Fiddler crabs are intertidal animals that
live in mangrove forests, tidal creeks, sandbars, mudflats or,
occasionally, stone or boulder beaches. They can occur in huge numbers,
with thousands of individuals living in small, adjacent territories. Males
and females live intermixed and each individual has its own burrow and a
small area of surface sediment around it. The burrow is extremely
important. It is a refuge during high tide, and during low tide it is a
source of water for keeping the gills wet, it is an escape from predators,
it is the site of mating and incubation. The space
around the burrow is used for feeding and courting.
Typical Fiddler crab social system
Males wave their enlarged claw to attract females for mating. When a female is ready to mate (i.e. she has ripe ovaries), she will leave her own burrow and wanders through the population of waving males. She visits several males before selecting a mate (up to 24). A visit consists of a direct approach to the male, he then darts into his burrow, the female follows him in, spends a few seconds underground and then reappears to continue wandering. When the female accepts the male as a mate, she does not reappear, but rather the male emerges about 5 minutes later and gathers up sand to plug the burrow entrance. He plugs it from the inside, sealing him and the female underground. Mating occurs in the terminal chamber of the burrow and the following day, the male emerges. He reseals the burrow entrance with the female underground. The male then leaves the area and wanders around to find an empty burrow or to fight another male for its burrow. The female remains underground for the following two weeks while she incubates her eggs. She does not emerge to feed during this time.
Females time mating so that egg release coincides with a Spring tide. The eggs are released during a nocturnal high spring tide so that the tiny, swimming larvae are washed far out to sea. They develop out there for several weeks and then wash into shore. They settle out of the water column when they detect the odour of adult crabs of their own species. They undergo a final moult to become minute little crabs, and then start their lives on the mudflats.
Fiddlers as a study species
Fiddlers are very active during diurnal low tides. They are constantly busy waving, fighting, mating, feeding or cleaning up around their burrows. Because they occur in such high numbers, data collection is extremely simple. They are easy to watch, either with binoculars or even just by eye. You can catch and mark them with little effort, and they can be ‘caged’ on the mudflat by surrounding them with mesh walls pushed into the sediment. They are the ideal animals for manipulative experimental work.
Females breed every month (two weeks of incubation, two weeks of feeding) and males are ready to mate virtually every day, so you can collect mating data with ease. Fighting is also common between males and between females. Males very rarely fight females or steal burrows from females, although I have no idea why not.
The best part about working on fiddler crabs is that you get to spend the field season in a pleasant tropical environment, sitting on a beach and getting so much data you feel positively smug. Also, you are constrained by the tides, so you can only collect data for a maximum of six hours a day. Bummer!
I can only think of one disadvantage to studying fiddlers. You cannot follow consecutive generations or measure the success of the offspring. They get washed out to sea and may reappear at any of the surrounding mudflats depending on the currents (this means that populations are not genetically isolated). You can easily catch the offspring as they are released, but they are difficult to rear in the lab.
There are about 100 species of fiddler crabs worldwide. They all belong to the genus Uca, family Ocypodidae, order Decapoda. There are approximately 20 species in Australia, 11 of which are endemic. Nine species occur in the Darwin area:
We have worked mainly on Uca mjoebergi, Uca capricornis and Uca dampieri. We are all behavioural ecologists. We study the mating, fighting, signalling and ecology of the fiddlers in their natural environment. This involves both observational and experimental work. Four examples of the type of work we do are:
THE MATE CHOICE PROCESS
We have recently teamed up with Stephen Sims who has built us robotic crabs. This means we can now run controlled choice experiments in which we offer a female a choice between two robotic crabs that differ in wave rate, claw size, wave timing, claw colour or any other factor we wish to manipulate. This has opened up a whole range of detailed manipulative projects that look at exactly what it is that females are selecting.
FIGHTING NEIGHBOURS AND STRANGERS
Fiddlers are amazing little animals. They are fun to watch, easy to study and they do really intersting things. It is strange that ALL biologists don't study them. They are much better than birds or mammals or fish or snakes.