Determinazione dell'età nei cefalopodi: riconoscimento degli incrementi periodici nel becco superiore di Octopus vulgaris (Cuvier, 1797)

The aim of this work is to contribute to age estimation of common octopus, Octopus vulgaris (Cuvier, 1797), fished in the Bay of Napoli (Mediterranean Sea, Italy). Studies on age determination in marine species allow to examine the effective connection existent between environmental and physiological dynamics and resource exploitation, assessing the impacts of fishing activity on the ecosystem. Data about growth rates provide valuable information on the biology of a species, both at an individual and population level, thus are fundamental for a correct and conscious management, and this is particularly important for the most harvested species. The cephalopod mollusk O. vulgaris is widely distributed all over the world and represents one of the main marine resources for human consumption, but during the last years both production and trade showed a worrying decline. The common octopus shows prodigious motor and behavioral skills of surprising complexity if compared with the taxonomic position. The biology of this species is well known and widely treated in scientific literature, but information about growth rates and age determination appears still incomplete and contradictory; without an accurate idea of age, parameters like population age structure, mortality rate, longevity, productivity and recruitment processes rely on assumptions derived only from morphological assessments and catch data. Using potentially inaccurate methods may have disastrous implications for the continued sustainability of octopus fisheries. Recent studies on age determination in cephalopods species focus on growth increments of hard internal structures (e.g. stylets, statoliths, beaks or gladius) with the aim to delineate an accurate methodology similar to those used for other taxa (e.g. corals, bivalves, teleost fishes or mammalians). In O. vulgaris various are the internal structures analyzed during last decades for a direct age determination method: statoliths (Young, 1960), beaks and radula (Nixon, 1969; Raya & Hernández-González, 1998; Hernández-Lopez et al., 2001), eye lenses (Gonçalves, 1993), stylets (Hermosilla et al., in press). The present study focuses on counting and analyzing growth increments of the beak of O. vulgaris; this is formed by two horny structures, the upper and lower beaks (or “jaws” or “mandibles”), embedded in a muscular mass and constituted by protein matrix and chitinous fibers. This structure was studied till the end of Sixties, but it was only in 2001 that Hernández-Lopez et alii explored a new methodology, based on count of superficial microstructures (called “rings”), to ageing the common octopus, suggesting a daily trend of deposition. This methodology has been replicated for the aim of this study and applied on a wider sample of beaks (735), collected between 2002 and 2009 from octopus catched in the Bay of Napoli. Daily periodicity of growth increments was validated in wild caught specimens maintained under controlled conditions by marking the beaks and comparing the number of rings produced with the time elapsed between marking event and sacrifice. Subsequently we correlated the rings density (in terms of number of increments deposited per mm of beak length) to seawater temperatures, with the aim to investigate the seasonal pattern of deposition