Slow muscle power output of yellow- and silver-phase European eels (Anguilla anguilla L.): Changes in muscle performance prior to migration

Eels swim in the anguilliform mode in which the majority of the body axis u ndulates to generate thrust. For this reason, muscle function has been hypo thesised to be relatively uniform along the body axis relative to some othe r teleosts in which the caudal fin is the main site of thrust production, T he European eel (Anguilla anguilla L.) has a complex life cycle involving a lengthy spawning migration. Prior to migration, there is a metamorphosis f rom a yellow (non-migratory) to a silver (migratory) life-history phase, Th e work loop technique was used to determine slow muscle power outputs in ye llow- and silver-phase eels. Differences in muscle properties and power out puts were apparent between yellow- and silver-phase eels. The mass-specific power output of silver-phase slow muscle was greater than that of yellow-p hase slow muscle, Maximum slow muscle power outputs under approximated in v ivo conditions were 0.24 W kg(-1) in yellow-phase eel and 0.74 W kg(-1) in silver-phase eel. Power output peaked at cycle frequencies of 0.3-0.5 Hz in yellow-phase slow muscle and at 0.5-0.8 Hz in silver-phase slow muscle. Th e time from stimulus offset to 90% relaxation was significantly greater in yellow- than in silver-phase eels, The time from stimulus onset to peak for ce was not significantly different between life-history stages or axial loc ations, Yellow-phase eels shifted to intermittent bursts of higher-frequenc y tailbeats at a lower swimming speed than silver-phase eels. This map indi cate recruitment of fast muscle at low speeds in yellow-phase eels to compe nsate for a relatively lower slow muscle power output and operating frequen cy.