Recurrent growth cycles in juvenile Octopus vulgaris beaks: evidence from spectral analysis

Individuals of Octopus vulgaris were collected year‑round using juvenile octopus collectors deployed off Vilanova i la Geltrú (NW Mediterranean), between winter 2021 and spring 2022. A total of 111 early benthic octopuses ranging from 0.23 to 64.30 g were analysed. Age estimation based on daily beak growth increments in wild early benthic O. vulgaris provided, for the first time, a reconstruction of growth patterns spanning the full early ontogeny, from hatching through planktonic development to settlement. For each individual, daily increment‑width time series were ordered chronologically by age in days (d) and analysed using a frequency‑domain approach based on Fast Fourier Transform (FFT). Power spectra were computed to quantify periodic components in beak increment growth variability. Periods were derived as the inverse of spectral frequency, and only biologically relevant cycles (3–45 d) were retained. The FFT analysis identified 183 significant cycles (i.e., periodic signals or spectral peaks) across the full sample. Of these, 56% were concentrated within the 3–<6 d range (i.e. ≥3 and <6 d) and a further 19% in the range 6–<9 d, indicating that short‑term periodicity dominates the temporal structure of daily beak growth increments. Longer periods (≥9 d) were comparatively rare and showed weak and inconsistent recurrence. Both dominant bands exhibited high cumulative spectral strength and extremely low combined probabilities (p< 0.001), supporting the presence of recurrent modulation in growth intensity rather than stochastic noise. Stratified analyses showed high recurrence (≥20 significant cases) of the dominant 3–<6 d cycle across multiple seasonal groupings, both by hatching and capture date. In contrast, the 6–<9 d period showed elevated recurrence only in specific seasonal subsets. Because previous studies in O. vulgaris have demonstrated that beak growth increments are formed daily, the detected cycles reflect structured variability in growth intensity. The apparent absence of temporal synchrony among individuals suggests that these cycles are unlikely to be linked to discrete external events. Possible interpretations, proposed as preliminary working hypotheses, include endogenous growth rhythms, hormonal cycles, short‑term variability in food availability (potentially influenced by trophic or coastal oceanographic processes), or a combined control of the biomineralisation process by both environmental factors and internal biological rhythms.