Securing genetic integrity in freshwater pearl mussel propagation and captive breeding

Securing genetic integrity is of key importance in conservation-oriented captive breeding programs releasing juveniles into the wild. This is particularly true for species such as the endangered freshwater pearl mussel (Margaritifera margaritifera) for which a number of captive breeding facilities has been established in Europe. The core objective of this study was to compare the genetic constitution of 29 cohorts of captive-bred freshwater pearl mussels from five different breeding facilities in Austria, France, Luxembourg and Germany, with their original 14 source populations from nine major European drainages, based on microsatellite markers. Captive-bred mussels represented 11 different genetic clusters, suggesting an important contribution of the breeding stations to securing the genetic diversity of the species. In almost all cases, the cultured offspring closely resembled the genetic constitution of the source mussels as revealed from the STRUCTURE analysis and the generally high assignment of offspring to the original source populations. The majority of captive-bred cohorts had an increased inbreeding coefficient and decreased genetic variability compared to their source populations as measured by AR and HO. Highest numbers of deformed juveniles coincided with very low levels of HO < 0.05. Since erosion of genetic diversity in captive breeding was mostly evident in individual year-cohorts, long-term breeding over multiple years can minimize such effects. The systematic selection of priority populations for conservation, effective breeding strategies avoiding effects of in- and outbreeding by genetically informed selection of parent individuals, and a network of collaboration among the different breeding facilities would be very useful to increase resilience and effectiveness.

Effects of dissolved oxygen concentrations on growth, survival and gonadal development of freshwater pearl mussel Lamellidens marginalis

The present study was conducted to evaluate the effects of dissolved oxygen concentrations on growth, survival and gonadal development of freshwater pearl mussel (Lamellidens marginalis). Twenty adult L. marginalis were exposed to each of the five different concentrations of dissolved oxygen (DO) i.e (T0, no aeration), and T1, T2, T3 and T4 with one, two, three and four air stone aerators, respectively in fifteen glass aquaria each containing 10L of pond water. DO concentrations were significantly higher in all the treatments compared to the control but there were no significant differences among the treatments. There were significant differences in the growth and survival rates between control and treatments. The relative growth rates (%) were significantly higher in the highest oxygen concentration (T4) compared to other treatments. The survival rate was zero in control (T0), whereas it was 66.67%, 70.00%, 71.67% and 73.33%, respectively for T1, T2, T3 and T4. On day-30, oocytes in T0 were found to be smaller in size, with developing acini and no previtellogenic oocytes; whereas in T1, T2, T3 and T4, previtellogenic oocytes and larger acini were evident. On day-60, vitellogenic oocytes along with few under-developed oocytes started to detach from the acini walls, as well as some already started migration to germinal duct on day-90. This indicates that ovaries of all the treatments were ripe and were ready to spawn between day-60 and -90. On day-30, secondary spermatocytes were observed in testicular acini in case of T0, whereas more developed and dense spermatids were evident in T1, T2, T3 and T4. These results indicate that testes were in late development stages in T0, whereas they were ripe in all the treatments. The current findings indicate that hypoxia slows down growth, reduces survival rate and retards development of reproductive organs in L. marginalis.

Higher mortality of the less suitable brown trout host compared to the principal Atlantic salmon host when infested with freshwater pearl mussel (Margaritifera margaritifera) glochidia

The freshwater pearl mussel (Margaritifera margaritifera) is a highly host-specific parasite, with an obligate parasitic stage on salmonid fish. Atlantic salmon (Salmo salar) and brown trout (Salmo trutta f. trutta and Salmo trutta f. fario) are the only hosts in their European distribution. Some M. margaritifera populations exclusively infest either Atlantic salmon or brown trout, while others infest both hosts with one salmonid species typically being the principal host and the other a less suitable host. Glochidial abundance, prevalence and growth are often used as parameters to measure host suitability, with the most suitable host species displaying the highest parameters. However, it is not known if the degree of host specialisation will negatively influence host fitness (virulence) among different host species. In this study we examined the hypothesis that glochidial infestation would result in differential virulence in two salmonid host species and that lower virulence would be observed on the most suitable host. Atlantic salmon and brown trout were infested with glochidia from two M. margaritifera populations that use Atlantic salmon as their principal host, and the difference in host mortality among infested and control (sham infested) fish was examined. Higher mortality was observed in infested brown trout (the less suitable host) groups, compared to the other test groups. Genetic assignment was used to identify offspring from individual mother mussels. We found that glochidia from individual mothers can infest both the salmonid hosts; however, some mothers displayed a bias towards either salmon or trout. We believe that the differences in host-dependent virulence and the host bias displayed by individual mothers were a result of genotype × genotype interactions between the glochidia and their hosts, indicating that there is an underlying genetic component for this parasite-host interaction.

Effect of substrate particle size on burrowing of the juvenile freshwater pearl mussel Margaritifera margaritifera

Juveniles of the endangered freshwater pearl mussel (FPM, Margaritifera margaritifera) live burrowed in stream substrate for the first years of their life. Fine sediments block water exchange within substrate and may cause juvenile mortality and recruitment failure. To better understand the connection between success of juvenile FPM and substrate particle size, it would be important to understand behavioural responses of FPM to varying substrate sizes at this critical life stage. We placed newly detached FPM juveniles in a 7-mm layer of sieved sand sorted into five sizes (< 120, 120–200, 200–250, 250–500 and 500–650 µm) each with 10 replicate dishes, 10 juveniles per dish, with burrowing status monitored for 96 h. Mean dish-specific proportion burrowed (PB) was significantly affected by substrate size, increasing from 52% in the finest sand to 98% in the coarsest sand. Furthermore, the significant substrate × time interaction was observed due to dropped PB (30-34%) in finest sand at 2–4 h time points. Thus, results suggest a clear behavioural response by juvenile FPM to substrate size, with fine sediments triggering surfacing behaviour. Surfacing may indicate stress, can increase predation risk, and expose to drift and/or enable drift of juveniles.