Fiddler crabs are small crabs of the genus Uca, the ones in the Ria Formosa are Uca tangeri. There are other crabs in the Ria Formosa but it will be the Fiddler Crabs that catch your attention with their extreme cheliped asymmetry.
In other words you will be amazed by the male crabs’ one very large claw!
As we discovered though, winter visitors are unlikely to see Fiddler Crabs in the Algarve, as they have a thing about really warm days. 
Surface temperature has to be at least 18degrees. However from late March until October you should spot them if you are a careful walker at low tide.
They are called Fiddler Crabs because of the effect they create when waving their large claw – when there are lots of them it looks like lots of fiddlers in an orchestra.
The waving takes place in earnest during the courtship phase, but even when they are just moving them at other times you will get a feel for the orchestra effect.
When not on the surface they are in their burrows, apparently they move burrows regularly, with 20% moving on within 7 days, and 90% within two weeks. 
The burrows as well as their homes during high tide and the winter, are also a pre-requisite for successful mating. In the summer months at low tide if you keep very still and very quiet, spot them fighting over a burrow. If you are visiting over the Winter then the best you can hope for is to spot the holes to their burrows, which are opened up by the crabs periodically to refresh the air inside.
The crabs are usually brown but during the ‘courtship’ phase they change colours, and you will see shades of orange and purple which look very striking against the green algae on the mudflats.
Most of my information has come from Birgit Wolfrath’s research paper – certainly worth a read.
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