Soft shell crabs are a seasonal Venetian treat, as they are in the US. In Venice, they are referred to as moleche. These crabs are a different species than found here in the US, they are smaller (about 2-3 inches), and are available twice a year - in the fall and spring.
Males and females both shed their old shell in the spring, while only males molt in the fall. There is a 5-6 hour window of opportunity from when the crabs shed their hard shell, before the new, soft shell hardens again with contact with water. Timing therefore is crucial to the whole enterprise.
The moleche fishermen of Venice (molecanti) are masters as managing the molting process of their harvest. There is only a very brief 5-6 hour period in which the shells are soft enough to eat, as continued contact with water will harden them in a matter of hours. The trick is to identify those crabs that are just about to molt from those that are not; the former are stored in tanks until they have molted, at which point they are taken to markets such as the amazing fish market at Rialto. The latter are held back until their molting time has come. The crab stocks were almost depleted in the 1980s. Today they are farmed in various locations around Venice, and it now is an important industry in the region.
Every fish restaurant in Venice will feature soft shell crabs on its menu during the season. We were lucky enough to be in Venice last September with one of our Chefs on Bikes trips, and were treated to some wonderful Venetian soft shell crabs are our welcome dinner at Villa Margherita. They were simply sauteed, and served in a very traditional manner with grilled polenta.