Introduction
One of the traits of bivalves that affects aquaculture, and specially mytiliculture , consists in the existence of large differences in growth performances between individuals. A great proportion of such differences appear to be genetically determined and hence to be heritable, and they are turned into measurable differences in the energy balance of individuals. However, genetic clues are not clearly identified and more studies are needed at different biological organization levels to better manage the mussel production. Under high organic content rations , the differences between fast and slow growing individuals are determined by distinct inter-individual capacities to acquire and process food particles, with a higher metabolic efficiency5. The size of the digestive gland has also been reported to constitute a gross measurement of the digestive6 and growth7 performances, and different gill-surface areas seem to point out a different food acquisition capacity5. Y et the underlying structures are still unknown. The aim of these s tudy was to stablish the histological bases of the differential physiological performance between differentially growing individuals of Mytilus galloprovincialis.
Methods
Juvenile mussels ( shell length of 10.98 ± 0.52 mm ) sampled from an intertidal rocky shore were maintained in the laboratory under conditions of continuous feeding with I. galbana . After three months, mussels were subdued to a two-week acclimation under two different food-rations: high (H: 50 000 part · mL-1) and low (L: 10 000 part · mL-1). To analyse the differential growth of mussels, the largest 40 (fast-growers – F), the 40 medium-sized mussels (intermediate growers – I) and the smallest 40 (slow-growers – S) were selected and characterized in terms of energetic physiology and histology. The main components of the energy balance (clearance rate -CR- (L·h-1) and routine metabolic rate -RMR- (mL O2·h-1)) were measured following standard procedures6. Once the physiological measurements were completed, dissections were performed for histological examination. To determine the status of the digestive gland, the mean epithelial thickness (MET), mean luminal radius (MLR), mean digestive diverticula radius (MDR) , and the connective to diverticula ratio (CTD) were quantified. In addition, the adipogranular cell density and the gill integrity were calculated.
Results
Fast mussels were 3.5 times heavier than S ones (1.35 ± 0.21 g vs. 0.37 ± 0.08 g), and had almost 70 % larger shell lengths (24.73 ± 1.11 mm vs. 14.57 ± 1.11 mm).
Physiological measurements . Both growth condition (F, I or S) and food ration (H or L) exerted a significant effect on the CR of mussels: CR was two times higher at low food ration (0.411 ± 0.16 L·h-1 ) as compared with high food ration (0.233 ± 0.15 L·h-1 ); and fast growers attained two times higher values than slow growers (0.429 ± 0.19 vs. 0.216 ± 0.12, respectively). Medium growers (I) had intermediate CR values that were not significantly different to those in F and S mussels. In good correspondence with CR values, the gill surface-area of F mussels was significantly higher than that of S ones, ir respective of the ration. However, no significant effects on RMR were recorded.
Histological assessment . Significant differences in the gills were associated with the growth condition factor. F and I mussels had a higher organization level of the epithelium with an evident higher frontal and latero-frontal cilia density than S mussels. In addition, i rrespective of the ration, S mussels had lower MDR and MET values than F and I ones ( MDR: 23.53 ± 0.89 vs. 26.27 ± 1.23 and 25.72 ± 1.91 ; MET: 7.50 ± 2.45 vs. 13.44 ± 1.98 and 12.29 ± 2.09). Mussels fed high food ration achieved significantly higher MDR and MLR of the digestive diverticula than those fed with low ration. T he adipogranular cell index was two times higher in F and I mussels than in S individuals, and no significant effect of food ration was recorded .
Discussion
Both fast and slow growing individuals showed a similar regulatory r esponse to the particle concentration increase through CR reduction, a phenomenon that has been comprehensively addressed and interpreted in bivalves as a mechanism allowing the regulation of ingestion rate and gut passage time1,2. In this context, the food acquisition and processing capacities of fast and slow growers differ considerably. This has to do, to a great extent, with the different capacity of their gills, digestive gland and energy-storage capacity , as observed herein. Previous studies3,4,5 have shown that both inter-specific and size-related intra-specific differences in CRs may be explained by corresponding differences in gill surface areas. The data from the present study not only agrees with that , but also has revealed that gills of slow growers possess a lower cilia density a nd a disorganised structure overall . Histological analysis showed that F individuals have a greater digestive capacity, thanks to a higher digestive diverticula to connective tissue proportion, and larger and thicker digestive diverticula, which could lay bare higher intracellular digestive and absorption capacities. In addition, fast growers showed a higher level of investment in storage in the form of adipogranular tissue, which is expected to reflect an increase in gamete production. These evidences could be of a mayor importance since the selection of animals with enhanced digestive capacity and growth conditions could be an improvement in aquaculture management.
References
1Bayne, B. L., Hawkins, A. J. S., & Navarro, E. (1987 ). J. Exp . Mar. Biol. Ecol., 111, 1-22.
2Navarro , E., Iglesias, J. I. P., Ortega, M. M., et al. (1994 ). Physiol. Zool., 67, 468-496.
3Pouvreau , S., Jonquières , G. & Buestel, D. (1999 ). Aquac., 176, 295-314.
4Honkoop, P. J. C. (2003 ). Oecol., 135, 176-183.
5Prieto , D., Urrutxurtu , I., Navarro, E., et al. (2018 ). Mar. Environ . Res., 140, 114-125.
6Ibarrola , I., Etxeberria, M., Iglesias, J. I. P., et al. (2000 ). J. Exp . Mar. Biol. Ecol., 252, 199-219.
7Albentosa , M., Sánchez-Hernández, M., Campillo, J. A., et al. (2012 ). Comp. Biochem . Phys . A., 163, 286-295.