Michele Melchior

Context: Phenology plays a key role in shaping population dynamics, community structure and evolutionary adaptations. For freshwater mussels that rely on a parasitic larval (glochidia) phase on fish, shifts in reproductive phenology driven by environmental conditions may result in mismatches between glochidia release and host fish availability. Aims: We investigated intra- and interspecific reproductive timing variations in sympatric Echyridella aucklandica and E. menziesii, and identified thermal cues (accumulated degree days, ADD) associated with brooding and glochidia maturation. Methods: Brooding activity and glochidia maturation were assessed fortnightly–monthly over 1 year within four New Zealand streams. Results: The previously unknown phenology of E. aucklandica showed earlier brooding (May–July) and longer gravidity (9–11 months) than for E. menziesii (August; 6–7 months). Both species exhibited peak brooding in late austral spring–summer (November–December). ADD played a key role in regulating the timing of brooding onset in both species, as evidenced by the narrow ADD range observed across sites, and the relationship between ADD and brooding onset in both species. Conclusion: The demonstrated link between ADD and reproductive phenology has broad implications in the context of climate change. Specifically, it raises concerns about potential timing mismatches in glochidia release and host-fish availability, which could affect the survival and reproductive success of freshwater mussels.

Larvae (glochidia) of Unionida mussels form obligate symbiotic associations with host fish to complete their life‐cycle. To identify previously unknown host fish associations for the threatened New Zealand Echyridella aucklandica , glochidia infestation on fish by two mussel species was quantified in four Waikato streams. A host for E. aucklandica was identified for the first time, with glochidia found to attach and encyst on the common smelt Retropinna retropinna but not on other fish species caught. In contrast, E. menziesii was found on a range of fish species but not on the two R. retropinna caught. Encysted E. aucklandica glochidia showed evidence of growth on the gills of host fish. These preliminary results provide critical knowledge of host–glochidia interactions in sympatric New Zealand unionid species to support integrated conservation management targeting both unionid communities and host fish populations.

We measured δ ¹⁵ N, δ ¹³ C, and percent carbon (C) and nitrogen (N) on foot and remaining soft tissue of the two native freshwater mussel species Echyridella aucklandica and E. menziesii (Hyriidae: Unionida) from three Waikato, northern New Zealand, streams to investigate differences among sites, sexes and species. Mean differences in δ ¹⁵ N and δ ¹³ C between foot and remaining soft tissue (mainly guts, gills and gonads) were 15 N and δ ¹³ C displayed marked differences among sites for both species, with δ ¹⁵ N variations consistent with the level of catchment land‐use intensification. Variation in δ ¹³ C and δ ¹⁵ N among replicate mussels was low indicating few individuals are required to precisely characterise isotope signatures of local populations. The lack of evidence of trophic resource partitioning among species, and consistent differences in isotope ratios between sites, suggests that the more widespread and abundant E. menziesii may provide an effective baseline integrator of catchment activities that alter carbon supplies and nitrogen sources.

We quantified trophic overlap between the invasive, non‐native catfish brown bullhead (Ameiurus nebulosus) and the New Zealand native shortfin eel (Anguilla australis) in four peat and riverine lakes using stable isotope (δ¹³C and δ¹⁵N) and gut content analyses. Across all lakes and fish sizes over the austral spring–summer period, shortfin eel guts were dominated numerically by fish prey (57% occurrence cf 42% in brown bullhead), while Diptera larvae were most commonly encountered in guts of brown bullhead (45% cf 14% in eels). Significant differences in % composition of animal contents in guts were detected between fish species and sampling occasions (n = 4) but not between lakes. In contrast, stable isotope signatures of brown bullhead and shortfin eel did differ significantly between lakes but not between sampling occasions, indicating enduring sources of nutrition despite apparently differing ingestion patterns over time. The R mixing model MixSIAR indicated that shortfins likely assimilated higher proportions of fish prey carbon compared to brown bullheads, which appeared to show greater assimilation of invertebrates, consistent with the results of gut content analyses. Isotopic niche regions, calculated in nicheROVER using probabilistic ellipses, indicated that shortfin eels occupied at least c.60% of brown bullhead trophic niche, which occupied less than 30% of eel trophic niche in all but one lake. These estimates suggest that brown bullhead has higher potential to influence shortfin eel nutrition than vice versa, or that a broad trophic niche occupied by eels provides resilience to the effects of overlapping consumption patterns with invasive omnivores.