Effects of phosphate on the reproductive symbiosis between bitterling and freshwater mussels: implications for conservation

Abstract

Summary We examined the effects of phosphorus, an important pollutant in fresh waters, on mussels Anodonta anatina and embryos of European bitterling Rhodeus sericeus amarus. These species are involved in an unusual symbiosis, with the fish depositing eggs inside the mussels' gills, where the embryos develop for 3–6 weeks until the larvae leave the mussel. High concentrations of phosphate (500 μg L−1 and higher) caused a significant increase in the rate of premature expulsion and emergence of bitterling embryos from the mussels, with a concomitant increase in fish mortality. At concentrations of 750 μg L−1 and higher, mussels reduced their frequency of ventilation and movements through the substratum. These results have implications for understanding the nature of the symbiosis between the fish and their mussel hosts, showing that mussels have control over bitterling developing in their gills. The premature expulsion of bitterling embryos at high phosphate levels is detrimental to the fish that they harbour. Recommendations for the conservation of bitterling and species of mussels, which are threatened or endangered over parts of their ranges, include reductions in levels of phosphorus in natural habitats and the need for low levels in reintroduction sites for the fish and their mussel hosts.