The Roman Republic was the first to understand the importance of professional soldiers dedicated to melee combat onboard of ships. During the First Punic War, Roman crews remained inferior in naval experience to the Carthaginians and could not hope to match them in naval tactics, which required great fleet maneuverability and tactical experience. The Romans therefore employed a novel weapon which changed sea warfare to their advantage — they equipped their ships with the corvus, a long pivoting plank with a beak-like spike on the underside for hooking onto enemy ships, possibly developed earlier by the Syracusans against the Athenians during the Sicilian Expedition of the Peloponnesian War. Using it as a boarding bridge, Roman infantrymen were able to invade an enemy ship, transforming sea combat into a version of land combat, where the Roman legionaries had the upper hand. During the early Principate, a ship's crew, regardless of its size, was organized as a centuria. Crewmen could sign on as naval infantry (called Marinus), rowers/seamen, craftsmen and various other jobs, though all personnel serving in the imperial fleet were classed as milites ("soldiers"), regardless of their function; only when differentiation with the army was required, were the adjectives classiarius or classicus added. The Roman Navy's two fleet legions, I Adiutrix and II Adiutrix, were among the first distinct naval infantry units.