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This special issue represents a summary and development of the 5th International Sclerochronology Conference (ISC) held in Split, Croatia in June 2019.

Over the past two decades, the field of sclerochronology has been rapidly developing, with scientists devoting significant efforts to studying the physical and chemical variations in hard tissues of aquatic organisms. Most of this research has been limited to certain taxa and geographic areas. Although growth increments in fish otoliths are used for sclerochronology purposes, relatively little has been done in the Mediterranean Sea. According to the literature, the chemical composition of otoliths from Mediterranean fish species has primarily been used for analyzing migration patterns, habitat use, and population structure of commercially important fish species. To the best of our knowledge, there are no studies on fish growth chronology construction conducted in the Mediterranean Sea. In order to identify the opportunities for sclerochronology research on fish from the Mediterranean, we used FishBase to identify potential candidate species with a sufficiently long lifespan and clearly defined growth increments for growth chronology construction and otolith chemistry research. We also present the challenges and limitations for sclerochronology research, including: (i) very few fish species in the Mediterranean Sea have a longevity of several decades ; (ii) issues associated with reliable age determination for certain long-lived fish species ; (iii) a general lack of understanding and effort to constructed and manage otolith collections ; and (iv) limitations imposed by the availability of funding, expertise, and instrumentation. Despite these challenges, fish sclerochronology research has strong potential in the Mediterranean and adjacent seas. Recent studies in the Adriatic Sea have resulted in the construction of bivalve chronologies and the geochemical analysis of shells, providing important time-series data for comparative analysis and a multispecies approach. Furthermore, studies conducted in other parts of the world have demonstrated great potential for the use of fish otoliths in monitoring environmental variability and the effects of pollutants and disturbance.

Using morphometric measurements and wavelets functions, the asterisci otoliths of curimatã, Prochilodus nigricans were analysed to identify the variation in shape and growth increment of individuals from Solimões, Japurá and Negro rivers of the Amazon basin, Brazil. The morphometric and morphological analyses did not reveal evidences of population segregation among rivers, but variations were found in the estimation of otolith growth increment. Also, the otolith shape showed a high variability between individuals, identifying four morphotypes. Morphotype 1 shows a more oval shape with a posterior zone clearly rounded; Morphotype 2 shows the posterior zone rounded, but the anterior end is more elongated; Morphotype 3 shows a completely different shape, elliptic-pentagonal and Morphotype 4 shows the posterior zone rounded, but the anterior end is more elongated and it is the pattern with antirostrum and rostrum more pointed and a deep notch. Therefore, the otolith shape exhibited a phenotypic plasticity that it was not associated with the metabolism of otolith growth. Whereas the otolith shape indicated a homogeneity in the sound perception through Amazon basin, the growth rates revealed an adaptive mechanism to environmental conditions or migratory process of this species. RESUMO Medidas e funções wavelets dos otólitos asteriscus de curimatã, Prochilodus nigricans foram analisados para identificar a variação da forma e do incremento de crescimento dos indivíduos dos rios Solimões, Japurá e Negro da bacia Amazônica, Brasil. As análises morfométricas e morfológicas não revelaram evidências de segregação populacional entre os rios, mas foram encontradas variações na estimativa do incremento do crescimento dos otólitos. Além disso, a forma do otólito mostrou uma alta variabilidade entre os indivíduos, identificando quatro morfotipos. O Morfotipo 1 mostra uma forma mais oval, com zona posterior levemente arredondada; Morfotipo 2 mostra a zona posterior arredondada, mas a extremidade anterior é mais alongada; Morfotipo 3 mostra uma forma completamente diferente, elíptico-pentagonal e Morfotipo 4 mostra a zona posterior arredondada, mas a extremidade anterior mais alongada e ele tem padrão com antirostrum e rostrum mais pontiagudos e abertura profunda. Portanto, a forma do otólito exibiu uma plasticidade fenotípica que não foi associada ao metabolismo do crescimento do otólito. No entanto a forma do otólito indicou uma homogeneidade através da percepção do som na bacia Amazônica, e as taxas de crescimento revelaram um mecanismo adaptativo às condições ambientais ou ao processo migratório da espécie.

It has recently been confirmed that some species of decapod crustacean retain their gastric mill and calcified region of the eyestalks throughout their moults. It had previously been assumed that crustacea lost all growth structures that could potentially record age information, such as the bones and otoliths in fish, through moulting. In this study, a novel preparation method was used for observing growth increments within these calcified structures of Nephrops norvegicus and Cancer pagurus (Linnaeus, 1758). This method involved: boiling, drying, resin embedding, sectioning, and polishing the gastric mill and eyestalks. Clear and readable growth increments were observed in longitudinal sections of the mesocardiac ossicle of the gastric mill for N. norvegicus, and longitudinal sections of the zygocardiac ossicle of the gastric mill for C. pagurus. Growth increments were also observed in longitudinal sections of the calcified region of the eyestalk for N. norvegicus. Validation is needed to confirm the periodicity of these growth increments.

15 páginas, 7 figuras, 1 tabla, A study on morphological, structural and biochemical composition of Sepia officinalis and Loligo vulgaris statoliths and statocyst endolymph was undertaken with the aim of determining the major factors affecting the deposition process of statolith formation and to clarify the cause for the poor definition of the growth increments in S. officinalis statoliths. It is suggested that the different biochemical composition of the statocyst endolymph found in the two species accounts for distinct statolith crystallisation processes, which results in a different microstructure. This explains the better definition of growth increments in L. vulgaris statoliths comparing with those of S. 21 21 officinalis. The protein content as well as Ca and Mg concentrations in the endolymph are 21 more implicated in growth increments formation than Sr ion concentration. Moreover, the daily variations of the three factors mentioned, allowed us to formulate a working hypothesis to explain the daily deposition of growth increments: a dark ring (rich in organic matter) is deposited during daylight whereas a light ring (rich in CaCO ) during darkness. These results are discussed in the 3 light of alternative hypotheses explaining the deposition mechanisms in statoliths, This work is a part of a PhD thesis by V. Bettencourt, funded by FCT-Portugal (Praxis /BD/4561/94) and supported by CICYT (MAR 95-1919-C05). [SS]