Analogues of red and white muscle in squid mantle.

In five species of squid, varying in life-style from fast-swimming pelagic predators to sluggish benthic forms, the circular muscle of the mantle was found to be 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 both the inner and outer edges of the mantle. In these bands, mitochondrial abundance and the ratios of oxidative to glycolytic enzyme activities were high. It is proposed that this metabolic differentiation is analogous to the development of red and white muscles in vertebrates and that it serves a similar function, white muscle mainly supporting burst-type swimming and red muscle sustaining steady-state oxidative work.