Can beak shape help to research the life history of squid?

Abstract

Elucidating biological shape and concomitant growth increments is an important precursor for understanding life history and chronology. Logarithmic spiral and deviations from spiral are proposed for investigating growth variation in upper squid beaks. Twenty‐three beaks of two large squid species Kondakovia longimana and Mesonychoteuthis hamiltoni were photographed to a resolution between 20 and 30 pixels per mm in the sagittal plane, then digitised within a standard deviation of 0.05 mm. Estimating logarithmic spirals by nonlinear least squares provided a close fit (r² > 98%) to every beak. Deviations from spiral ranged up to 1 mm on individual beaks, and were significantly non‐random, often with a prominent spike then dip midway (20 to 40 mm) from a beak tip. Species differed both in the distance of spike and dip from the beak tip and also the distance between the spike and dip. Change from inshore to oceanic life or in trophic level are postulated causes of these non‐random deviations. The method of fitting spirals and deviation from spiral growth is a quick, nondestructive complement to sectioning. Uncertainty on original squid size, maturity and gender restrict conclusions on the beaks to being preliminary in nature. Further research is recommended to confirm and extend the new concept of spiral shape and growth deviations for investigating squid lifestyle.

Keywords:

• logarithmic spiral
• nonlinear least squares
• growth deviation
• oegopsid