What is the starters command signal in swimming?

In heat one: one long whistle means for the swimmers to , then the official says "take your mark". A gun, whistle, or beep may be heard as the signal for "GO".

In further heats: as the last swimmer from the heat before reaches the flags, 3 short whistles are heard to signal the next heat of swimmers that it is time for the next heat to get ready. The same commands as above then apply.