Physiological basis of inter-population, inter-familiar and intra-familiar differences in growth rate in the green-lipped mussel Perna canaliculus

In New Zealand, three quarters (by value) of national aquaculture exports are based on the production of an endemic species, the green-lipped mussel, Perna canaliculus, yet there remains a paucity of information describing its underpinning biology. The majority of seed stock is still derived from wild capture from two main geographic locations, however commercial hatchery production has just begun. An established selective breeding programme, now in its 7th generation, has created family lines selected for desirable production traits, including growth rate. The current study therefore used animals derived from selectively bred lines, as well as animals produced in the hatchery from the two geographically distant populations used for commercial spatcatching, which are observed to have differences in growth rates, to explore the key energetic and morphological parameters that may determine fast and slow growth and growth efficiency. Metabolic rates, clearance and assimilation parameters, and morphological characteristics were established for mussels from 3 distinct levels of relatedness; comparing populations, full sib families (selected a priori for fast or slow growth rate), and interindividual differences between siblings within families displaying heterogeneous growth rates. Scaling parameters were also determined for metabolic rate, clearance rate, and gill-surface area in the two wild populations of mussels, for comparison of the allometry of the populations, and for size standardization of data. No significant inter-population differences were found in the scaling exponent for any of the parameters, indicating that the size-dependency of physiological rates was effectively the same for mussels from the different geographic areas. However, in contrast, significantly different mass-specific clearance rates were seen in mussels from the two different populations, indicating enhanced energy balances for mussels derived from the Golden Bay population. In the inter-and intra-family experiments, significant differences were found between fastgrowing and slow-growing mussels both in terms of anatomy (condition index and relative size of the digestive gland), and in physiological traits such as clearance rate, routine metabolic rate, scope for growth, growth efficiency and metabolic costs of feeding and growth. These results point towards a relatively higher food processing capacity in the gut of faster-growing mussels, and a generally higher metabolic cost of feeding and growth in slower growing mussels. Overall these results reveal the strength of the genetic factors in determining inter-individual variations in physiological performance affecting growth potential, and reinforce the notion that selection of breeding stock both with regard to origin or family provides considerable scope for improvement in mussel production.