Mass Mortality Events in Freshwater Pearl Mussel (Margaritifera margaritifera) Populations in Sweden: An Overview and Indication of Possible Causes

Abstract

The freshwater pearl mussel Margaritifera margaritifera is an endangered species in Sweden with more than 600 known populations distributed in 16 out of 21 counties. Only approximately one-third of these populations are considered viable and healthy with signs of recent juvenile recruitment. From 2011 to 2017, research documented an increased mortality in this species, of up to 100% in some populations, but no etiological cause of these mortalities has been identified. With this paper, we provide current knowledge of locations where mass mortality of freshwater pearl mussel has been found in Sweden and discuss possible causes. Postmortem sampling and histopathological findings from two counties in 2016-17 detected lesions in digestive glands indicating a reduced capacity for nutrient uptake. Results from these macroscopic and microscopic investigations also indicate a reduction in, or a lack of, reproductive output compared with reference populations.

Identification of a new gregarine parasite associated with mass mortality events of freshwater pearl mussels (Margaritifera margaritifera) in Sweden

“…After a mass mortality event (MME) of M. margaritifera in Teåkersälven first detected in 2017, a general decline in the population was continuously observed (Wengström et al, 2019), culminating at about 65% in 2019 (Table S1).…”

“…Swedish populations of M . margaritifera have been closely monitored due to their critical status, with an overview of mortality events including some microscopic as well as macroscopic descriptions performed in 2016 and 2017 (Wengström et al., 2019). From populations experiencing mass mortality events (MME), patterns of pathological changes in gonads, foot, digestive gland and mantel are described.…”

“…From populations experiencing mass mortality events (MME), patterns of pathological changes in gonads, foot, digestive gland and mantel are described. However, no clear conclusion on the etiology or possible causes for the observations and histopathological changes could be drawn (Wengström et al., 2019). In the Swedish locality of Teåkersälven, where new MME were detected in 2017, we collected a few moribund mussels and performed initial histopathology, which revealed the presence of cysts with similarities to gregarines (Apicomplexa).…”

Pathology and infectious agents of unionid mussels: A primer for pathologists in disease surveillance and investigation of mortality events

“…Most descriptions of degeneration and necrosis in Unionidae involve the digestive gland, 85,87,211,222 where they may be grossly evident as pale green-brown discoloration of the gland. 205 Microscopically, degeneration manifests as cell swelling, cytoplasmic vacuolation (indicating lipidosis or lysosomal swelling), or lipofuscin accumulation. 37 Necrosis is microscopically characterized by cell sloughing, leaving a denuded basement membrane and luminal cell debris (Fig.…”

The Status of Mussel Health Assessment and a Path Forward

“…More recent reports of mussel declines in the Pacific Northwest have come from the Crooked River, Oregon (2014 and 2018), and Chehalis River, Washington (2018) (E. Blevins et al, unpublished data), again, without explanation. In Europe, unexplained die-offs have been reported in populations of the Freshwater Pearly Mussel (M. margaritifera) in Sweden (Wengström et al 2019).…”