1. Key Points

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The crystalline lenses in fish eyeballs record their life history (Fig. 1) in a manner similar to how tree rings record annual growth.

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A method was developed to examine the global distribution area and trophic history of a target fish by analyzing the nitrogen isotope ratios of the amino acids that are the main components of the crystalline lens. This method will allow the life history of the fish to be reconstructed at the highest temporal resolution currently possible, revealing “when and in which ocean the fish had lived.”

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This method can be applied to almost all marine organisms. Therefore, it is expected to play an important role in marine ecological research and for fisheries resource management, e.g., by identifying and determining production regions.

2. Overview

In collaboration with Professor Shin-ichi Ito of the Atmosphere and Ocean Research Institute at the University of Tokyo and Principal Investigator Michio Yoneda of Japan Fisheries Research and Education Agency, Researcher Yota Harada, Director Naohiko Ohkouchi, and their colleagues at the Biogeochemistry Research Center of the Research Institute for Marine Resources Utilization at the Japan Agency for Marine-Earth Science and Technology developed a method to study the ecology of fish from its crystalline lens.

The aim for the sustainable use of marine resources necessitates a thorough understanding of the ecology of marine organisms. However, our knowledge of the ecology of marine organisms remains insufficient compared to that of the ecology of terrestrial organisms. Observation of marine organisms, such as fish, is challenging as they move through vast, three-dimensional oceanic spaces over long distances. Bio-logging, a method of attaching a GPS or other recording devices to fish, has been widely used in recent years. Nevertheless, such methods have limitations based on the size of the fish and the battery life.

To overcome these limitations, the current study developed a method to analyze fish ecology using the crystalline lens of their eyeballs. The crystalline lens of a fish eyeball begins forming in the egg. The lens grows incrementally by forming new layers, such as how trees add growth rings annually, and these are retained throughout their lives. Thus, the growth layers can be collected in chronological order, in a manner like peeling an onion skin (Video 1). The crystalline lens contains an amino acid called phenylalanine, which can only be acquired through feeding. Analyzing the nitrogen isotope ratios therein could likely allow the distribution and trophic history of fish to be estimated .

In this study, we used crystalline lens analysis to investigate the typical migration pattern of chub mackerel. We predicted that these fish had spent their larval stage in subtropical waters with a low nitrogen isotope ratio (off the Izu Peninsula) and moved to the subarctic waters with a high nitrogen isotope ratio (off the coast of Sanriku) as they grew. (Figs. 1 and 2).

The method developed in this study can be applied to almost all marine organisms with crystalline lenses, and it is expected to play an important role in marine ecological research and fisheries resource management.

The findings of this study published in Frontiers in Marine Science on February 3, 2022 (Japan time).

Title：Compound-specific nitrogen isotope analysis of amino acids in eye lenses as a new tool to reconstruct the geographic and trophic histories of fish

Authors：Yota Harada¹, Shin-ichi Ito², Nanako O. OGAWA¹, Chisato Yoshikawa¹, Naoto F. Ishikawa¹, Michio Yoneda³, Naohiko Ohkouchi¹

Affiliation：
1. JAPAN AGENCY FOR MARINE-EARTH SCIENCE AND TECHNOLOGY
2. Atmosphere and Ocean Research Institute, The University of Tokyo
3. Japan Fisheries Research and Education Agency (FRA)

Doi：10.3389/fmars.2021.796532