Age Determination of the Common Octopus (Octopus vulgaris) Using Growth-Increment Counts in the Beak: Methodology and application to samples collected off the west coast of Libya.

Abstract

Estimating the age of octopuses has long been one of the more stubborn problems in cephalopod biology, mainly because these animals have no bones, scales, or otoliths of the kind fisheries scientists rely on for fish. What Octopus vulgaris does have is a hard, chitinous beak that grows in layers, and under a microscope those layers show up as a sequence of fine concentric lines – growth increments – that are laid down at a rate close to one per day. This paper sets out, in practical terms, how the beak of the common octopus is prepared, sectioned, and read for ageing purposes, and applies the method to a sample of 150 specimens. We describe specimen handling, beak extraction, embedding and sectioning of the lower beak hood, microscopic reading of the increments, and the statistical treatment of the resulting age estimates, including their relationship with mantle length and body weight. Mean estimated age across the sample was 257.8 ± 125.0 days (range 60–496 days), and the age-length relationship was well described by a Gompertz growth curve (DML∞ = 18.08 cm, k = 0.0119 day⁻¹, r = 0.88). Inter-reader precision, expressed as the Average Percent Error, was low enough to support routine use of the technique. We discuss the strengths and limitations of beak ageing relative to the more commonly used statolith method, and outline where the technique is most useful in practice.Note on the dataset used in this paper: no live specimens were sampled for this document. The 150-specimen dataset used throughout was generated to match the size, weight, and growth-rate ranges reported in the published literature on O. vulgaris beak ageing, purely to illustrate how the data are collected, read, and analyzed. Anyone applying this protocol to a real fishery or laboratory sample should substitute their own beak-reading measurements for the figures and table given here.