Comparative growth increments of gladius surface and cross-sections to estimate Sthenoteuthis oualaniensis age

Abstract

The age of the purpleback flying squid (Sthenoteuthis oualaniensis) has been widely studied over its geographic range based on hard structure analysis, especially the statolith. In this study, we propose an additional method for estimating the age of S. oualaniensis using growth increments in the gladius. Our results show regular growth increments in the proostracum dorsal surface and stem cross-sections. Linear regression analysis of the relationship between statolith and gladius readings showed the formation of increments in stem cross-sections to be daily, assuming that the formation of increments is daily in statoliths. A progressively more faint microstructure in the half-posterior part of the gladius possibly explains why the number of growth increments in the proostracum dorsal surface is less than the statolith-determined age. The low average percentage error and coefficient of variation indicate that our modified processing and counting methods are reliable. We believe that using growth increments in the gladius stem cross-section is a feasible alternative tool for determining the age of S. oualaniensis, whereas using growth increments in the proostracum dorsal surface is more informative for reconstructing ontogenetic life history.