Positive buoyancy in eel leptocephali: an adaptation for life in the ocean surface layer |
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Authors: | Katsumi Tsukamoto Yoshiaki Yamada Akihiro Okamura Toyoji Kaneko Hideki Tanaka Michael J Miller Noriyuki Horie Naomi Mikawa Tomoko Utoh Satoru Tanaka |
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Institution: | (1) Ocean Research Institute, The University of Tokyo, Nakano, Tokyo 164-8639, Japan;(2) IRAGO Institute Co., Ltd., Tahara Aichi, 441-3605, Japan;(3) Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan;(4) Fisheries Research Agency, National Research Institute of Aquaculture, Minamiise Mie, 516-0193, Japan |
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Abstract: | Many planktonic organisms have adaptations such as floats or lighter substances to obtain buoyancy to help them remain in
the surface layer of the ocean where photosynthetic primary production occurs and food is most abundant. The almost totally
transparent eel larvae, called leptocephali, are a unique member of the planktonic community of the surface layer, but their
ecology and physiology are poorly understood. We conducted a comparative study on the specific gravity of planktonic animals
including 25 taxa of 7 phyla of marine invertebrates and 6 taxa of leptocephali (vertebrate) to gain a broad perspective on
the buoyancy of the eggs and larval stages of the Japanese eel. The specific gravity values of the various freshly caught
marine invertebrate taxa varied widely from 1.020 to 1.425, but leptocephali had some of the lowest values (1.028–1.043).
Artificially cultured live leptocephali had even greater buoyancies with specific gravities of 1.019–1.025 that were close
to or lower than seawater, and their buoyancy showed ontogenetic changes among the different early life history stages. Leptocephali
appear to have a unique mechanism of buoyancy control by chloride cells all over body surface through osmoregulation of body
fluid contained in the extracellular matrix of transparent gelatinous glycosaminoglycans filling their bodies. This adaptation
is likely a key factor for their survival by helping them to remain in the surface layer where food particles are the most
abundant, while being transparent for predator avoidance. The ontogenetic change in buoyancy of eel eggs, leptocephali and
glass eels likely enhances their larval survival, transport, and recruitment to terrestrial freshwater habitats. |
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