Abstract
Incrementally grown, metabolically inert tissues such as fish otoliths provide biochemical records that can used to infer behavior and physiology throughout the lifetime of the individual. Organic tissues are particularly useful as the stable isotope composition of the organic component can provide information about diet, trophic level and location. Unfortunately, inert, incrementally grown organic tissues are relatively uncommon. The vertebrate eye lens, however, is formed via sequential deposition of protein-filled fiber cells, which are subsequently metabolically inert. Lenses therefore have the potential to serve as biochemical data recorders capturing life-long variations in dietary and spatial ecology. Here we review the state of knowledge regarding the structure and formation of fish eye lenses in the context of using lens tissue for retrospective isotopic analysis. We discuss the relationship between eye lens diameter and body size, describe the successful recovery of expected isotopic gradients throughout ontogeny and between species, and quantify the isotopic offset between lens protein and white muscle tissue. We show that fish eye lens protein is an attractive host for recovery of stable isotope life histories, particularly for juvenile life stages, and especially in elasmobranchs lacking otoliths, but interpretation of lens-based records is complicated by species-specific uncertainties associated with lens growth rates.
Highlights
Retrospective chemical analyses of inert, incrementally formed tissues provide valuable insights into the trophic and spatial ecology of animals (Best & Schell, 1996; Bird et al, 2018), population connectivity and stock structure (Campana, 1999) and interactions between animals and climate (Trueman, MacKenzie & Palmer, 2012)
L. nasus (n = 30) lenses and associated body size data were collected from sharks incidentally by-caught by off-shore commercial gill-net fisheries within the Celtic Sea between 2011 and 2014, landed under dispensation in association with dedicated Cefas-led scientific fishery studies (Bendall et al, 2012; Ellis et al, 2016)
We initially identified the following issues that need to be addressed before eye lens tissues can be used as an effective target for ontogenetic stable isotope analyses: (1) The relationship between lens diameter and body size should be tested across taxa
Summary
Retrospective chemical analyses of inert, incrementally formed tissues provide valuable insights into (for instance) the trophic and spatial ecology of animals (i.e., their trophic geography) (Best & Schell, 1996; Bird et al, 2018), population connectivity and stock structure (Campana, 1999) and interactions between animals and climate (Trueman, MacKenzie & Palmer, 2012). The trace element and stable isotope chemistry of otolith aragonite has been used extensively to provide a chronological chemical record of the water inhabited by the fish (e.g., Kalish, 1991; Thorrold et al, 1997; Campana, 1999; Høie, Otterlei & Folkvord, 2004; Tzadik et al, 2017), addressing questions such as stock mixing and nursery origin of Atlantic bluefin tuna (Thunnus thynnus) (Schloesser et al, 2010), ontogenetic depth migration in the roundnose grenadier (Coryphaenoides rupestris) (Longmore et al, 2011) and migration in Atlantic salmon (Salmo salar) (Hanson et al, 2010) Such inorganic records do not provide information on trophic ecology. Variations in stable isotope compositions of carbon, nitrogen and sulfur within incrementally-grown tissues can be interpreted with respect to changes in diet or trophic level and/or variation in the isotopic composition of primary production across areas or habitats (the isotopic baseline)
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