Each ship was commanded by a "trierarchus", whose exact relationship with the ship's centurion is unclear. Squadrons, most likely of ten ships each, were put under a "nauarchus", who often appears to have risen from the ranks of the "trierarchi". The post of "nauarchus archigubernes" or "nauarchus princeps" appeared later in the Imperial period, and functioned either as a commander of several squadrons or as an executive officer under a civilian admiral, equivalent to the legionary "primus pilus". All these were professional officers, usually "peregrini", who had a status equal to an auxiliary centurion (and were thus increasingly called "centuriones " after ca. 70 AD). Until the reign of Antoninus Pius, their careers were restricted to the fleet. Only in the 3rd century were these officers equated to the legionary centurions in status and pay, and could henceforth be transferred to a similar position in the legions.