Congruence in stable isotope values among two sympatric freshwater mussel species in northern New Zealand streams

ABSTRACT

We measured δ¹⁵N, δ¹³C, 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 δ¹⁵N and δ¹³C between foot and remaining soft tissue (mainly guts, gills and gonads) were <0.5‰, whereas %C and %N were higher in foot tissue and C:N values were lower. No differences were detected between mussel species, but δ¹⁵N and δ¹³C displayed marked differences among sites for both species, with δ¹⁵N variations consistent with the level of catchment land-use intensification. Variation in δ¹³C and δ¹⁵N 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.

KEYWORDS:

• Unionida
• Echyridella aucklandica
• Echyridella menziesii
• δ¹⁵N
• δ¹³C
• carbon:nitrogen ratio

Acknowledgements

We thank landowners Brent Pini and Nora Palmer (Paepaeruru Trust) for access to the Ohautira site, Anjana Rajendram and Judy Hoult for performing the stable isotope analyses, and two anonymous referees for helpful reviews.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The study was funded by The University of Waikato.