Beak growth pattern of purpleback flying squid Sthenoteuthis oualaniensis in the eastern tropical Pacific equatorial waters

Cephalopod beaks maintain a stable morphology, implying that they can be used to explore ecological influences on squid life history. Understanding the beak growth pattern can help us to improve knowledge of the trophic characteristics of squids and to estimate squid biomass. Sthenoteuthis oualaniensis is widely distributed in eastern tropical Pacific equatorial waters and has been caught commercially by Chinese squid jigging fleets since 2011. In this study, we randomly took 220 samples of S. oualaniensis with mantle lengths (ML) of between 119 and 351 mm and body weights (BW) of between 45 and 1975 g and measured six beak morphological variables for each sampled squid. The relationships between ML and all of the beak variables were power functions, except for upper lateral wall length (ULWL), upper crest length, and lower lateral wall length, which showed linear relationships with ML. The relationships between BW and the six beak variables were best fitted with power functions, and these functions can be used to estimate squid biomass from beak variable values. All of the beak morphological variables varied according to the maturity stage of the squid. Results of a post hoc comparison suggested that the values of beak morphological variables for immature squids (maturity stages I and II) showed significant differences from the corresponding values for mature squids (maturity stages III–V). These differences may result from changes in diet that occur during maturation, which affect the relevant mandibular muscle strength. The most common pigmentation stages (PS) encountered were II–V. The relationships of PS to ULWL and lower wing length were best described by exponential functions. Beak morphology and pigmentation of S. oualaniensis tended to change markedly with ontogenetic stage. It is easy to separate mature and immature squids based on their PS. This study provides important biological information on S. oualaniensis.