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/

Captive breeding of European freshwater mussels as aconservation tool: A review

1. Freshwater mussels are declining throughout their range. Their importantecological functions along with insufficient levels of natural recruitment haveprompted captive breeding for population augmentation and questions about the usefulness and applicability of such measures. 2. This article reviews the current state of captive breeding and rearing programmes for freshwater mussels in Europe. It considers the various species, strategies, andtechniques of propagation, as well as the different levels of effort requiredaccording to rearing method, highlighting the key factors of success. 3. Within the last 30 years, 46 breeding activities in 16 European countries have been reported, mainly of Margaritifera margaritifera and Unio crassus. Some facilities propagate species that are in a very critical situation, such as Pseudunio auricularius, Unio mancus, and Unio ravoisieri, or multiple species concurrently. Insome streams, the number of released captive-bred mussels already exceeds the size of the remaining natural population. 4. Rearing efforts range from highly intensive laboratory incubation to lowerintensity methods using in-river mussel cages or silos. Most breeding efforts are funded by national and EU LIFE(+) grants, are well documented, and consider the genetic integrity of the propagated mussels. Limited long-term funding perspectives, the availability of experienced staff, water quality, and feeding/survival during early life stages are seen as the most important challenges. 5. Successful captive breeding programmes need to be combined with restoration ofthe habitats into which the mussels are released. This work will benefit from anevidence-based approach, knowledge exchange among facilities, and an overall breeding strategy comprising multiple countries and conservation units. aquaculture, captive breeding, conservation translocation, freshwater mussel culturing, Margaritifera margaritifera, propagation, reintroduction, Unio crassusCaptive breeding of European freshwater mussels as aconservation tool: A reviewpublishedVersio

The conservation status of the world's freshwater molluscs

With the biodiversity crisis continuing unchecked, we need to establish levels and drivers of extinction risk, and reassessments over time, to effectively allocate conservation resources and track progress towards global conservation targets. Given that threat appears particularly high in freshwaters, we assessed the extinction risk of 1428 randomly selected freshwater molluscs using the IUCN Red List Categories and Criteria, as part of the Sampled Red List Index project. We show that close to one-third of species in our sample are estimated to be threatened with extinction, with highest levels of threat in the Nearctic, Palearctic and Australasia and among gastropods. Threat levels were higher in lotic than lentic systems. Pollution (chemical and physical) and the modification of natural systems (e.g. through damming and water abstraction) were the most frequently reported threats to freshwater molluscs, with some regional variation. Given that we found little spatial congruence between species richness patterns of freshwater molluscs and other freshwater taxa, apart from crayfish, new additional conservation priority areas emerged from our study. We discuss the implications of our findings for freshwater mollusc conservation, the adequacy of a sampled approach and important next steps to estimate trends in freshwater mollusc extinction risk over time

Research priorities for freshwater mussel conservation assessment

Freshwater mussels are declining globally, and effective conservation requires prioritizing research and actions to identify and mitigate threats impacting mussel species. Conservation priorities vary widely, ranging from preventing imminent extinction to maintaining abundant populations. Here, we develop a portfolio of priority research topics for freshwater mussel conservation assessment. To address these topics, we group research priorities into two categories: intrinsic or extrinsic factors. Intrinsic factors are indicators of organismal or population status, while extrinsic factors encompass environmental variables and threats. An understanding of intrinsic factors is useful in monitoring, and of extrinsic factors are important to understand ongoing and potential impacts on conservation status. This dual approach can guide conservation status assessments prior to the establishment of priority species and implementation of conservation management actions.NF-R was supported by a post-doctoral fellowship (Xunta de Galicia Plan I2C 2017-2020, 09.40.561B.444.0) from the government of the autonomous community of Galicia. BY was supported by the Ministry of Science and Higher Education (no. 0409-2016-0022). DLS was supported by the G. E. Hutchinson Chair at the Cary Institute of Ecosystem Studies. AO was supported by the Russian Foundation for Basic Research (no. 17-44-290016). SV was funded by European Investment Funds by FEDER/COMPETE/POCI- Operacional Competitiveness and Internacionalization Programme, under Project POCI-01-0145-FEDER-006958 and National Funds by FCT-Portuguese Foundation for Science and Technology, under the project UID/AGR/04033/2013. NF-R is very grateful to the University of Oklahoma Biological Survey for providing space to work in the U.S. and especially to Vaughn Lab members. Authors are very grateful to Akimasa Hattori, Allan K. Smith, Andrew Roberts, Daniel Graf, David Stagliano, David T. Zanatta, Dirk Van Damme, Ekaterina Konopleva, Emilie Blevins, Ethan Nedeau, Frankie Thielen, Gregory Cope, Heinrich Vicentini, Hugh Jones, Htilya Sereflisan, Ilya Vikhrev, John Pfeiffer, Karen Mock, Mary Seddon, Katharina Stockl, Katarzyna Zajac, Kengo Ito, Marie Capoulade, Marko Kangas, Michael Lange, Mike Davis, Pirkko-Liisa Luhta, Sarina Jepsen, Somsak Panha, Stephen McMurray, G. Thomas Watters, Wendell R. Haag, and Yoko Inui for their valuable contribution in the initial selection and description of extrinsic and intrinsic factors. We also wish to thank Dr. Amanda Bates, Chase Smith, and two anonymous reviewers for comments on earlier drafts of this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government

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

This study tested a culture system for rearing Margaritifera margaritifera at the Freshwater Pearl Mussel Ark in Windermere and investigated the effects of substrate size (0.25-1 mm and 1-2 mm) and cleaning regime (weekly and monthly) on survival and growth. At 362 days (12 months) and 758 days (25 months), a total of 1207 and 518 juveniles were reared successfully in the system described here. 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). Survival rates in this investigation were comparable to, if not better than, other published studies culturing M. margaritifera of this age. Juveniles from most treatments did not display size-dependent over-winter survival, but a significant correlation was found between shell length and survival in the 0.25-1 mm weekly treatment which was the least suitable treatment. Additionally we examined the effects of flow rate, dissolved oxygen concentration, ammonia concentration and biofilm as factors which may affect growth and survival of juveniles between July - September 2015. These results provide an insight into findings from the initial work and discussion is offered on the challenges for practically applying this method to captive rearing at the Ark in future

Substrate parameters affecting propagation of juvenile freshwater pearl mussels margaritifera margaritifera (bivalvia: margaritiferidae)

Interstitial habitat conditions are of critical importance to species inhabiting the hyporheic zone, particularly for moderately immobile species incapable of escaping poor habitat conditions. The critically endangered freshwater pearl mussel (Margaritifera margaritifera Linnaeus, 1758) has seen increasing propagation effort over the last three decades, often with mixed success. This study aimed to investigate parameters with the potential to affect juvenile survival in captivity by considering a range of habitat conditions within the substrate of a previously described propagation system using different substrate size classes (0.25–1 and 1–2mm) and cleaning regimes (weekly and monthly). Juvenile survival was highest in larger substrates, likely because of higher flow through larger pore spaces. This provided higher dissolved oxygen delivery in 1–2mm substrates cleaned weekly (8.26 ± 0.19 mg/L) and monthly (8.24 ± 0.44 mg/L), compared with 0.25–1mm substrates cleaned weekly (7.98 ± 0.44 mg/L) and monthly (6.78 ± 1.27 mg/L). The amount of organic material trapped in the substrate did not differ between treatments but the high concentrations of inorganic phosphorus liberated from ashed organic matter indicated phosphorus storage in phytoplankton. High dissolved oxygen concentrations and good water replacement between the water column and the substrate are crucial for survival in captive freshwater pearl mussels.N/

Applying morphometrics to choose optimal captive brood stock for an endangered species:A case study using the freshwater pearl mussel, Margaritifera margaritifera (L.)

To maximize captive breeding success for the globally endangered freshwater mussel Margaritifera margaritifera, morphometrics was applied to develop a tool for selecting optimal brood stock. There was high discrimination between brooding and non-brooding individuals and the presence of brood explained the variation in the percentage of mussels with a typical brooding morphology. Brooding individuals were significantly wider than non-brooding individuals. However, after reclassifying those non-brooding individuals with morphology highly indicative of brooding individuals using Mahalanobis distance modelling, only shell curvature along the ventral region differed significantly. The Mahalanobis model explained more variation in shell morphology than a model based on field observations, highlighting that shell morphology is a good predictor of brooding mussels. In addition, it could be argued that an identified novel morph is that of hermaphroditic M. margaritifera, which has developed in response to historic low population density. This is the first application of a non-invasive, morphometric technique to optimize captive breeding programmes for an endangered species. Since a greater number of species are under threat of extinction from climate change, there will be a demand for captive breeding programmes, emphasizing the importance of this study.</p

Diagram of gill position and cross-section through a lamella showing ascending and descending limbs of a filament.

<p>A: Gill position within a valve showing positioning of the anterior adductor (aa) and posterior adductor muscles (pa), budding zone (bz) and filaments (fi). B: A dorso-ventral section though a eulamellibranch showing the foot (ft), gill base (gb), inner demibranch (id), interlamellar junction (ilj), interlamellar space (ils), mantle (m) and outer demibranch (od). Used with permission from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193637#pone.0193637.ref038" target="_blank">38</a>].</p

Scatter plot of the number of inner ‘vs’ outer demibranch filaments.

<p>The number of inner demibranch filaments is able to predict the number of outer demibranch filaments and accounted for 99% of the explained variability in number of outer demibranch filaments.</p