Juvenile biology and captive rearing of the freshwater pearl mussel Margaritifera margaritifera

Captive breeding of the freshwater pearl mussel (Margaritifera margaritifera) is an important short-term strategy to conserve this critically endangered species. The aim of this thesis was to improve current knowledge of the factors affecting juvenile M. margaritifera in a captive setting, and to develop understanding of juvenile anatomy, ontogeny and the ecological requirements of juveniles in captivity. The substrate requirements of newly-excysted juveniles were investigated in an experimental flow-through system (Chapter 3) by analysing differences in survival and growth in two different substrate size clasts (0.25 - 1 mm or 1 - 2 mm), and cleaning regimes (weekly or monthly). Factors potentially affecting juvenile survival and growth were further investigated in Chapter 4. Results indicate that dissolved oxygen and flow were crucial for juveniles in this system. Investigations of juvenile anatomy and ontogeny (Chapter 5) using scanning electron microscopy have greatly improved our knowledge of the timing of key developmental stages, such as the onset of gill reflection. Analyses of gill ciliation suggest the species is capable of retaining very small particles (<2 μm diameter), offering a potential reason for why M. margaritifera is so sensitive to turbid and enriched conditions. Improving the efficiency and effectiveness of monitoring juveniles in captivity should be an objective for all rearing programmes. Batch marking of juveniles through immersion in calcein (Chapter 6) was shown to offer a quick and reliable method and has the potential to save rearing programmes time and money whilst improving juvenile monitoring. The findings of these investigations should inform other captive rearing programmes in order to improve juvenile survival. Rearing efforts should focus initially on ensuring sufficient flow and dissolved oxygen for post-excystment juveniles, before tailoring systems to ensure low-stress conditions for transforming juveniles.