An animal hard to miss on a sandy beach is the crab. But surprisingly you can also find these little creatures scuttling about in the wet slushy mud, waving their arms about if you happen to be visiting a mangrove region or an estuary. These are fiddler crabs and the waving ‘arm’ is one of their claws.
Like the insects, crabs are also arthropods but belong to a completely different class called Crustacea, which also includes lobsters, prawns, and shrimps. Fiddler crabs are among the best studied ones and belong to the family Ocypodidae. Another type of crab called Ghost crab also belongs to this same family of fiddler crabs. There are currently about 97 species of fiddler crabs worldwide.
Fiddler crabs are small, semi-terrestrial and live in the intertidal regions. In fact, fiddler crabs are one of the first creatures you will meet in mangroves. The mudflats are a favourite place of theirs, where they live in burrows. Their burrows are located in the intertidal zone, and at low tide the crabs come out for feeding and courting. Large crabs, fish, some mammals and birds prey upon these crabs. A particular species of snake, commonly called the mangrove snake, goes into the burrows of these crabs to hunt them out. Those that live in sandy regions do not survive well in the marshy areas, for they feed on sediment particles of certain size.
For the classroom
- Two topics that would make a good project for outdoor study for class 7 or 8 students are:
- Do right-clawed fiddler crabs engage in significantly more fights than left-clawed ones?
- Is there a difference in the speed or rate of feeding between male and female fiddler crabs?
- Study of shapes of the major claw in fiddlers: There is diversity even in the shape of the major claw. This could be an interesting study.
- Use these diagrams of the crab and the major claw to add details of colour and pattern, in order to keep track of the crabs you will observe during your study.
Crabs and maths: Observe how far the crabs move away from their burrows. Estimate the distance. Observe them again, mark the spots and measure using a tape. Determine whether your estimation was correct.
The author is a consultant for science and environment education. She can be reached at firstname.lastname@example.org.