Dietary specialization among individual squid: using Illex argentinus as a case and meta-analysis for other squid species

Squid, which occupy a similar role to teleost fish as open water predators, are opportunistic foragers. However, there is a lack of empirical evidence related to whether individual squid specialize their diets to optimize fitness. We investigated whether individual squid have specialized diets and what factors impact any specialization using the Argentine shortfin squid as a case study species, coupled with a meta-analysis for other squid species. Hutchinson's n-dimensional hypervolume concept was used to estimate individual dietary niches based on stable isotope and fatty acid analyses. Individual squid showed a high degree of dietary specialization, with individual specialization indices typically greater than 0.70, and pairwise niche overlap less than 0.5, with adults having greater specialization in dietary niche. For the Argentine shortfin squid, higher reproductive investment and water temperature increased individual dietary specialization. Individual dietary specialization probably reduces interindividual competition, optimizes food resource use and increases fitness and hence net energy gain for reproduction. The existence of dietary specialization at the individual level provides insight into the life history of squid.