Timo J. Marjomäki

The decline of endangered freshwater pearl mussel (FPM, Margaritifera margaritifera ) has been attributed to juvenile mortality caused by low concentrations of dissolved oxygen in the stream substrate resulting from fine sediments (siltation) that impede water exchange in the interstitial microhabitat of juveniles. If low oxygen concentration causes recruitment failure of FPMs, knowledge on the oxygen tolerance of juvenile FPMs is essential for the conservation of the species, as it will justify conservation efforts improving water exchange in the bottom gravel. However, the tolerance of low oxygen of FPM juveniles has not been directly studied. Juvenile FPMs (9–11 months old) were exposed in individual chambers equipped with optical oxygen measurement spots to different levels of dissolved oxygen at 19 °C and their viability was monitored for 10 days to assess the acute oxygen tolerance of juvenile FPMs. Oxygen concentration ranged between 8.8 and 6.2 mg L −1 in the high oxygen treatment (control), 5.0–0.4 mg L −1 in the medium treatment, and 1.3–0.04 mg L −1 in the low oxygen treatment (near‐anoxic conditions). Viability of juvenile FPMs depended on the concentration of available dissolved oxygen, such that all juveniles exposed to near‐anoxic conditions were classified as non‐viable, whereas all mussels exposed to high and medium concentrations were viable at the end of the 10 day experiment. Juveniles differed in their ability to tolerate near‐anoxic conditions, so that some individuals survived only 1 day and others survived up to 9 days. This study provides the first direct experimental evidence on the oxygen sensitivity of FPM juveniles and suggests that >10‐day events of very low dissolved oxygen at summer temperatures are fatal to juvenile FPMs, supporting the view that actions preventing low oxygen episodes in the substrate are essential for recruitment, and conservation, of FPMs.

Several northern freshwater fishes have gone through rapid adaptive radiation after the last glacial period, resulting in new species or intraspecific morphs with distinct life histories. Parasite infections can promote adaptive radiations and spatiotemporal differences in patterns of infections can potentially reveal incipient or ongoing speciation processes. We investigated intraspecific differentiation in a freshwater gadoid fish, burbot ( Lota lota ), by exploring differences in parasite infections between two potential life‐history morphs in Lake Konnevesi, Central‐Finland, one reproducing species characteristically in shallow littoral waters in February and the other possibly in deep profundal zone roughly a month later. By conducting a sampling campaign on reproducing fish over two consecutive years, we found significant differences in infections between the fish captured from littoral and profundal sites. More specifically, larval trematode and cestode infections were consistently less abundant in profundal fish, tracking long‐term exposure differences in shallow waters. In contrast, trophically transmitted metazoan infections in the fish gut, reflecting shorter‐term differences in feeding, showed higher variation between sampling years rather than depths. We also found suggestive evidence of higher trematode‐inflicted tissue damage per parasite in the profundal fish, implying lower tolerance to the infection. These results offer further evidence that burbot captured from littoral and profundal sites represent differentiated life‐history morphs. We propose that ecological and evolutionary differentiation within burbot populations across its circumpolar distribution may be more widespread than previously acknowledged.