Sawfish, read in tooth and saw: Rostral teeth as endogenous chemical records of movement and life-history in a critically endangered species

The ecology of endangered and rare species can be difficult to study owing to their low abundances and legal limits on scientist’s ability to catch, sample, and track them. This is particularly true of sawfish (family Pristidae), whose numbers have declined precipitously, placing all five species on the IUCN Red List of Threatened Species worldwide. Best known for their distinctive, toothed rostrum, the ecology, movement and life-history of sawfish are poorly understood. Sawfish rostral teeth are modified placoid scales, which grow continuously throughout the life of the fish. This continuous growth, combined with their stable calcified makeup, makes sawfish rostral teeth a potential source of temporal records of chemical and isotopic changes through the life of the fish. Rostral teeth are often preserved in museums and as curios, potentially providing a source of life-history data to inform conservation actions without the need for field study, or as an important compliment to it. This is the first study to recover temporally explicit chemical data from sawfish rostral teeth. Using archived samples of largetooth sawfish (Pristis pristis) we show that multiple chemical tracers can be recovered from sawfish rostral teeth, and that these tracers can be used to understand movement across salinity gradients. We further show that sawfish rostral teeth contain repeated structures and indistinct banding which could potentially be used for ageing or growth analysis of fish.