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The naiads, large freshwater mussels (Unionida), have very long life spans, are large-bodied, and produce thousands to millions of larvae (glochidia) which typically must attach to host fish tissues to metamorphose into a juvenile mussel. Glochidia develop within a female's marsupial gill demibranch, thus their number is restricted by female size. However, larger mussels acquire more energy, which could be invested in either larger-sized glochidia, in a more glochidia, or a combination of both. The high level of host specialization seen in many naiads may constrain glochidial size and shape around a narrow optimum, while naiads that use a wide range of host fishes may be predicted to possess greater plasticity in glochidial morphology. In this paper, we investigated the relationship between maternal body size and progeny body size and shape, aided by modern digital microscopy. We analyzed the between- and within- species variation of glochidia size and shape relative to female size in four widespread species of European naiads: Anodonta anatina, Anodonta cygnea, Unio crassus and Unio tumidus. Whereas the total reproductive output is collinear with female body size, substantial differences between species in glochidia size were found within genus Anodonta, but not genus Unio where glochidial size is remarkably consistent. The glochidial shape, however, differed within both Unio and Anodonta. We interpret this constant within-species glochidial size in Unio as reflecting a constraint imposed by the likelihood of successful transmission onto and off from a narrow range of hosts, whereas their shape seems to be less constrained. The Anodonta species, inhabiting a wide spectrum of habitats and using more than twice the number of fish hosts than Unio spp., have larger glochidia with greater variation in size and shape. Our results suggest that measures of glochidial variability may also serve as an indicator of host specificity in other naiads. [ABSTRACT FROM AUTHOR]

The life cycle of unionids is characterized by a obligatory period of larval parasitism on a fish host, any disturbance of which might cause a large-scale decline in the mussel population. Because the probability of fish infestation is so important, we modelled what would happen to a population (in terms of population growth and probability of extinction), if the same number of glochidia were released in one or more separate spawning events, by a hypothetical mussel population living in conditions differing in the probability of fish infestation (a "neutral" scenario, a "good" one and three variants of a "bad" one). The single brood strategy was the best in the "good" scenario. However, when the frequency of unfavourable stochastic events increased ("bad" scenarios), all strategies led to population decline, the single brood strategy being the worst. In "good" and moderately "bad" conditions the double brood strategy performed better than the other multiple brood strategies, but as infestation conditions deteriorated, a greater number of spawning events ensured a slower population decline and longer persistence. Our model can facilitate a better understanding of this problem and set up a framework for further tests in other unionid species and their environmental conditions. [ABSTRACT FROM AUTHOR]

Freshwater mussels are one of the most endangered groups of animals. Translocation is a common conservation measure for protecting them, but the success of this procedure varies widely, and the reasons for failure are frequently unknown.The dispersal and mortality of freshwater mussels was studied using a novel method: the radio tracking of Unio crassus translocated to new sites during a reintroduction project for this threatened species in the River Biała (Poland).Individual mussels were equipped with radio transmitters (10 mussels at each of four sites during low water in autumn, and 10 mussels at each of 11 sites during the spring flood). After being released into the water, the mussels generally moved away from the bank, and then performed erratic movements, dispersing over a larger area on flat parts of the riverbed, finally congregating on steep parts of the river bed in shallow water (0.20 m) near the bank.Mussel mortality was surprisingly low: all survived the autumn trial, and 80% of the radio‐tracked adults survived the spring flood. Some of them were displaced quite far within the channel (>100 m), apparently suffering no harm. The confirmed cases of mortality were a few individuals that had become stranded on gravel bars formed during the flood that were now drying out.Long‐distance displacement was more frequent in lotic or regulated parts of the channel and in the parts where the channel morphology was unstable during the flood.Radiotelemetry should be used to test the suitability of receptor sites before translocation, in order to find places where mussels will choose to stay, and to standardize mussel detectability when monitoring relocation success. [ABSTRACT FROM AUTHOR]

Abstract: Freshwater mussels (Unionoida) are globally threatened. The key to the development of effective conservation strategies for this group is to unravel their complicated life cycle, including the obligatory larva, which parasitizes the external tissues of fish. Artificial infestation was conducted for three fish species – Barbus barbus, Chondrostoma nasus, and Squalius cephalus – in order to study the prevalence, site, and intensity of infestation, in relation to the fish species and their individual body size. Fish gut contents were also examined, in order to analyse the possibility of glochidia consumption. Glochidia of Unio crassus are considered parasites of fish gills; however, this study revealed that U. crassus glochidia were attached to the fins of all the host species tested. The number of glochidia attached to C. nasus fins was four times higher than the number of glochidia attached to gills. The prevalence of fin infestation was higher for C. nasus and S. cephalus than for B. barbus. Infestation intensity was highest for C. nasus and lowest for B. barbus, and was also dependent on fin type. The number of glochidia attached to fins demonstrated a nonlinear (log10) relationship with fish body length. Inspection of dissected fish guts showed that all of the fish species tested foraged on glochidia: S. cephalus and B. barbus more frequently than C. nasus. Using predatory fish host species with non‐predatory species in mixed assemblages for artificial rearing of mussels led to increased mortality of fish and decreased infestation intensity. The best conservation practices for freshwater mussels should therefore be based on single fish species that are easy to infest and that do not prey on glochidia. [ABSTRACT FROM AUTHOR]