Fiddler crab

Fiddler crabs are semiterrestrial marine crabs belonging to the family Ocypodidae. This family contains about one hundred different species. They are well known for extreme sexual dimorphism. This term describes how males and females of the same species look very different. In males, one major claw is significantly larger than the minor claw. Females have two claws that are the same size. The name "fiddler crab" comes from the way the small and large claws look together. They resemble a musical fiddle.

These crabs live in various saltwater wetlands. You can find them on sea beaches and brackish intertidal mudflats. They also inhabit lagoons, swamps, and salt marshes within mangroves. Their range is quite large and global. They are found in West Africa and the Western Atlantic. They also live in the Eastern Pacific and the Indo-Pacific. Some species live in the Algarve region of Portugal. Most are small crabs. The largest species is Afruca tangeri, which is slightly over two inches or 5 cm across.

Fiddler crabs are omnivorous opportunists. This means they will eat anything with nutritional value. They feed by bringing chunks of sediment to their mouths. They sift through the sediment to extract organic material. This material includes algae, microbes, fungus, or detritus. Once they finish eating, they deposit the leftover sediment as small sand balls near their burrows. This feeding process may make them ecosystem engineers. They rework the sediment by upturning the top layer. Scientists are still debating if this turnover changes nutrient levels or aeration in the mud.

To grow, these crustaceans must perform ecdysis. This is the scientific name for the process of moulting. During ecdysis, hormones trigger the crab to shed its exoskeleton. This is a very stressful and vulnerable time. The new shell is very soft, so crabs hide in burrows until it hardens. If a crab loses a leg or claw, a new one appears during the next moult. Males can regenerate a lost major claw on the same side. However, the regenerated claw is often substantially weaker than the original. This is called dishonest signalling because the claw looks strong but performs poorly.

Crabs build two main types of burrows. Temporary burrows are built by both males and females. These are used during high tide or at night to hide from predators. Males also build breeding burrows within their territories. These are used for copulation and for the female to deposit and incubate her eggs. Larger males may have multiple breeding burrows to mate with many females. Females often prefer males with the widest burrows.

Males use complex signals to attract mates. They perform a waving display by raising the major claw and dropping it in a beckoning motion. There are two types of waving. Broadcast waving is a slow pace used when no females are visible. Directed waving is faster and occurs when a male spots a female. Sometimes, many males perform synchronous waving at the same time. While this is a cooperative behavior, it does not benefit the individual male. Females prefer to mate with the male who is leading the wave.

Coloration also plays a role in their lives. Females are traditionally more colorful than males. Some species, like Tubuca capricornis, can change color rapidly under stress. Females use their visual systems to select mates. They generally prefer males that are more brightly colored. Beyond waving, crabs build structures like chimneys, hoods, pillars, semidomes, mudballs, or rims. About 49 species in the Ocypodidae family build these sedimentary pillars. Females are more likely to be attracted to a male who has a pillar outside his burrow.