The first millimetre - rearing juvenile freshwater pearl mussels (Margaritifera margaritifera L.) in plastic boxes

The last remaining population of Margaritifera margaritifera in the Our River (Luxembourg, Europe) has come close to extinction. It consists predominantly of adult animals and will disappear within a few years without assistance. Juvenile mussels cannot survive in the river bed interstices, which are clogged by fine sediments, and rearing methods are needed to help them through this critical period. The objectives of this study were to elucidate the best rearing conditions for juvenile mussels with respect to the food type and density of individuals in order to breed them in the laboratory to a size at which survival in the river is likely. Different food mixtures (combinations of algae, detritus and crushed red bloodworms) were fed to juvenile mussels in plastic boxes containing 500mL of river water during a period of 110days. To understand fluctuations in the concentrations of nitrate, nitrite and ammonium between water exchanges, these ions were analysed over a period of 8days. The best rearing results were achieved with a combination of detritus and algae. This treatment resulted in a growth rate of 189% (up to 1.13mm (SD ±0.30) per box) with a survival rate of 80% (101 dead mussels (SD ±163.71) per box). The optimum number of mussels per box was 200. When detritus was added to the boxes, levels of nitrite and ammonium were reduced by more than 50% compared with the initial value within 8days. Without detritus, ion concentrations increased noticeably (ammonium >50%, nitrite >150%), probably explaining higher mortality rates. Juvenile mussels have food reserves for less than 8days after excystment and need additional food as soon as possible after dropping from the host fish. Juvenile mussels showed the most favourable survival rate and growth rate if fed with a mixture of algae and detritus. Detritus functions not only as a food source but also as a biologically active compound which reduces harmful ions such as ammonium and nitrite in the boxes.