The other members of the Fish family were Sturgeon, the Siemens and Halske T52 Schlüsselfernschreibmaschine (‘Cipher Teleprinter Machine’), Thrasher was probably the Siemens T43, a one time tape machine. (Sturgeon, on the other hand, was not an attachment but a combined teleprinter and cipher machine.) At the sending end of a Tunny link, the operator typed plain language (the ‘plaintext’ of the message) at the teleprinter keyboard, and at the receiving end the plaintext was printed out automatically by another teleprinter (usually onto paper strip, resembling a telegram).
The transmitted ‘ciphertext’ (the encrypted form of the message) was not seen by the German operators.
One carried the radio equipment, which had to be kept well away from teleprinters for fear of interference.
The other carried the teleprinter equipment and two Tunny machines, one for sending and one for receiving.
Using the Bletchley convention of representing a pulse by a cross and no pulse by a dot, the letter C, for example, is •••. gave each link a piscine name: Berlin-Paris was Jellyfish, Berlin-Rome was Bream, Berlin-Copenhagen Turbot (see right-hand column).