Abstract
Margaritiferidae is a small freshwater bivalve family with 16 species. In spite of a small number of taxa and long-term history of research, several gaps in our knowledge on the freshwater pearl mussels still exist. Here we present the discovery of host fishes for Margaritifera dahurica, i.e. Lower Amur grayling, sharp-snouted lenok, and blunt-snouted lenok. The host fishes were studied in rivers of the Ussuri Basin. The identification of glochidia and fish hosts was confirmed by DNA analysis. The life cycle of M. dahurica and its glochidia are described for the first time. The SEM study of glochidia revealed that the rounded, unhooked Margaritifera dahurica larvae are similar to those of the other Margaritiferidae. Margaritifera dahurica is a tachytictic breeder, the larvae of which attach to fish gills during the Late August – September and finish the metamorphosis in June. Ancestral host reconstruction and a review of the salmonid - pearl mussel coevolution suggest that the ancestral host of the Margaritiferidae was a non-salmonid fish, while that of the genus Margaritifera most likely was an early salmonid species or their stem lineage. The overfishing of lenoks and graylings appears to be the most significant threat for this rare mussel species.
Highlights
The study of parasites and hosts which have been connected to each other for tens of million years contributes significantly to our understanding of the evolutionary process[1,2,3,4]
The glochidia of M. dahurica fit well within the size range of larvae of other freshwater pearl mussels, with exception of P. auricularius having the largest larvae among the Margaritiferidae (Supplementary Table 1)
Our findings reveal that M. dahurica is a tachytictic breeder with a long-term parasitic stage which passes throughout the winter (Fig. 4)
Summary
The study of parasites and hosts which have been connected to each other for tens of million years contributes significantly to our understanding of the evolutionary process[1,2,3,4]. M. dahurica is listed in “The IUCN Red List of Threatened Species” under the “Data Deficient” category[20] It means that reliable data on the population trends and abundance of this species is yet to be obtained. Klishko and Bogan[23] listed seven salmonid species in the Amur River drainage which could be the potential hosts for glochidia of this freshwater pearl mussel species. Lenok or Manchurian trout Brachymystax lenok (Pallas, 1773) was observed to be infested by glochidia in the Komissarovka River (tributary of the Khanka Lake, Amur Basin) but the larvae were not identified with certainty[9].The unknown reproductive timing of M. dahurica make the task of host identification complicated. Mussels with a bradytictic strategy spawn eggs to marsupia in the late summer, develop larvae during the fall and overwinter glochidia in the marsupia. It was reported that M. dahurica from the Ingoda River, Transbaikalia, Russia, starts to release glochidia when the water temperature become lower than 8–10 °C, which takes place at the end of September in the area[33]
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