Unusual Development

Males with two large claws (a.k.a.Super-males)

The most commonly observed unusual development is the rare finding of a male with two large claws, rather than the typical one large and one small. The reason this is the most commonly observed is not necessarily that it is the most common in nature, just that it is the easiest to identify as this strange quirk of morphology can readily be seen through binoculars (unlike the others, which all require more direct examination of the reproductive organs). Males with two large claws are presumed to be at a disadvantage with respect to obtaining food, as feeding is generally done with the small claws, but as individuals with this condition are occasionally found in nature, they clearly can find a way to survive. There is some suggestion that this state is seen more commonly in laboratory-reared individuals where there may be reduced selection pressure.

A secondary issue is some of these studies have focused on very young juveniles, where the asymmetry may not be entirely set. It is not uncommon to find tiny crabs with two larger claws where these later transition to standard asymmetry as the further develop. This is very different than the rarer instances of full-sized adult males with two large claws.

Males with two small claws

Occasionally when examining large populations of fiddler crabs, male crabs have been found with two small claws largely indistinguishable from those of the females. Males with this condition would be at no survival disadvantage with respect to feeding, but might presumably have difficulty attracting mates. For very small juvenile male fiddler crabs, this is the standard condition before asymmetry develops. Males and females are largely indistinguisable at very small sizes as both asymmetry and other secondary sexual characteristics have not yet developed.

Females with one large and one small claw

A single instance of an otherwise fully functional female crab (based on exmination and dissection of the ovaries) with a single large claw and a small claw (otherwise identical to those in male fiddlers) has been reported in the literature. As with the previous case, this condition would not be obvious in wild populations as the crab would otherwise superficially appear male in the field.

Intersex crabs

Intersex crabs are those with a seeming mix of male and female features, generally such that the crab is neither clearly male nor female, even upon examination of the reproductive organs. This is a slightly different (although arguably overlapping) category than the previous two in which the reproductive organs appeared to be unequivocally male or female, with only the asymmetry of the claws reversed from normal.