engleski

Age determination of the short-finned squid Illex coindetii using statolith analysis

Statolith analysis is widely used method for investigating age and growth of cephalopods, especially squids. Statoliths are paired calcareous structures mostly composed of aragonite and a small percentage of organic matter, located in cephalopod equilibrium organs called statocysts. Statoliths are result of biomineralization, with dark rings deposited during daylight and light rings during darkness. It is suggested that statolith increments are formed daily, therefore one increment represents one day. A total of 500 Illex coindetii individuals, with dorsal mantle length range from 55 to 216 mm, were sampled in the eastern part of the central Adriatic Sea to study statolith microstructure. Statoliths were surgicaly extracted from the cephalic cartilage and stored dry in plastic vials. Before grinding, concave side of statoliths were mounted on microscope slide using thermoplastic resin. Grinding procedure consisted of proper grinding and polishing both sides of statolith, using lapping film sheets with 30, 12, 5 and 0, 05 µ grades. The total number of increments visible in the statolith microstructure were counted under a light microscope at 200 and 400x magnification, starting from the natal ring in the direction from statolith nucleus to the edge of the dorsal dome. The hatching dates of each squid was back-calculated from date of capture and increments counts. The life span of the Adriatic Illex coindetii was estimated at around six months. The youngest male had a total of 52 days (77 mm DML) and youngest female a total of 56 days (82 mm DML). The maximum counted age was 186 days (162 mm DML) for males and 180 days (188 mm DML) for females. No statistically significant difference in statolith increment counts was found between the sexes (p = 0.657), with mean values of 108 days (SD = 22.44) and 109 days (SD = 25.66) for males and females, respectively. The oldest individual was caught in spring and youngest during the summer season. Obtained results indicate the existence of seasonality in squid hatching time in the eastern Adriatic Sea.

age, growth, cephalopods, Adriatic Sea

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