Diet of European freshwater pearl mussels Margaritifera margaritifera (Mollusca: Bivalvia: Unionoida) in small river (Republic of Karelia, Russia)

The freshwater pearl mussel Margaritifera margaritifera (Linnaeus,1758) is endangered in Europe and is now listed in the Red Data Book of many countries and regions. The diet of the species in the Syskyänjoki River (a tributary of Lake Ladoga) has been studied. The contents of the intestine generally correspond to the composition of seston, and include organic detritus, filamentous and unicellular algae, fragments of invertebrates and macrophyte tissues mixed with silt and sand. The total biomass of the intestinal contents of varied from 0.8 to 30.6 mg per organism (absolutely dry weight). Margaritifera margaritifera consumes a wide range of particles, from 0.5 μm3 (bacteria and unicellular algae) to 200 000 μm3 (fragments of invertebrates and macrophyte tissues). About 90–95% (by volume) of the intestinal contents was consisted by fine organic detritus. The food composition did not differ significantly for mollusks of different sexes and size. In the intestinal contents, 63 taxa of algae were identified. The number of algal species in the content of one intestine varied from 3 to 17, with their abundance from 250 to 9560 cells per organism. The most abundant and constant in the contents of the intestines are unicellular algae. Diatoms are the most diverse, they make up 50.8% of the total number of species.

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.

Identification and Functional Analysis of the Sex-Determiner Transformer-2 Homologue in the Freshwater Pearl Mussel, Hyriopsis cumingii

Transformer-2 (Tra-2) is an upstream regulatory element of the sex regulation mechanism in insects and plays a critical role in sex formation. To understand the role of tra-2 in Hyriopsis cumingii, the full-length Hctra-2 (1867 bp) was obtained from the gonads, and sequence alignment with other species showed that HCTRA-2 protein had a highly conserved RRM domain. Phylogenetic analysis showed that the HCTRA-2 protein was a close relative to of the mollusks TRA-2 protein. The qRT-PCR of tissue-specific expression pattern showed that the Hctra-2 was abundant in gonads, and the expression in testes was higher than that in ovaries (p &lt; 0.01). It suggests that Hctra-2 may play a potential regulatory role in gonadal development of H. cumingii. In the early gonadal development, the Hctra-2 expression was the highest on the third day after fertilization and increased slightly from 4 months to 5 months, which may be related to the embryonic sex determination and early gonadal development. In situ hybridization showed that Hctra-2 mRNA signals were present in both male and female gonads. After silencing Hctra-2 by RNAi, the expression levels of Hcfem-1b and Hcdmrt were changed. It is speculated that there may be a certain relationship between them, which plays an important role in the sex regulation of H. cumingii. Our research will help to deepen our understanding of the shellfish sex determination mechanisms.

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.