Current Biology Vol 16 No 2R40

Quick guide

Fiddler crabsJochen Zeil1, Jan M. Hemmi1and Patricia R.Y. Backwell2

What are fiddler crabs? Fiddlercrabs belong to the genus Uca.They are members of theocypodid family of brachyurancrabs, the most recent marineanimals to have invaded land.They spend the first part of theirlife as aquatic plankton and onlysettle in the intertidal zone aftertheir last larval moult. Adults livein burrows on intertidal mud- andsand-flats within dense, mixed-age, mixed-sex and mixed-species colonies (Figure 1). Eachadult defends his or her ownburrow and a small area aroundit. They are active on the surfaceduring low tide, feeding on algae,bacteria and detritus in thetopsoil. It is thought that fiddlercrabs can live for up to sevenyears and adults of the largestspecies can reach a body size ofabout 5 cm. The crabs grow bymoulting which, under favourableconditions, they do about everyeight weeks.

Fiddler crabs are highly socialanimals with a rich behaviouralrepertoire. They communicate byvisual and vibratory signals; theyhave complex territorialinteractions and flexible courtshipand mating systems. Somespecies carry individually distinctcolour patterns and some otherseven build mud or sandstructures as homing aids and toenhance or limit socialinteractions. As their commonname suggests, one of the mostobvious behaviours in a fiddlercrab colony is claw waving: maleswave their one enlarged claw toattract females for mating and torepel intruders from their territory.The massive claw can weigh halfa male’s body weight and is alsoused as a weapon. Interestingly,handedness differs amongspecies: in most species there areequal numbers of left- and right-handed males, but in a fewspecies virtually all the males are

right-handed. We do not know yetwhat determines handedness norwhat are its social consequences.

Fiddler crabs have two distinctmating strategies, with somespecies exhibiting both forms. Inone strategy, females leave theirburrows and move through thecolony visiting many males beforechoosing a mate. Males wavevigorously to attract thesefemales to their burrow, wheremating takes place undergroundand where the female willincubate her eggs. In the otherstrategy, mating takes place atthe entrance to the females’burrow, and it is the males thathave to search for and locate thefemales. Little or no wavingprecedes surface mating. In somespecies that have both matingsystems, the relative proportionof each type depends on the risksof wandering: females stopsearching for suitable mates ifpredation pressure becomes toohigh, leaving the males to riskmoving across the mudflat insearch of receptive females.

What is special about them?Fiddler crabs exhibit manyadaptations to life on land and —for an invertebrate — showsurprising behavioural complexity

and flexibility; they are excessivecommunicators that can set themudflat in motion with their mass-waving displays; their stalkedeyes are highly specialized forvision in a flat world; and theirminiature societies areexceptionally accessible fordetailed observation and analysis.

What decisions do fiddlercrabs have to make? Like mostsocial animals living in dynamicenvironments, fiddler crabsconstantly have to makedecisions. They need to feed,maintain their burrows, establishand maintain neighbourhoodrelations, avoid predators andpursue mating opportunities.While we do not know how theymake these crucial decisions, wedo know that they are expertsurvivors with complex andflexible responses to the manycompeting interests they face. Weknow that they are exquisitelysensitive to bird-like objectsflying overhead and to crab-likeobjects approaching their burrow.We also know that they careabout their neighbourhood — tothe extent that they come to theaid of weaker neighbours trying tofight off wandering burrowsnatchers. They are capable of

Figure 1. View across a Uca vomeris colony at Bowling Green Bay, Queensland, Aus-tralia. Inset shows a male Uca polita (left) and a female Uca vomeris (right).