Ontogeny of juvenile freshwater pearl mussels, Margaritifera margaritifera (Bivalvia: Margaritiferidae).

The gills of juvenile freshwater bivalves undergo a complex morphogenesis that may correlate with changes in feeding ecology, but ontogenic studies on juvenile mussels are rare. Scanning electron microscopy was used to examine the ultrastructure and ontogeny of 117 juvenile freshwater pearl mussels (Margaritifera margaritifera) ranging in age from 1–44 months and length from 0.49–8.90 mm. Three stages of gill development are described. In Stage 1 (5–9 inner demibranch filaments), only unreflected inner demibranch filaments were present. In Stage 2 (9–17 inner demibranch filaments), inner demibranch filaments began to reflect when shell length exceeded 1.13 mm, at 13–16 months old. Reflection began in medial filaments and then proceeded anterior and posterior. In Stage 3 (28–94 inner demibranch filaments), outer demibranch filaments began developing at shell length > 3.1 mm and about 34 months of age. The oral groove on the inner demibranch was first observed in 34 month old specimens > 2.66 mm but was never observed on the outer demibranch. Shell length (R2 = 0.99) was a better predictor of developmental stage compared to age (R2 = 0.84). The full suite of gill ciliation was present on filaments in all stages. Interfilamentary distance averaged 31.3 μm and did not change with age (4–44 months) or with size (0.75–8.9 mm). Distance between laterofrontal cirri couplets averaged 1.54 μm and did not change significantly with size or age. Labial palp primordia were present in even the youngest individuals but ciliature became more diverse in more developed individuals. Information presented here is valuable to captive rearing programmes as it provides insight in to when juveniles may be particularly vulnerable to stressors due to specific ontogenic changes. The data are compared with two other recent studies of Margaritifera development.N/

Effects of substrate size and cleaning regime on growth and survival of captive-bred juvenile freshwater pearl mussels, Margaritifera margaritifera (Linnaeus, 1758)

The freshwater pearl mussel is critically endangered and most English populations are at risk of extinction unless conservation measures are implemented immediately. The study objectives were to test a culture system for rearing Margaritifera margaritifera in captivity, and to investigate the effects of substrate size (0.25–1 and 1–2 mm) and cleaning regime (weekly and monthly) on survival and growth. In total, 1207 and 518 juveniles were reared to 362 (12 months) and 758 days (25 months), respectively. After 362 days, survival was significantly higher in 1–2 mm substrate treatments cleaned monthly (55 ± 6 %) and lowest in 0.25–1 mm substrate cleaned weekly (14 ± 3 %). Growth was significantly higher in 1–2 mm substrates cleaned weekly (length = 1.15 ± 0.21 mm) and lowest in 0.25–1 mm substrates cleaned monthly (length = 0.83 ± 0.23 mm). Juveniles from most treatments did not display sizedependent over-winter survival, but a significant correlation was found between shell length and survival in the 0.25–1 mm weekly treatment. This low-maintenance system utilised features of previously described systems and growth and survival rates were comparable to, if not better than, other studies culturing M. margaritifera. The system could be scaled up to rear significant numbers of juveniles in captivity