Effects of ration on protein-nitrogen flux and mantle muscle growth in a sepiolid

Growth can be viewed as a result of a series of processes operating at different levels of organization: changes at the whole animal level, in the relative size of organs, the generation and growth of muscle fibers, and the physiological process of synthesizing protein. The aim of this study was to understand the process of growth at different levels, whole animal and cellular, to recognize how ration level influenced the growth dynamics of the cephalopod Euprymna tasmanica. Somatic growth of the squid, E. tasmanica (Pfeffer, 1884), when exposed to two set rations of mysid shrimp (high, 10% body weight per day and low, 3% body weight per day), was examined at the whole-body and cellular level using various methods. Whole-body growth was assessed using weight and dorsal mantle length (DML) relationships, gonad to somatic indices (GSI) and nitrogen budgets created from measurements of protein-nitrogen flux. Cellular growth was described using growth correlates (RNA and protein ratios) and histological analyses of mantle muscle block widths and frequency distributions of fiber sizes and densities. Daily ammonia excretion rates were not explained by feed consumption two or three days prior to measurement, suggesting that temperate sepiolids may take longer than 72 h to digest meals. Excretion was affected by body size with larger squid excreting more ammonia than smaller squid. The high variability in growth rates among squid was not explained by ration, even though females consumed twice the amount of mysids than males and had larger gonads in comparison to males of the same weight. There was no significant difference in specific growth rates or gono-somatic index between genders. RNA: protein did not correlate with fractional growth rates in E. tasmanica. Therefore, this method was not considered a useful growth correlate under the present experimental conditions. Cellular growth of squid mantle muscle tissue occurred through the generation of new fibers (hyperplasia) and the increase in size of existing fibers (hypertrophy). Small circular and radial fibers (<2 ˜í¬¿m) were present in the mantle in all rations and DML size classes, which suggested that fiber recruitment was continuous. Circular muscle fibers were seen to branch off, thereby creating new circular fibers within muscle blocks. Muscle blocks became larger as animals increased in size. Mantle block widths were not affected by ration, but did vary significantly among mantle regions. The anterior region of the mantle was smaller than the mid and posterior regions with higher relative frequency of smaller fibers (<2 ˜í¬¿m) suggesting that growth was initiated in the anterior of the animal. Overall, the variability in growth rates among E. tasmanica was not explained by differences in food consumption, somatic size, or muscle blocks width. This suggested that other factors may be responsible. This preliminary study on E. tasmanica provided a framework to which other biotic and abiotic factors can be explored to explain growth dynamics.