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

Abstract

ABSTRACT1. 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.2. 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.3. Different food mixtures (combinations of algae, detritus and crushed red bloodworms) were fed to juvenile mussels in plastic boxes containing 500 mL of river water during a period of 110 days. To understand fluctuations in the concentrations of nitrate, nitrite and ammonium between water exchanges, these ions were analysed over a period of 8 days.4. The best rearing results were achieved with a combination of detritus and algae. This treatment resulted in a growth rate of 189% (up to 1.13 mm (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 8 days. Without detritus, ion concentrations increased noticeably (ammonium >50%, nitrite >150%), probably explaining higher mortality rates.5. Juvenile mussels have food reserves for less than 8 days after excystment and need additional food as soon as possible after dropping from the host fish.6. 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.

Influence of stock origin and environmental conditions on the survival and growth of juvenile freshwater pearl mussels (Margaritifera margaritifera) in a cross-exposure experiment

“…Only juveniles collected during the peak collection period were used in the experiment to avoid the introduction of a bias by using incompletely developed juveniles. Since not all mussels excysted at the same day, freshly dropped off A c c e p t e d M a n u s c r i p t juveniles were maintained for a maximum of 14 days and supplied with food following Eybe et al (2013) until they were randomly transferred to the mesh cages. Per study stream, five standard mesh cages, so-called Buddensiek cages (Buddensiek, 1995), were installed.…”

Conservation status of the freshwater pearl mussel Margaritifera margaritifera in Portugal

“…Some successful captive breeding programs for M. margaritifera already exist in Europe, and so, these techniques are already available to be used in Portugal, taking in account the best practices that should include genetic information, careful determination of habitat requirements (from the macro to the microscale) and assessment of suitable food resources (e.g. Preston et al, 2007;Bolland et al, 2010;Gum et al, 2011;Wilson et al, 2012;Eybe et al, 2013;Scheder et al, 2014).…”

Captive breeding of Margaritifera auricularia (Spengler, 1793) and its conservation importance

“…The culture of juveniles of M. auricularia was based on the ‘detritus boxes’ method developed by Eybe et al () for M. margaritifera . The water for these boxes was collected from the Ebro River main channel upstream of the city of Zaragoza and filtered through a 7‐μm sieve in 2014 and 2015 and an 18‐μm sieve in 2016.…”

“…Silica sand with a particle size of 400–800 μm was used as substrate (following Liberty et al, and preliminary tests). This sand was previously washed and dried in a furnace at 150°C for 24 h. Detritus used in the boxes was collected weekly by trampling a flooded margin of the Ebro River containing abundant aquatic and riparian vegetation upstream of Zaragoza (Eybe et al, ). The collected sample was filtered through an 18‐μm sieve, and 25 ml of the resulting water was added to each box.…”