Comparative Biochemistry and Physiology Part A 120 (1998) 551 – 556 Movement of water in fish otoliths R.W. Gauldie *, C.E. Thacker, I.F. West, L. Wang Hawaii Institute of Geophysics and Planetology, School of Ocean, Earth Sciences and Technology, Uniersity of Hawaii, 2525 Correa Road, Honolulu, HI 96822, USA Received 28 March 1996; received in revised form 12 March 1998; accepted 1 May 1998 Abstract The diffusion of small organic and inorganic solvent molecules and larger dye molecules into fish otoliths has long been utilized in clearing and staining otoliths to facilitate age estimation studies. This study quantifies the diffusivity of water into otoliths of Beryx splendens as a general analog of small molecule diffusivity in otoliths. The effects on the optical properties of otoliths of the diffusion of solvent and dye molecules into otoliths are also discussed. © 1998 Elsevier Science Inc. All rights reserved. Keywords: Otolith; Diffusion; Water; Beryx splendens 1. Introduction Recent studies [21,22] have shown that the chemical composition of fish otoliths change when they are soaked in water or ethanol because many ions (Na + , Cl - , etc.) are not bound in the crystal structure of the otolith and diffuse out of the otolith into surrounding fluid. Similarly, direct measurements have shown that the large 222 Rn molecule, the intermediate daughter product in the radioactive decay of 226 Ra to 210 Pb, also diffuses out of otoliths [9]. Diffusion of ions out of the otolith draws attention to the reverse process, diffusion of ions and molecules into the otolith. There has always been indirect evidence for the ready diffusion of water and other solvents into otoliths. Otoliths are often treated with substances such as wa- ter, oil, solvents and dyes to facilitate reading their optical patterns. The diffusion of water into otoliths causes an increase in light transmission, hence the use of the term ‘clearing’ in the otolith literature. The optical patterns of teleost otoliths are widely used both for aging and extracting life history patterns of fish. Numerous optical techniques exist for reading the in- formation stored in otoliths; they range from viewing whole otoliths under a light microscope, through exam- ination of broken and burned sections under oil, to counting microincrements [2,15,26,29]. Otoliths may also be cleared with substances such as water, glycerin, alcohols or xylene to increase the contrast of internal features [2,15,18]. These procedures are effective be- cause the crystal structure of the otolith is porous and rapidly takes up fluids. Dyes that react with calcified tissues have also been used to stain otoliths. Calcium- specific stains such as alizarin red and calcein, as well as the antibiotic tetracycline, produce bands on otoliths that are discernible under visible as well as ultraviolet light [1,7,23,28]. Non-calcium-specific stains such as fluorescein have been used to increase the internal contrast of otoliths [5]. Flain [5] also reported clearing of otoliths rendered opaque by formalin by soaking them for at least a week in a 3% solution of trisodium orthophosphate. These techniques are used not only to facilitate estimates of age, but also to mark fish for later identification. This paper provides a quantitative mea- sure of the uptake of water molecules into the otolith that can be used as an analog of small molecule diffu- sion in otoliths. * Corresponding author. Present address: 76 Inglis Street, Seatoun, Wellington, New Zealand. Tel.: +64 4 3885486. 1095-6433/98/$19.00 © 1998 Elsevier Science Inc. All rights reserved. PII S1095-6433(98)10065-X