Culturing freshwater pearl mussel <i>Margaritifera margaritifera</i>: a breakthrough in the conservation of an endangered species.

1. The freshwater pearl mussel <i>Margaritifera margaritifera</i> L. is globally endangered and is threatened by commercial exploitation, pollution and habitat loss throughout its range. Captive breeding would be a valuable tool in enhancing the status of <i>M. margaritifera</i> in the UK. 2. We have developed a semi-natural system for successfully infecting juvenile brown trout with glochidial <i>M. margaritifera</i>, and culturing juvenile mussels in experimental tanks where glochidial <i>M. margaritifera</i> can excyst from fish gills and settle into sediment. 3. Infected fish had less than 1% mortality. Levels of infection varied among fish. Two yearly cohorts of juvenile <i>M. margaritifera</i> were identified from samples of sediment taken from each experimental tank. Individuals range in size from 1.4 mm (2000 cohort) to >3 mm in length (1999 cohort). 4. The number of juvenile <i>M. margaritifera</i> present in the two experimental tanks are estimated to be between 3600 (tank A) and 0 (tank B) for the putative 1999 cohort and between 6000 (tank A) and 13 000 (tank B) for the putative 2000 cohort. 5. This pioneering method for large-scale cultivation of juvenile <i>M. margaritifera</i> is intermediate between the release of infected fish into rivers and the intensive cultivation systems developed in continental Europe and the USA for other species of unionid. This is the first time that large numbers of <i>M. margaritifera</i> have been cultured and represents a significant breakthrough in the conservation of this globally endangered Red Data List species. The method is straightforward and is most cost-effective when undertaken alongside established hatchery processes.