Evidence that freshwater mussels attempt temporal partitioning of their host fishes.

Freshwater unionid mussels, which have parasitic glochidia larvae, often occur in multispecies mussel beds where they likely compete for host fishes. The temporal dynamics of glochidia release was examined at 2-h intervals over ten, 24-h periods in the Sydenham River, Ontario, Canada from late August through September 2020 using a rosette water sampler. A total of 6104 glochidia from 17 species were identified morphometrically and these were dominated numerically by four species (Eurynia dilatata; Ortmanniana ligamentina; Epioblasma triquetra; and Cyclonaias tuberculata). The total glochidia abundance was greatest during the night, 20:00 h (± 1 h) local solar time (LST) in part because of the dominant species. Conversely, the abundance of glochidia from individual species was not uniform temporally among days (p < 0.001) nor time-of-day (p < 0.001) but appeared to coincide with the reported diel activity periods (i.e., diurnal, crepuscular, and nocturnal) of the host fishes for 10 species. The abundance of glochidia peaked at different times in unionid species that shared hosts (five cases), suggesting an attempt to partition host fishes temporally. Whereas temporal partitioning of hosts could ultimately provide a mechanism by which multiple species of unionids coexist in the same habitat, this is remarkable because glochidia release occurs over longer temporal scales (i.e., weeks), and there is limited gill-space on host fish, especially small individuals that may not have acquired immunity. Recognizing the temporal dynamics of glochidial abundance in multispecies mussel beds will inform conservation approaches that address mussel-host fish relationships in this imperiled taxon.