Post larval red muscle growth in rainbow trout (Oncorhynchus mykiss)

International audienceThe myotome of fish consists of red and white muscles which differ in function and metabolism. The red muscles are separated from white muscle and located as a band along the side of the fish and composed of slow myofibers with an aerobic metabolism. In contrast, the white muscles are located more in depth with fast myofibers of anaerobic metabolism, and correspond to the major part of the fish’s musculature. To better understand the overall and relative growth of red muscles during the post larval development, its proportion in transversal cuts, the number and the size of the red myofibers from 10g to 2kg body weight trout was measured. Our results showed in ~35cm trout that the proportion of red muscles increased linearly on the antero-posterior axis from 1.5-2% (head) to 7% (caudal fin). Red muscle proportion at the anal fin position increased from 4,7% to 6% between 10g and 100g fish respectively, and then was maintained up to 2kg. Histology analysis revealed that red myofibre mean diameter only increased between 500g and 2kg fish (30 versus 45 µm respectively). In contrast the number of red myofibers dramatically increased between 10g and 500g fish (8260 versus 114 000 fibers), and then was maintained in fish up to 2kg (153 000 fibers). Altogether, these results indicate that from 10g up to 500g, red muscle growth occurs mainly by a mechanism of hyperplasia, and continue by hypertrophy at least up 2kg in trout