Quantifying physiological influences on otolith microchemistry

Abstract

Summary1. Trace element concentrations in fish earstones ('otoliths') are widely used to discriminate spatially discrete populations or individuals of marine fish, based on a commonly held assumption that physiological influences on otolith composition are minor, and thus variations in otolith elemental chemistry primarily reflect changes in ambient water chemistry. 2. We carried out a long-term (1-year) experiment, serially sampling seawater, blood plasma and otoliths of mature and immature European plaice (Pleuronectes platessa L.) to test relationships between otolith chemistry and environmental and physiological variables. 3. Seasonal variations in otolith elemental composition did not track seawater concentrations, but instead reflected physiological controls on metal transport and biokinetics, which are likely moderated by ambient temperature. The influence of physiological factors on otolith composition was particularly evident in Sr/Ca ratios, the most widely used elemental marker in applied otolith microchemistry studies. Reproduction also triggered specific variations in otolith and blood plasma metal chemistry, especially Zn/Ca ratios in female fish, which could potentially serve as retrospective spawning indicators. 4. The influence of physiology on the trace metal composition of otoliths may explain the success of microchemical stock discrimination in relatively homogenous marine environments, but could complicate alternative uses for trace element compositions in biominerals of higher organisms.

Otoliths as individual indicators: a reappraisal of the link between fish physiology and otolith characteristics

“…This conclusion has recently been explored further in an elaborate rearing experiment that allowed Sturrock et al (2014Sturrock et al ( , 2015 to test of the effect of physiological factors on the otolith chemistry of adult marine fish. Blood samples, physiological data and water chemistry were sampled over the course of a year and related to temporally matched profiles of otolith element : calcium ratios.…”

Ecogeochemistry potential in deep time biodiversity illustrated using a modern deep-water case study

“…Critically, the trace element composition recorded in otoliths and other biominerals reflects both variations in the trace element composition of the water and the physiological condition of the individual. Biological traits such as reproductive status, metabolic rate and somatic growth rate may all influence the trace element composition of biominerals [19][20][21][22]. Consequently, isolated populations may display chemically discrete trace element compositions even when the ambient waters they inhabit are chemically homogeneous.…”

“…Interpretation of biomineral trace element concentrations therefore requires careful consideration of likely physiological influences. For instance, comparisons across life stages are likely to introduce physiological variability, particularly in Sr/Ca ratios [21,22,26]. Matching individuals by species and life stage is highly recommended.…”

Where do elements bind within the otoliths of fish?

“…Similarly, heterogeneity in the protein content of opaque and translucent regions of the otoliths (Hüssy et al 2004;Fablet et al 2011) will likely result in seasonally variable elemental concentrations (e.g. Kalish 1989;Sturrock et al 2015). Subsampling discrete regions of the otolith (e.g.…”