collection of photos, videos, and references to be a bit more species-information-centric. The maps that ended up on the site were constructed in the roughest way possible.

The base data that went into each map started with the maps from Crane’s 1975 monograph.

Map 9 from Crane (1975) depicting the range of four fiddler crab taxa.

Using this as the starting range, I pored through the literature post-1975 (more or less) to look for publications that might have information altering the ranges presented by Crane. Once I’d established an updated range for a species, a background map of the coastline was drawn for the appropriate area using the first version of my spatial software PASSaGEThis map was imported into a graphics program (Photoshop? MS Paint?) and the range was hand drawn with a transparent brush and saved as a raster image (GIF of all things) for display on the site. Not precisely high-tech, but it got the job done.

Original hand drawn range map of Uca maracoani.

One obvious problem with this approach was updating a map often required redrawing it from scratch, particularly if the range needed to be reduced rather than expanded. Not the most efficient approach and not the best quality.

2. The Interactive Maps

At some point between 2011 and 2013, the entire website was rebuilt from the ground up; this is when the site was transitioned from hand-coded to dynamically created from a data back-end. How to handle the range maps was an interesting question, and after some exploration Google Maps seemed to be a good solution. By creating a custom KML layer for each species, I could use the Google Maps API to insert an interactive map into the webpages.

The challenge was how best to create the KML layers with the species ranges. The solution I came up with had a couple of different parts. One key element was to take advantage of the fact that (adult) fiddler crabs are essentially restricted to marine coastlines (they go a little bit inland into river mouths and estuaries and the like, but on the  global scale this is still within the margin of error of the “coast”). Juveniles presumably get a bit more into the ocean, but even there the limited data suggests most of them seem to stick close to the shore.

Thus, a set of coastline data can provide the entire framework on which to build a fiddler crab range: basically, any given piece of coastline can either be in the range or out of the range, and you don’t have to particularly worry about any space in between coastlines (whether land or open ocean). To create these, I imported a full set of world country borders from within Google Earth (Google Earth made it easy to quickly display, check, and update distributions without fighting with the vagaries of the online Google Maps API). For each country whose coastline had fiddler crabs, I exported the individual country to a KML file, then manually removed the parts of the boundaries that represented interior land borders, leaving only coastlines. Countries with borders that included multiple major oceanic regions were split into their constituent parts (e.g., an Atlantic Panama coast and a Pacific Panama coast). All of these coast outlines were re-imported into Google Earth. For each species it was then just a matter of copying the appropriate country borders into a new folder to represent that species; when only part of a country was in the species range, a copy of that country’s coastline was again exported, manually trimmed to the correct range, then re-imported. While seemingly a lot of up front work, for most species it became a fairly quick and easy method to produce the ranges. Each species had its own folder within Google Earth and the whole set was exported as a single KML file. I then wrote a program to extract all of the individual maps from this file, standardizing certain style elements, and exporting each one to its own individual file for integrating with the Google Maps API on the website. This also allowed the creation of the map which overlaps all of the species ranges with high transparency to get a worldwide view of species density.

Uca maracoani range displayed on Google Map embedded in website.

Updating or editing the maps, while still a bit of work, was substantially easier than with the original ones since it only required changing a subset of the coastline data, rather than redrawing an entire map by hand.

Interestingly, a few people complained about the new maps. As rough as they were, the old ones displayed the ranges in a fairly simple cartoon form which is lost with the more complicated Google Map backdrop; also, the new images are not easily exportable for use in another format (beyond doing a screen grab, as I had to do to display the above figure).

3. The New Static Maps

Recently, as part of some potential and planned updates to the site, I came to the realization that the inability to automatically export the maps to an image for use outside a webpage had become somewhat problematic. After a lot of time spent trying (and failing) to come up with ways to automatically export the layered Google Map, it finally occurred to me that the obvious solution was to back away from the Google Map approach entirely for non-web-based use. In fact, part of the solution was to go back to the beginning. As mentioned above, I’ve written code in the past that can draw a background map given a set of lines or polygons describing coastal outlines (data readily available from a number of online sources). And although it was never part of the original plan, the KML file that is being used to add the Google Map layers has all of the boundary data for the ranges in an already parsable format. Combining these into the site creation code with a drawing module that could export directly to a file suddenly allows us to automatically draw higher quality cartoon maps, much like the site originally had, but automated from the code rather than hand painted. (1) Use the world map data to draw a nice representation of key coastlines. (2) Draw the range data from the KML file on top of this in a different color and with a slightly thicker line to help make it stand out. (3) Export as a vector file format (SVG) rather than raster file to allow scalability for high quality figures. I still use the Google Maps and KML layers on the website for interactivity, but these new maps can be seen and downloaded by clicking on the link directly below the corresponding Google map (the link can be seen in the previous figure).

New range map for Uca maracoani.

On these new maps I decided to use a background map with country boundaries to help make the ranges more obvious, and filled in the land areas to make the maps clearer for species with more limited ranges, but unlike with the original maps, these decisions can be quickly changed and the maps redrawn with minimal effort since they are created programmatically. The range data itself is identical to that from the Google Maps.

For display purposes outside of webpages, these newer maps are quite nice. But all is not perfect if we actually start thinking about using these ranges as data rather than just visual guides. To be continued in part two…

The Indus Fiddler Crab, Uca sindensis (Fiddler Crab of the Week)

For our very first Fiddler Crab of the Week post, our random generator choose the Indus Fiddler Crab, Uca sindensis. Not the easiest species to start with.

The Indus Fiddler crab is a relatively unknown species found on the coasts of Pakistan, Iran, Iraq, and Kuwait. It has also been reported from the United Arab Emirates, near the Oman border (Ismail & Achmed, 1993), so it is feasible that it could be found on the southern shores of the Persian Gulf as well. It is arguably the most abundant species in Pakistan and most of what is known about its biology comes from the recent work and lab of Dr. Noor Us Saher at the Center of Excellence for Marine Biology at the University of Karachi.

It is a smallish fiddler crab (adult male carapace between 9 and 17mm wide, females a little smaller), lacking some of the more conspicuous colors of other species. It is generally grey in color; the carapace has grey and white bands, with the legs a bit darker grey-brown. The large claw of the male is described as generally pink, with a bit darker-purple toward the arm, shifting toward white toward the top and along the dactyl (the upper/movable finger), and more red toward the pollex (the lower/fixed finger). Unfortunately, I cannot find a photo of a living specimen, but some of the basic color can still be seen in this collected specimen.

A specimen of Uca sindensis from Pakistan. Photograph by Sahir Odhano.

This crab is usually found on open mudflats or muddy-sand in the upper intertidal zone, away from the waterline during low tide. In Pakistan it is often found intermixed with another species, Uca iranica; other fiddler crab species found on the same beaches are less likely to directly intermix.

Very little has been reported in the literature about its behavior, although like many species its waving display appears to involve much more than just the large claw. From minimal observations, Jocelyn Crane (1975, p. 108) stated:

“Wave obliquely vertical to lateral straight, the major cheliped not fully unflexed. Minor makes corresponding motion. Several ambulatories on minor side raised in turn during display.”

Phylogenetically, the Indus Fiddler Crab falls into the general category of “broad-front” species from the Indo-West Pacific (IWP) region. It was originally thought to be closely related to (and a subspecies of) Uca inversa, but now not only is recognized as a separate species, but thought to belong to an entirely different group of broad-front crabs (the subgenus Austruca).

So that’s the first Fiddler Crab of the Week entry. It would figure that my first one would involve one of the least-well known species on the planet (although not the very least).

Fiddler Crab of the Week

Starting next week, I’m going to pick one fiddler crab species at random every week to write a post about (no duplicates). With more than 100 species, it’ll take a bit more than two years to do them all, which if nothing else should give me some motivation to keep posting. It’ll be interesting to see which ones are easy to write about and which ones take a little more effort to find interesting tidbits.

Visual Chronologies of Names

One of the things to realize about the site is that it is now as much a research project as it is an information portal (it didn’t start this way…maybe I’ll write up the history of the site at some point). The research project end is a bit poorly defined, but could roughly be viewed as “what types of things can I say, infer, or visualize about fiddler crabs if I collect a whole lot of data on the back end.” Yeah…not very specific.

One idea that had been floating in the back of my mind for quite awhile was how to to visualize changes in taxonomic name usage over time. If one name replace another, how do we show that visually? Not surprisingly, others have thought about this and come up with some solutions. In late December I was sitting in an airport preparing to leave the country for the holidays when I stumbled across a blog post by Rod Page, Taxonomic name timelines for BHL, about just such a question. He mentioned an earlier project called Synonyms that would produce nice looking name chronology charts by mining data from the Biodiversity Heritage Library; unfortunately, Synonyms had gone defunct and Rod had hacked together a rough replacement (as is his wont to do). The Synonyms graphs were exactly what I had been trying to come up with so immediately made note to try to do this with my own data when I got back.

It only took me a few hours in early January to extract the data I needed and get some basic-Synonymy style charts plotted. It took quite a bit longer to fiddle with their formatting to get them “just right.” Let’s look at an example.

For a simple case, let’s compare the names of the genera. For most of their history, fiddler crabs have been placed in one of two genera: for most of the 19th century they were considered to be Gelasimus; since the start of the 20th century they have mostly been placed in Uca. Most of the other genera which have included fiddlers (23 in total) make up a tiny proportion of references relative to these two. Here are their chronology plots (I’m still struggling to come up with a good name for these):



These charts represent the number of publications per year that used each genus as an accepted name (whether a publication named one species or ten, it only counted as one valid use of the name).

Caveat: the database is still largely incomplete after about 1977, so the parts of the figure from the last 40 years are not representative of the full pattern.

A few things stand out from looking at these charts  (some of which I knew already):

  1. Generally, references to fiddler crabs skyrocketed starting in the 1950’s (this is when fiddlers really started being  used as model organisms to study certain aspects of behavior and physiology).
  2. It took about 15 years for the transition to Gelasimus to Uca to really take hold; from the late 1890’s through the late 1910’s, both genera were used in roughly equal numbers.
  3. Although Gelasimus was technically abandoned by taxonomists at the start of the 20th century, the genus continued to be used at a small, but steady pace at least through the 1970’s. It’s hard to see on the graph, but there has been at least one use of the genus as valid as recently as 2006.

Something not obvious from this chart: most of the uses of Uca prior to 1897 are actually references to a different type of crab. For most of the 19th century, Uca was used for crabs in the genus we now call Ucides (the somewhat tortured history of these names has been discussed by others and is summarized in the Systematics section of the main website).

To some extent, these figures aren’t quite as exciting as I’d hoped they’d be, but they still visually illustrate the name usage in a direct and attractive way. These charts are not live on the website yet, but the capability to add them is now built-in to the code and graphs depicting name usage for binomials and specific names, as well as directly comparing synonyms will all be made available at some point in the future, once a few more kinks are worked out.

Once again, into the breach…

On occasion in the past I’ve attempted to start a blog and each attempt has ended rather abruptly with rather few posts. We’re going to try this again and see what happens.

This particular blog is attached to my fiddler crab website ( and will mostly be focused on thoughts about these little critters and their biology and evolution, the website, the science, or related issues such as biodiversity informatics and cyber taxonomy. Or maybe just other stuff as it occurs to me if I find I have nowhere else to vent.

The look of the blog is amazingly bare bones right now, but we’ll see about sprucing it up into something slightly more visually appealing as I get into the hang of this (or not).

I’ll probably tackle the first real posts in the next day or so. I’ve got a couple of topics already in mind…