Abstract
The mineral component of fish otoliths (ear bones), which is aragonitic calcium carbonate (CaCO3), makes this structure the preferred sample choice for measuring biological carbon and oxygen-stable isotopes in order to address fundamental questions in fish ecology and fisheries science. The main drawback is that the removal of otoliths requires sacrificing the specimen, which is particularly impractical for endangered and commercially valuable species such as Atlantic bluefin tuna (Thunnus thynnus) (ABFT). This study explores the suitability of using the first dorsal fin spine bone of ABFT as a non-lethal alternative to otolith analysis or as a complementary hard structure. The fin spines of freshly caught ABFT were collected to identify carbonate ions within the mineral matrix (i.e., hydroxyapatite) and to determine the nature of the carbonate substitution within the crystal lattice, knowledge which is crucial for correct measurement and ecological interpretation of oxygen and carbon stable isotopes of carbonates. Fin spine sections were analyzed via X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, and Fourier Transform InfraRed (FTIR). The XPS survey analysis showed signals of Ca, O, and P (three compositional elements that comprise hydroxyapatite). The Raman and FTIR techniques showed evidence of carbonate ions within the hydroxyapatite matrix, with the IR spectra being the most powerful for identifying the type B carbonate substitution as shown by the carbonate band in the v2 CO32− domain at ∼872 cm−1. The results of this study confirmed the presence of carbonate ions within the mineral matrix of the fin spine bone of ABFT, showing the feasibility of using this calcified structure for analysis of stable isotopes. Overall, our findings will facilitate new approaches to safeguarding commercially valuable and endangered/protected fish species and will open new research avenues to improve fisheries management and species conservation strategies.
Highlights
Use of otoliths found in teleost fish species is preferred by fishery biologists as they provide time-calibrated archives of isotopic information (δ13C and δ18O ) within the mineral matrix that can be used to address fundamental questions in fish ecology and fisheries science related to migration, habitat use (Campana, 2005; Elsdon et al, 2008; McMahon, Berumen & Thorrold, 2012) and stock structure (Patterson, Mcbride & Julien, 2004)
The presence of carbon and nitrogen and the high proportion of oxygen (O/Ca and O/P ratios were higher than expected in hydroxyapatite) in all tuna fin spines analyzed in comparison with the synthetic HA (Lu et al, 2000) is certainly evidence of the presence of collagen Type I, an organic matrix that makes up the protein component of bone and is assimilated from the carbon and nitrogen contained in the protein constituents of a consumer’s diet (Koch, 2007; Rey et al, 2009)
The major contribution of this study is the confirmation that the mineral matrix of fin spine bone regularly showed carbonate ions that substitute for phosphate (B-type) as carbonate ions were detected within all the ABFT fin spines analyzed
Summary
It is not practical for commercially valuable species such as Atlantic bluefin tuna since it greatly affects the appearance of a fish, diminishing its market value Alternative hard tissues such as the first dorsal fin spine bone may provide valuable and complementary chemical information, so sampling these tissues represents a non-lethal, minimally invasive sampling method (Zymonas & McMahon, 2006). The mineral component of bone, Hydroxyapatite (Ca10(PO4)6(OH)2), is a type of calcium phosphate apatite, which is deposited onto collagen fibrils, providing strength to the bone structure and serving as an ion reservoir (LeGeros, 1981) In biological tissues, this mineral matrix is complex, rarely stoichiometric, and usually calcium-deficient.
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