Muscle cellularity, enzyme activities, and nucleic acid content in meagre (Argyrosomus regius) Ioannis Mittakos, M. Dolores Ayala, Octavio López-Albors, Kriton Grigorakis, Dimitrios Lenas, Fotini Kakali, and Cosmas Nathanailides Abstract: Anatomical and biochemical indices of axial muscle growth were monitored in farmed meagre (Argyrosomus re- gius (Asso, 1801)), a species with larger ultimate size. Within the first 19 months of a production cycle, body mass ex- ceeded 1300 g. The specific daily growth rate ranged from a winter low of 0.2% to a summer high of 1.3%. Axial muscle RNA:DNA ratio decreased and cytochrome c oxidase levels increased from spring to winter, indicating a metabolic reorgan- isation of this tissue in response to winter temperature lows. Body mass correlated positively with increased lactate dehydro- genase activity and myofibre size (hypertrophy). The DNA:protein ratio, the myofibre density, and the percentage of small myofibres (0–150 μm 2 ) decreased towards the end of the production cycle. However, small myofibres persisted even after the first 20 months of rearing. Compared with commonly cultivated species in the Mediterranean region, meagre exhibits delayed onset of puberty, larger ultimate size, and growth rate that is supported by the recruitment of new muscle fibres. This is in agreement with the hypothesis of a relationship between ultimate size and muscle growth dynamics. Key words: meagre, Argyrosomus regius, fish, muscle, growth, hyperplasia, LDH, metabolism. Résumé : Des indices anatomiques et biochimiques de croissance des muscles axiaux ont été mesurés dans le temps chez le maigre (Argyrosomus regius (Asso, 1801)) d’élevage, une espèce d’assez grande taille ultime. Dix-neuf mois après le début du cycle de production, la masse corporelle dépassait 1300 g. Le taux de croissance spécifique quotidien oscillait entre un minimum hivernal de 0,2 % et un maximum estival de 1,3 %. Le rapport ARN:ADN des muscles axiaux a diminué alors que les teneurs en cytochrome oxydase c ont augmenté du printemps à l’hiver, témoignant d’une réorganisation métabolique de ces tissus en réponse aux faibles températures hivernales. La masse corporelle était corrélée positivement à l’augmenta- tion de l’activité de la lacticodéshydrogénase et de la taille des myofibres (hypertrophie). Le rapport ADN:protéines, la den- sité de myofibres et le pourcentage de petites myofibres (0–150 μm 2 ) ont diminué vers la fin du cycle de production. Toutefois, des myofibres de petite taille demeuraient même après les 20 premiers mois d’élevage. Comparativement aux es- pèces couramment cultivées dans la région méditerranéenne, le maigre présente un début de puberté tardif et une taille ul- time et un taux de croissance plus importants, favorisés par le recrutement de nouvelles fibres musculaires. Ces constatations concordent avec l’hypothèse voulant qu’il y ait un lien entre la taille ultime et la dynamique de la croissance musculaire. Mots‐clés : maigre, Argyrosomus regius, poisson, muscle, croissance, hyperplasie, LDH, métabolisme. [Traduit par la Rédaction] Introduction The growth of fish axial musculature is affected by envi- ronmental parameters, as well as intrinsic aspects of fish growth (Carpenè and Veggetti 1981; Stoiber and Sänger 1996; Valente et al. 1999; Wilkes et al. 2001; Johnston et al. 2003; Alami-Durante et al. 2007; Lohne et al. 2012). For ex- ample, the number and the size of fish skeletal muscle fibres increase throughout their life span (Hudson 1969; Johnston 1980), but the ratio of muscle hyperplasia and hypertrophy varies and is governed by developmental, genetic, and envi- ronmental factors (Weatherley et al. 1988; Veggetti et al. 1990). As a result, fast growth of aquaculture species is usu- ally associated with increased muscle cellularity and an in- creased rate of muscle hyperplasia (Nathanailides et al. 1996; Johnston et al. 2000; Periago et al. 2005). There are several biochemical parameters that can be used as indices of fish growth and metabolism. For example, changes in the metabolic enzymes and nucleic acids of fish muscle are fre- quently associated with changes in metabolic capacity and growth (Sherwood et al. 2002), with fish exhibiting increased activity of mitochondrial enzymes in periods of fast growth or after a temperature-induced compensatory response. Also, an increase in the DNA content (mg DNA·g –1 of muscle) of growing fish can reflect changes in the number of cells, whereas an increase in the RNA content can reflect changes Received 28 March 2012. Accepted 23 July 2012. Published at www.nrcresearchpress.com/cjz on 19 September 2012. I. Mittakos, D. Lenas, F. Kakali, and C. Nathanailides. Department of Aquaculture and Fisheries, Faculty of Agricultural Sciences, Epirus Institute of Technological Educational, Igoumenitsa 46100, Greece. M.D. Ayala and O. López-Albors. Department of Anatomy and Comparative Pathology, University of Murcia, Murcia 30100, Spain. K. Grigorakis. Hellenic Centre for Marine Research, Institute of Aquaculture, Agios Kosmas Hellinikon, Athens 16777, Greece. Corresponding author: Cosmas Nathanailides (e-mail: Cosmasfax@yahoo.com). 1270 Can. J. Zool. 90: 1270–1277 (2012) doi:10.1139/Z2012-097 Published by NRC Research Press Can. J. Zool. 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