Morphological variation provides a method of estimating thermal niche in hatchlings of the squid Loligo forbesi (Mollusca: Cephalopoda)

Abstract

Morphological variation was investigated in hatchling squid Loligo forbesi Steenstrup 1856 reared at three constant temperatures. Twenty-seven morphometric and chromatophore characters were measured on 180 hatchlings. Hatchlings were randomly sampled from 15 egg strings, representing six egg mops and spread across two spawning seasons. Size at hatching varied inversely with temperature and was significantly reduced at 16°C (mean mantle length 2.87±0.04 se mm) compared to 8°C (3.26±0.03 se mm). Hatchlings reared at 12°C developed significantly narrower mantles than those reared at 8 or 16°C. Principal components, stepwise elimination and canonical variates analyses performed using all 27 measured characters revealed that nine characters were vital in discriminating between temperature groups. Canonical structure values associated with eight of these characters were used in the construction of a canonical equation. By measuring the same eight characters on field-sampled L. forbesi, this equation can be used to estimate mean thermal history of wild hatchlings. Field-application of the techniques developed during the present study may allow assessment of realized thermal niche in a mobile marine invertebrate.