Analogues of red and white muscle in squid mantle

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Abstract
In 5 spp. of squid [Loligo opalescens, Illex illecebrosus, Ommastrephes sp., Berryteuthis magister and Symplectoteuthis oualaniensis], varying in life-style from fast-swimming pelagic predators to sluggish benthic forms, the circular muscle of the mantle was metabolically and structurally differentiated into inner, middle and outer zones. In the middle zone, mitochondrial abundance and the ratios of oxidative to glycolytic enzyme activities were low. This zone was sandwiched between thinner bands of muscle, lining the inner and outer edges of the mantle. In these bands, mitochondrial abundance and the ratios of oxidative to glycolytic enzyme activities were high. This metabolic differentiation is analogous to the development of red and white muscles in vertebrates, and it serves a similar function, white muscle mainly supporting burst-type swimming and red muscle sustaining steady-state oxidative work.