Travelling in Microphis (Teleostei: Syngnathidae) otoliths with 2D XRF maps: twists and turns on the road to strontium incorporation

Tropical island streams of the Indo-Pacific are home to freshwater fish, including pipefish species (Microphis spp.). A diadromous life cycle comprising a marine transition phase would be expected, being a compelling response to cope with hostile features of labile and fragmented habitat for species sustainability. Otoliths, as reliable keepers of individual life, were used to uncover life history traits in four pipefish species, one of which is endemic to New Caledonia.

All four species meet the same methodological challenge given their tricky otoliths, small, fragile and mute for growth marks using basic observation tools. Sr is calcium substituent in the mineral lattice, driven by salinity conditions thus useful to study diadromous migrations. Synchrotron based scanning Xray fluorescence 2D high-resolution mapping allowed us to tackle global and hyperfine strontium heterogeneous distribution. We developed analytical imaging processes to retrieve biological information from otoliths from the data generated by Synchrotron analysis.

By studying four species we uncovered plasticity in their life cycle: all species were amphidromous, apart from some freshwater residents for the endemic species. Delving deeper into the topic of element incorporation dynamics throughout the otolith with 2D fine scale images outlined the heterogeneity of Sr distribution to a greater extent than what is generally assumed. In addition to the trivial Sr incorporation driven by environmental ionic conditions, there is a mosaic arrangement of Sr distribution, and we hypothesize that biological control – especially growth during early life stages - may sometimes overrule stoichiometry.

Synchrotron based X-ray fluorescence proves to be a highly effective method as it unraveled fine resolution 2D mapping of Sr distribution in the otolith. It led to uncover a trade-off expression between biological and chemical driving forces that govern element incorporation. It shows that it is worth studying otolith formation and element integration at imbricated scales and our methods and results cue for a strong basis to future works and prospects in otolith sciences.