The Periodicity and Formation of Otolith Increments in Salmo salar and Gasterosteus aculeatus

Wright, Peter John (1990) The Periodicity and Formation of Otolith Increments in Salmo salar and Gasterosteus aculeatus. PhD thesis, University of Glasgow.

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Teleost otoliths are calcareous structures deposited within the otic sacculi and are involved in mechanoreception. They are composed of aragonite crystals and a proteinaceous matrix. Accretion occurs through the successive deposition of a mineral rich and mineral deficient layer. In many fishes these two layers are laid down over a day, so producing a recognisable daily increment. This study investigated the formation of otolith increments and the influence of intrinsic and extrinsic factors on increment periodicity and size in two species of freshwater fish, the parr of Atlantic Salmon, Salmo salar and the three-spined stickleback, Gasterosteus aculeatus. A daily periodicity of increment formation, under a natural range of photoperiod and temperature, was confirmed for both species, by sequential sacrifice of fish of known age and fish with marked otoliths. Histological, histochemical and scanning electron microscopy techniques were used to investigate the formation of otoliths. Calcospherules (2-8mu) were found in dense layers over the apical surface of certain epithelial cells and on the otolith surface, suggesting that these structures were involved in otolith formation. Four types of non-sensory epithelia were identified within the sacculus; one of these (designated type II cells) appeared to secrete a fibrous material, containing sulphated glycoprotein, that was involved in crystal growth. A higher density of matrix fibres were found in the mineral- deficient layer than in the mineral-rich layer of the otolith increment. It was suggested that the high density of fibres has a limiting effect on crystal growth. A soluble calcium-binding protein, capable of inhibiting calcification in vitro, which may have been involved in the periodic inhibition of otolith mineralisation, was extracted from the otolith. The influence of extrinsic factors on increment periodicity was investigated in a series of experiments in which light, temperature and feeding frequency were systematically varied. Daily increments continued to be deposited even under constant conditions, indicating that increment deposition was endogenously controlled. However a 6L:6D photoperiod regime induced two indistinct increments per day in the otoliths of both salmon parr and sticklebacks. The effect of light/dark transitions on increment formation was investigated further in a series of in vivo 45Ca uptake experiments on salmon parr for a range of natural photoperiod regimes. These experiments demonstrated that a change from a net positive to a net negative flux from the otolith occurred at around three to five hours prior to dawn. A return to a net positive flux of calcium on to the otolith coincided with the dark:light transition indicating that the rhythm of increment formation was entrained to the light-dark cycle. An analysis of variations in total calcium concentrations in the plasma of salmon parr suggested that calcium levels declined around the time of no net deposition on to the otolith, although a regular cyclical decline was not evident in the limited data set. The possible influence of plasma calcium concentration on otolith calcification was investigated by examining the effect of induced hypocalcemia on in vivo calcium uptake on to the otolith. Induced hypocalcemia led to a decrease in the calcium concentration of the plasma and a net calcium efflux from the otolith, thus suggesting a positive relationship between plasma calcium concentration and otolith calcification. A comparison of somatic and otolith growth in salmon parr that grow throughout autumn and winter and smolt in their first year (S1) and those which almost cease feeding in autumn and smolt in their second year (S2), indicated an uncoupling of the two processes in S2 parr. However increment width was found to be positively correlated to oxygen consumption, in both S1 and S2 salmon parr, indicating that otolith accretion was related to metabolic rate. Consequently the uncoupling observed, may be explained by a decline in somatic growth during a metabolically favourable period. The results of this study were discussed in relation to previous concepts of the process and control of otolith increment formation.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Zoology
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78111
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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