Size-dependent change in body shape and its possible ecological role in the Patagonian squid (Doryteuthis gahi) in the Southwest Atlantic
Cephalopods are a versatile group with several mechanisms in place to ensure the success of future generations. The Patagonian long-finned squid (Doryteuthis gahi) populations on the southern Patagonian shelf are believed to be genetically homogenous, but the mechanisms connecting them geographically and temporally are unresolved. Individual growth is highly variable within cephalopod populations and is likely to affect individual patterns of migration and, thus, population connectivity as a whole. Therefore, this study aimed to make inferences about population structure by analysing the size at which individuals were mature and aimed to describe the intrapopulation growth (allometric) trajectories of body shape, using landmark-based geometric morphometric techniques to describe phenotypes. Samples were collected from June 1999 to November 2017 around 52°S and 58°W. Smoothing curves from binomial generalised additive models (GAMs) suggested two size modes of maturity in females and one or multiple modes in males dependent on year and season. There was a gradual elongation of the mantle and an increase in the relative fin size throughout ontogeny. Shape scores from geometric morphometric shape coordinates revealed a continuous non-linear allometric trajectory with a significantly different slope angle for males exceeding 20.1 cm dorsal mantle length (DML). At the extreme of this continuum, the largest ‘super-bull’ form had a substantially more elongated body shape, a heavier fin and a larger fin area compared to the rest of the population, a body shape associated with enhanced swimming performance which may help to maintain population connectivity. The prevalence of these rare super-bulls in the fishery varied widely between years, suggestive of phenotypic plasticity. This study provides evidence that the D. gahi population on the southern Patagonian shelf has a complex population structure with high intraspecific variation.
This study was supported by funding from the Falkland Islands Government. We are grateful to the scientific observers from the Falkland Islands Fisheries Department for sample collection and Beverley Reid for collecting traditional morphometric measurements and to three referees for their comments which greatly improved the manuscript. We thank the Director of Fisheries, John Barton, for supporting this work. The MASTS pooling initiative (Marine Alliance for Science and Technology for Scotland, funded by the Scottish Funding Council and contributing institutions; Grant reference HR09011) is gratefully acknowledged. Thanks are due to FCT/MCTES for financial support to CESAM (UID/AMB/50017/2019) through national funds and ERDF co-financing, under the Partnership Agreement for the PT2020 and Compete 2020 programs.
Compliance with ethical standards
Conflict of interest
The authors have no conflict of interest to declare.
Research involving human participants and/or animals
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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