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172. An orchestra of East African Fiddler Crabs (Cranuca occidentalis)


Mudflats are unique ecosystems that are exposed at low tide, providing an opportunity for the observation of the diverse organisms that inhabit these environments. One of the most fascinating creatures that can be observed in mudflats in the Western part of the Indian Ocean is the East African Fiddler Crab (Cranuca occidentalis). These small crabs are typically found in the intertidal zones of estuaries, mangroves, and mudflats along the eastern coast of Africa.

The East African Fiddler Crab is a detritivore, meaning that it feeds on detritus, which is composed of dead plant and animal matter. Detritivores play a crucial role in maintaining the health of wetland, marsh, and mangrove ecosystems, as they recycle nutrients and break down organic matter into simpler compounds that can be used by other organisms. The East African Fiddler Crab sifts through the sediment in search of food, using its specialised front claws to filter out organic matter from the mud. This process not only helps to clean the environment of detritus but also aerates the substrate, which promotes the growth of microorganisms that are important for the health of the ecosystem.

In addition to their role in maintaining the health of the ecosystem, the East African Fiddler Crab is also an important food source for many marine birds, such as egrets and herons. These birds are often seen feeding on the crabs during low tide, when they are exposed on the mudflats. The crabs are also preyed upon by other animals such as fish, reptiles, and mammals.

One interesting behaviour of the East African Fiddler Crab is its digging of burrows. The crabs construct their burrows in the mudflats using their specialised front claws, which are asymmetrical in size, with one claw being much larger than the other. The large claw is used for communication and territorial displays, while the smaller claw is used for digging and sifting through the sediment. The burrows serve as a refuge from predators, as well as a place for the crabs to retreat during high tide.

The East African Fiddler Crab is also known for its unique mating behaviour. During mating season, the male crabs wave their large claws in the air to attract females. The female then chooses a male based on the size of his claw and the quality of his burrow. Once a mate is chosen, the female enters the male's burrow, where she lays her eggs. The male then fertilises the eggs and guards them until they hatch.

Despite their importance in maintaining the health of mudflats and other wetland ecosystems, the East African Fiddler Crab is facing several threats. One of the major threats is habitat loss due to human activities such as coastal development and pollution. In addition, the crabs are also harvested for human consumption, which can have a significant impact on their populations if not managed sustainably.

Efforts are being made to conserve the East African Fiddler Crab and its habitat. One approach is the establishment of marine protected areas, which restrict human activities in certain areas to preserve the biodiversity of the ecosystem. Another approach is to promote sustainable harvesting practices, which ensure that the crab populations are not depleted beyond their ability to recover.


It was quite a challenge to film these little crabs (they were a mere centimeter/ 0,4 inch) . I had to lie down on the mudflat, trying to make myself as still as possible peeking through my viewfinder. At the slightest movement of my arm, my head or the camera they disappeared with lightning-fast speed into their burrows (sometimes called “chimneys”) only to reemerge many minutes later. It took a whole morning of filming to get less than 2 minutes of good quality footage.

Filming location:

This short underwater videoclip has been filmed in Mauritius 🇲🇺

More on this topic:

For another in-depth description about Fiddler Crabs please go to our vlog post 160 or click on this link: https://www.beyondscuba.com/post/tetragonal-fiddler-crab-gelasimus-tetragonon

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