Abstract
Females of freshwater mussels spawn eggs which, after fertilization, are brooded until the developing larvae reach a stadium ready to infest a fish host in order to complete the obligate parasitic phase of their life. We investigated the duration of the brooding period in the endangered thick-shelled river mussel Unio crassus in two rivers in southern Poland and identified the stadia of glochidia development which could be used to assess current larval developmental stages. The eggs were fertilized in marsupia. The duration of the brooding period shortened significantly during the season, from 35 days in early spring to 9 days in summer. The break between consecutive broods was usually short but very variable: from almost no break to 17 days. 20% of brooding events inspected every other day were miscarried at different stadia of larval development. The recorded differences in brooding time may significantly influence the reproductive output of individuals and populations, depending on the thermal conditions, which may be affected by both anthropogenic impact and global climate change. The study offers a practical scheme for assessing female gravidity advancement in Unio crassus, which may be very informative for the future application of conservation measures in this endangered European species.
Highlights
The most important ecological factor attracting the attention of scientists is global warming: for obvious reasons this is expected to influence the physiological processes and ontogeny of organisms
Unfertilized eggs were recorded in the marsupia of five individuals during the first survey; some of them had a small admixture of zygotes cleaving into two cells
The presence of unfertilized eggs in marsupia, which subsequently develop into next-stage embryos, suggests that fertilization in U. crassus takes place in the marsupia; the details of this process are unknown
Summary
The most important ecological factor attracting the attention of scientists is global warming: for obvious reasons this is expected to influence the physiological processes and ontogeny of organisms. The species of Unionidae produce large quantities of larvae called glochidia, which, to complete their development, need an obligatory phase of parasitic life, attached to fish gills or fins (Modesto et al, 2018). The probability of fish infestation determines a population’s prospects (Cmiel et al, 2019); the factors influencing glochidia production are still poorly understood. Taeubert et al (2014) showed that temperature can speed up the parasitic phase in U. crassus; one can predict that it will accelerate glochidia development It has been demonstrated that temperature influences the timing of glochidia release (Gascho Landis et al, 2012; Schneider et al, 2018) and that elevated water temperatures negatively influence the survival of glochidia, though not of juveniles (Pandolfo et al, 2010). Taeubert et al (2014) showed that temperature can speed up the parasitic phase in U. crassus; one can predict that it will accelerate glochidia development
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