The Female Uca Pugnax Fiddler Crab

Usually when one thinks of fiddler crabs they think of the males who have one large claw and one small but we cannot forget to consider the females (without which there would be no males).  The female uca pugnax fiddler crab is an essential part of her brackish marsh ecosystem.  Females generally eat more and in a shorter amount of time thanks to having two small claws that are commonly called "feeder claws".  They clean the sand and mud of detritus and dead things, thus making for a cleaner marsh.  Males also clean the sand of such debris but they are hindered by their large claw and therefore do not consume as much in the same time frame as the females do.  Due to the male's large claw which is an unfortunate hindrance and burden, he is also more prone to predation than the females. 

The females are the carriers of the eggs.  Once a female becomes fertilized by a male she will form an egg band (which looks like a small dark sponge) on the underside of her carapace.  This band is very noticeable.  The eggs are attached with a sticky substance.  In the wild the females release the eggs into the tides after about two weeks of incubation.  The eggs then hatch in the ocean and the baby crabs (also called larvae) become part of the zooplankton population.  Females can mate roughly every 4 weeks during spring and summer (and in captivity into later autumn, possibly even in to winter if the water temperature remains above 70 degrees Fahrenheit--this is a personal observation).  I must note that i have witnessed my female crab Fern eating the eggs off of her after they've been present for 2 weeks.  I am not 100% sure but that seems like a normal behavior that would occur if the female were not able to dispatch the eggs into the ocean.  I have also noticed that after each egg cycle she moults.

Fern, Female Uca Pugnax Fiddler Crab with Egg Sponge

Uca Pugnax females are burrowers and in the wild use those burrows to reside in while they are incubating (waiting for embryonic development) their eggs.  In captivity however the female may not be able to make a burrow so she may hide in an aquarium ornament or if she's not threatened by her tank mates, she may not hide at all.  Fern is not one to hide while she's carrying eggs and very much enjoys being out and about. She also enjoys staring at Bob in the other tank. I do feel Fern has a liking for Bob, it's a pity Bob is a jerk and cannot live with Fern and Steve.

Female crabs can be very social with other females and males but sometimes disputes do arise and females can become aggressive with both sexes.  Though it is somewhat rare for the females to get into altercations with males, it is not so rare for females to get a bit rowdy with other females.  Such disputes usually occur over territory and burrow/aquarium ornament rights but they usually don't involve injury.

While similar to the males, the females do have some differences.  If one observes the underbelly of the female they will see her carapace looks different than a male's.  There are also the obvious claw differences and sometimes the males will be darker or brighter in colour than the females.  In the aquarium setting females are more likely to escape because their climbing skills are a little better than the males' because the females do not have the hindrance of the large claw.

Fern attempting to climb the filter and aquarium wall

Fern side view (looking into Bob's tank)