Role of aerobic and anaerobic circular mantle muscle fibers in swimming squid: electromyography

Circular mantle muscle of squids and cuttlefishes consists of distinct zones of aerobic and anaerobic muscle fibers that are thought to have functional roles analogous to red and white muscle in fishes. To test predictions of the functional role of the circular muscle zones during swimming, electromyograms (EMGs) in conjunction with video footage were recorded from brief squid Lolliguncula brevis (5.0-6.8 cm dorsal mantle length, 10.9-18.3 g) swimming in a flume at speeds of 3-27 cm s(-1). In one set of experiments, in which EMGs were recorded from electrodes intersecting both the central anaerobic and peripheral aerobic circular mantle muscles, electrical activity was detected during each mantle contraction at all swimming speeds, and the amplitude and frequency of responses increased with speed. In another set of experiments, in which EMGs were recorded from electrodes placed in the central anaerobic circular muscle fibers alone, electrical activity was not detected during mantle contraction until speeds of about 15 cm s(-1), when EMG activity was sporadic. At speeds greater than 15 cm s(-1), the frequency of central circular muscle activity subsequently increased with swimming speed until maximum speeds of 21-27 cm s(-1), when muscular activity coincided with the majority of mantle contractions. These results indicate that peripheral aerobic circular muscle is used for low, intermediate, and probably high speeds, whereas central anaerobic circular muscle is recruited at intermediate speeds and used progressively more with speed for powerful, unsteady jetting. This is significant because it suggests that there is specialization and efficient use of locomotive muscle in squids.