Age studies based on daily growth increments in statoliths and growth lamellae in cuttlebone of cultured Sepia officinalis
One hundred and six individuals of Sepia officinalis were cultured throughout its life cycle in two different temperature regimes: 13–15°C and 18–20°C. The number of increments in the statoliths and the number of lamellae in the cuttlebone were read at known ages in different individuals. The formation of growth increments in the statoliths was linearly related with individual age, but it was independent of temperature. By comparison between the slope of these linear relationships and the bisecting line, the hypothesis "1 increment=1 day" was validated in individuals as old as 240 days. In specimens older than 240 days, the number of increments was underestimated due to the poor resolution of the later growth increments. The maximum number of days in captivity (420) was about 80% of the life span estimated for this species in the area studied. Preliminary validation was also obtained from statoliths marked with tetracycline. Increments between tetracycline marks were not visible. The mean distance between marks in ten statoliths was 44 µm (±6). Considering the same distance in statoliths of wild individuals, the mean number of increments counted was 37 (±6). This result is very close to the 40 days passed between both tetracycline marks. A straight line significantly different for each culture temperature defined the relationship between number of the cuttlebone lamellae and age. The number of lamellae in the cuttlebone does not correspond to the real age. The time necessary for the formation of one lamella was 8±2.3 days at 13–15°C, whereas the deposition of one lamella lasted 3.1±1.06 days at 18–20°C. In conclusion, the periodicity for lamellar deposition can be defined only if the temperature where the animal lived is considered.
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