Local adaptation studies and conservation: Parasite–host interactions between the endangered freshwater mussel Unio crassus and its host fish

Abstract

Parasite–host interactions can involve strong reciprocal selection pressure, and may lead to locally adapted specializations. The highly threatened unionoid mussels are temporary parasites on fish, but local adaptation has not yet been investigated for many species. Patterns of local adaptation of one of Europe's most threatened unionoids, the thick‐shelled river mussel (Unio crassus) were investigated. Eurasian minnows (Phoxinus phoxinus) from two rivers (separate drainage areas) were cross‐infested in the laboratory with sympatric and allopatric mussel larvae, while bullheads (Cottus gobio), inhabiting only one of the rivers, were infested with sympatric or allopatric mussel larvae. Larval encystment, juvenile mussel excystment and survival were measured. For one river, but not the other, juvenile excystment from P. phoxinus was highest when infested with sympatric mussels. The opposite pattern was found for C. gobio in this river, where juvenile excystment and post‐parasitic juvenile survival from allopatric C. gobio were highest. The results thus cannot confirm local adaptation of U. crassus to P. phoxinus in the study rivers, as excystment was not consistently higher in all sympatric mussel–host combinations, whereas there were potential maladaptive signs of U. crassus in relation to C. gobio. There was no loss of encysted larvae 3 days after infestation until juvenile excystment. Most juveniles were excysted between 17 and 29 days after infestation, and the numbers of excysted juveniles increased with fish size. The results have implications for parasite–host ecology and conservation management with regard to unionoid propagation and re‐introduction. This includes the need to (1) test suitability and adaptation patterns between U. crassus and multiple host fish species, (2) evaluate the suitability of certain unionoids and host fish strains after more than 3 days, and (3) determine whether large fish produce more juvenile mussels than smaller fish.

Reproductive ecology of the threatened and endemic freshwater mussel Lampsilis bracteata

“…Numerous previous studies have examined host fish suitability for mussels with artificial infestation in the laboratory, but fewer studies have investigated differences in host fish compatibility of mussels and fish of sympatric and allopatric river origin (e.g. Bingham, ; Caldwell, Zanatta, & Woolnough, ; Eckert, ; Riusech & Barnhart, ; Schneider, Nilsson, Höjesjö, & Österling, ; St. John White, Ferreri, Lellis, Wicklow, & Cole, ). Only one other study examined mussel–fish pairings of different populations within the same drainage basin, but it looked at variation of infection success rather than metamorphosis success (Douda et al, ).…”

“…Compatibility has been shown to decrease as isolation between the mussel source and fish source increased (St. John White et al, ) and host suitability was found to be higher on host fish within the natural distribution range of the mussel (Taeubert et al, ), but this tendency towards higher success on sympatric host fish is not universal. Margaritifera margaritifera in southern Norway were more abundant on allopatric than sympatric brown trout (Österling & Larsen, ), and a study on Unio crassus in two geographically separated rivers of southern Sweden suggested that not all populations of a species may show the same adaptive tendencies in respect to host fish compatibility (Schneider et al, ). No differences in host suitability between sympatric and allopatric mussel–fish pairings were found for Epioblasma triquetra in the Great Lakes basin (Caldwell et al, ).…”

“…Numerous previous studies have examined host fish suitability for mussels with artificial infestation in the laboratory, but fewer studies have investigated differences in host fish compatibility of mussels and fish of sympatric and allopatric river origin (e.g. Bingham, 2002;Caldwell, Zanatta, & Woolnough, 2016;Eckert, 2003;Riusech & Barnhart, 2000;Schneider, Nilsson, Höjesjö, & Österling, 2017;St. John White, Ferreri, Lellis, Wicklow, & Cole, 2017).…”

Effects of mussel and host fish density on reproduction potential of a threatened unionoid mussel: prioritization of conservation locations in management trade-offs

“…Although the distance between sites seems small, it is considered sufficient to evaluate small-scale effects of fish and mussel density on the reproductive potential of U. crassus. From these sites, P. phoxinus and C. gobio had been identified as primary host fish for U. crassus in a previous study (Schneider 2017), in which DNA analyses of glochidia encapsulated on fish gills and transformed juveniles identified U. crassus as the sole unionoid in the system. All three study sites are similar in their physical and chemical characteristics (Table 1).…”

Glochidial infestation of fish by the endangered thick‐shelled river mussel Unio crassus

“…In Europe, such programmes are under way for Spengler's freshwater mussel Margaritifera auricularia , the pearl mussel M . margaritifera (Gum, Lange, & Geist, ; Preston, Keys, & Roberts, ; Thomas, Taylor, & Garcia de Leaniz, ), and for the thick‐shelled river mussel Unio crassus (Eybe, Thielen, Bohn, & Sures, ; Schneider, Nilsson, Höjesjö, & Österling, ; Zając et al, ). These programmes face serious difficulties for large freshwater mussels owing to their complex life cycle, which is based on parasitic larvae (glochidia) that develop on the gills or fins (Kat, ) of host fish, depending on the type of glochidia.…”