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).
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:
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).
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:
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.