One of the most striking features of fiddler crabs is the extreme claw asymmetry found in males and one of the most common questions I get asked is whether males tend to be right- or left-handed. Most people seem to assume they are mostly one or the other, but in fact, most fiddler crab species appear to be evenly split 50:50 between right- and left-handed males. The exceptions are the species in the genus Gelasimus, where males are almost entirely right-handed (generally >95%).
An extremely common misconception is that when a male fiddler crab loses its large claw that the handedness switches and the small claws grows into a new large one. This is simply wrong! The crab will regenerate a large claw on the same side; the small claw remains small. This urban legend is so persistent that it's appeared (and been removed) from Wikipedia multiple times. Judith Weis recently wrote a paper about the origins and persistence of this myth (Weis 2019).
How handedness is determined has been studied by researchers numerous times, but the answer is still not entirely clear, as different studies have not always come to the same conclusion and there are a number of possible explanations for the variation in results. For the mostly right-handed species, right-handedness is likely genetically hardwired with the occasional left-handed individuals probably due to environmental damage at a critical developmental stage leading to the development of left-handedness. For the rest of the species, it is probably randomly determined by the environment, although there could be a genetic factor or a combination of both. Breeding experiments to examine heritability in fiddlers are not particularly easy or feasible. Some of the studies on this question include Morgan (1923), Morgan (1924), Vernberg & Costlow (1966), Yamaguchi (1977), Yamaguchi (1978), Ahmed (1978), Ahmed & Khan (1978), Krishnan (1992), and Yamaguchi & Henmi (2001).
Species information pages on this site include a link to information about the handedness for that species, including the predicted handedness based on taxonomy and the observed data that has been recorded (if any), including an overall estimate of the handedness based on the (usually limited) available data. Data has been updated to reflect our current taxonomy rather than the name used in the publication; some samples may include mixed species if certain species boundaries were not recognized at the time of original publication. Not all papers that record handedness have been entered yet, but many species have not been formally examined for handedness ratios, particularly with large sample sizes. Sometimes when they are commented upon, key data and specifics are not provided. For example, Altevogt & Davis (1979) report that 23 non-specified species have 50:50 ratios, and that more recent re-examination of three of these species still have 50:50 rations, while a fourth has a 50:50 from over 400 specimens examined. No actual counts are specified. They then state that in another species, only one left-handed male was found out of more than 1,000 examined. While useful general statements, these numbers lack the specificity one would want for computational analysis (For what it's worth, they also claim that the right-handed skewed species used to be 50:50 and shifted to the skew, but it is more likely that they never paid close attention in previous years.)