Lably
Skip to content

Fiddler crab

Genus of crabs

  • 13 min read
  • 4 chapters
Fiddler crab ( Minuca minax ) from NOAA CSC.
NOAA · Public domain

Related on Lably

Through Beach, Atlantic Ocean and Portugal

Lab · Class 2Skip CountingLab · Class 6–12Ancient Indian Ocean TradeLab · Class 6–10Maps & ProjectionsLab · Class 7–10Trade Routes

A smaller number of ghost crab and mangrove crab species are also found in the family Ocypodidae. This entire group is composed of small crabs, the largest being Afruca tangeri which is slightly over two inches (5 cm) across. Fiddler crabs are found along sea beaches and brackish intertidal mud flats, lagoons, swamps, and various other types of brackish or salt-water wetlands. Whilst fiddler crabs are currently split into two subfamilies of Gelasiminae and Ucinae, there is still phylogenetic and taxonomical debate as to whether the movement from the overall genus of Uca to these subfamilies and the separate 11 genera.

Like all crabs, fiddler crabs shed their shells as they grow. If they have lost legs or claws during their present growth cycle, a new one will be present when they molt. If the major claw is lost, males will regenerate one on the same side after their next molt. Newly molted crabs are very vulnerable because of their soft shells. They are reclusive and hide until the new shell hardens.

In a controlled laboratory setting, fiddler crabs exhibit a constant circadian rhythm that mimics the ebb and flow of the tides: they turn dark during the day and light at night.

Ecology and life cycle

Habitat and diet

Fiddler crabs primarily exist upon mudflats, sandy or muddy beaches as well as salt marshes within mangroves. Fiddler crabs are found in West Africa, the Western Atlantic, the Eastern Pacific, Indo-Pacific and Algarve region of Portugal.

Whilst the fiddler crab is classified as an omnivore, it does present itself as an opportunist and will consume anything with nutritional value. The crab will feed through bringing a chunk of sediment to its mouth and sifting through it to extract organic material. This crab will filter out algae, microbes, fungus or any form of detritus. Once finished consuming all the organic matter from the sediment, these crabs will then deposit them as small sand balls near their burrow.

Fiddler crabs are thought to potentially act as ecosystem engineers within their habitat due to the way they rework the sediment during feeding. Whilst these crabs do rework the sediment around them, upturning the very top layer and depositing it nearby, there is still debate that exists as to whether this turnover of sediment has any proven difference regarding nutrients and aeration of the sediment.

Burrows

Fiddler crabs are a burrowing species, where within their territory they may possess several burrows. There are two types of burrows that the fiddler crabs can build, either breeding burrows or temporary burrows. Temporary burrows are constructed by both males and females during high tide periods. These burrows are also constructed at night time when the crabs are no longer feeding and are hiding from predators. Breeding burrows are constructed by solely males, and will be constructed within the area that they have deemed their territory. These breeding burrows are constructed by male crabs so that the female and male crabs may copulate within the burrow, and the female may deposit and incubate her eggs within this area. Larger males who can more easily defend their territory will often have multiple suitable breeding burrows within their territory to enable them to mate with multiple female crabs. Female crabs are found to prefer to mate with males that have the widest burrows, however, carapace width and claw size does correlate with the width of the burrow, so could be a potential size bias.

Roles

Two types of fiddler crabs are found to exist within a given territory, a wandering female or male, and territory-holding male or females. When in a wandering state, this means crabs do not currently occupy a burrow. They will wander in order to look for territory which contains a burrow, or to look for a mate. Wandering females will look for a mate to copulate with, usually preferring to mate with a male that currently possesses a burrow. The female fiddler carries her eggs in a mass on the underside of her body. She remains in her burrow during a two-week gestation period, after which she ventures out to release her eggs into the receding tide. The larvae remain planktonic for a further two weeks.

The mating system of fiddler crabs is thought to be mainly polygynous, where the male crabs will mate with multiple females if they have the opportunity to, however, female fiddler crabs such as the Austruca lactea are known to also mate with multiple males.

Moulting

As they are a species of crustacean, they perform ecdysis, which is the process of moulting. When crabs moult, they produce hormones which trigger the shedding of their exoskeleton and regeneration of limbs. Moulting is already an extremely stressful time for fiddler crabs, as their shell becomes extremely soft, leaving them vulnerable to predation. When undergoing this moulting cycle, crabs will frequently hide within their burrows to avoid harm. When male crabs are undergoing the moulting process, if they are exposed to other male crabs in high grouping with consistent light, their ability to regenerate limbs will be impaired.

Claw

Whilst the crabs major claw does function as a tool for fighting and competition, it also plays a role in thermoregulation. As the claw is so large, and these crabs live in generally hot territory, so require strategies to keep themselves cool, particularly for wandering males without burrows. The presence of the major claw upon the male helps them keep their body temperature regulated, and decreases the chance of them losing or gaining too much heat in a given time period. The large claw draws away excess body heat from the core of the fiddler crab and allows it to dissipate. Heat is found to dissipate significantly faster when male crabs are performing waving at the same time.

Appearance

Fiddler crabs come in many different colourations and patterns, and are known to be able to change their colour over time. Fiddler crabs such as the Tubuca capricornis are capable of changing their colour rapidly when placed under significant stress. When fiddler crabs undergo moulting, they are seen to have reduced colouration after each sequential moult. Female fiddler crabs are traditionally more colourful than male fiddler crabs. Conspicuous colouring in fiddler crabs is dangerous as it increases predation rate, however, sexual selection argues for brightly coloured crabs. Fiddler crabs have finely tuned visual systems that aid in detecting colours of importance, which aid in selecting coloured mates. When given the choice, females prefer to pick males that are more brightly coloured in comparison to dull males.

Sign in to keep reading

3 more chapters are free with a Lably account.

More in Animals

Monotypic taxonTaxonomic group with only one subordinate taxonSpecies descriptionFormal description of a discovered speciesParaphylyType of taxonomic groupIUCN Red ListInventory of the global conservation status of biological speciesSubtropicsGeographic and climate zoneCommon nameName generally used for a taxon, group of taxa or organism(s)