Assessing the validity of stylets as ageing tools in Octopus pallidus

Abstract

A method to age octopus was determined through the quantification of growth increments within the stylet structure (highly reduced internal shells). To validate the periodicity of the increments, stylets were analysed from known-age laboratory-raised Octopus pallidus. The animals, which ranged from 3 to 8 months old, were exposed either to a simulated natural or to constant temperature regime. Transverse stylet sections were embedded in thermo-plastic cement and ground until a thin section was achieved. A pre-hatch region and first post-hatch increment was identified in the stylet microstructure. The number of increments, across all ages and temperature treatments, was in remarkably close agreement to age (number of days), clearly demonstrating that stylet increments are deposited daily throughout the lifecycle of O. pallidus. Morphometric analyses of the stylet indicated that increments were laid down regularly during stylet growth and that stylets grew in concert with body size, further supporting daily periodicity. This study successfully validates daily increments in stylets and thus demonstrate a method to age octopus, therefore, potentially having critical implications for future octopus research and the effective management of stocks worldwide.