The endangered thick-shelled river mussel (Unio crassus): a new host species for the European bitterling (Rhodeus amarus)

Abstract

Large freshwater mussels (Unionoida) are declining throughout the world. The European bitterling Rhodeus amarus (Bloch, 1782) female spawns its eggs inside the unionids' shells, where fertilisation and further embryonic development take place; thus its reproduction depends fully on the presence of large freshwater mussels. Unio crassus, previously regarded as one of the most numerous unionids in Europe, is now listed in the IUCN Red Data List as being globally endangered. Despite its previous prevalence, it was never reported as a host for the bitterling. A large population of U. crassus was studied in small river at the Świętokrzyskie Mts (Poland), where also electrofishing was conducted. In each bitterling territory located on the study plots, we found individuals of U. crassus, with the bitterling eggs or larvae developing on mussel's gills. That proves that this species can be also used by the bitterling for reproduction. We suggest that this relationship has not been reported to date due to the mussels' rarity and ongoing decline. However, it is also possible that the endangered mussel is a novel host for the bitterling, which is expanding its range throughout Europe.

The depressed river mussel Pseudanodonta complanata as an occasional host for the European bitterling Rhodeus amarus

“…These results match previous studies, as it is known that A. anatina, U. pictorum, and U. tumidus are hosts for the European bitterling (Balon, 1962;Wiepkema, 1961;Reynolds et al, 1997). Furthermore, it is also known to date that among co-occurring mussel species U. crassus (Tatoj et al, 2017), A. cygnea (Linnaeus, 1758) (Reynolds et al, 1997), Pseudunio auricularius (Spengler, 1793) (Soler et al, 1999), Microcondylaea bonellii (Férussac, 1827) (Sousa et al, 2020), U. mancus Lamarck, 1819, and Potomida litoralis (Cuvier, 1798) are suitable native European hosts for the European bitterling (Prié, 2017).…”

“…This study reinforces the hypothesis of Soler et al (2019), stating that Rhodeus amarus can utilise all European unionid species within its current range, including rare European species as occasional hosts, including Unio crassus (Tatoj et al, 2017;Lewisch et al, 2023) and Pseudanodonta complanata (Smith et al, 2004; this study). However, despite their capacity for using a wide range of hosts, field and laboratory studies show that European bitterling are choosy about which species of freshwater mussel they use for oviposition (Balon, 1962;Aldridge, 1997;Kondo et al, 1984;Smith et al, 2000;Reichard et al, 2010Reichard et al, , 2015Soler et al, 2019;Sousa et al, 2020).…”

Distribution Range of the Endangered Species Unio crassus Philipsson, 1788 in Serbia (Western Balkans Region), Historical and Recent Data

“…(Figure 6), in the littoral part of rivers, mostly in fine substrate but also on larger sediment fractions. Most European unionids are lowland species, whereas U. crassus can inhabit higher elevations than other unionids [19,30] and can even reach very high densities in mountainous rivers [50], which supports the hypothesis of a wider niche for habitat variables than expected [51].…”

Microcondylaea bonellii as a new host for the European bitterling Rhodeus amarus

“…In Drin River (41.890205, 19.571525), of a total of 7 M. bonellii analysed, 4 had bitterling embryos (57.1%) and in Gjadër River (41.905293, 19.581752), 8 of 14 M. bonellii analysed had bitterling embryos (57.1%). These levels of infestation are much higher than the ones described for U. crassus in Poland (4.25%) (Tatoj et al, 2017) but lower than the 100% described for Anemina globosula (Heude, 1878) individuals infested by Rhodeus ocellatus (Kner, 1866) embryos (Reichard et al, 2007). These high levels of infestation, in comparison to other European studies, may translate in a higher energetic cost for M. bonellii (Methling et al, 2019 and see below further discussion).…”