According to the National Cryptologic Museum, the Tunny and Sturgeon machines (referred to by American cryptologists as the "fish" machines) were both on-line cipher machines, which meant that a message could be simultaneously enciphered and transmitted, thus saving a great deal of time.

Although a British cryptanalytic attack made considerable progress, the results were far slimmer than against the Enigma, both because the difficulty of attack yielded fewer breaks, and because there was a lot less traffic sent over these systems.

One can only admire the great workmanship and the mechanics of this Tunny machine which is on display at the National Cryptologic Museum. (Photo by Jerry Proc)