| Argentinean shortfin squid Illex argentinus is a pelagic neritic cephalopod species in the southwest Atlantic Ocean.The squid is characterized by short lifespan,fast growth and semelparous reproduction.The squid is one of most important targeted species in global cephalopod fisheries,and also plays a key role as transient ‘biological pumps’ in the southwest Atlantic ecosystem.The squid exhibits feeding — spawning migration within the living range in the southwest Atlantic,in which it can migrate hundreds of or even thousands of miles between spawning ground and feeding ground to complete its life cycle.The squid adopts mixed income-capital breeding strategy,where the reproduction is mainly supported by the energy acquired and mobilized rapidly during the breeding season,coupled with using energy reserve when energy demand for reproduction increases significantly.The squid show spatial differences in feeding habits and also shift diets as they grow,leading to different trophic niches.Given the fact that species at higher trophic position have greater energy content,shifting diets to fish and cephalopods is expected to meet energy demand for reproduction.Herein,I.argentinus specimens were randomly collected from commercial jigging fisheries in the southwest Atlantic Ocean.By using tissue energy density technique and lipid extraction methodology,we analyzed the energy density of mantle,digestive gland,ovary and oviduct eggs for female I.argentinus before and after degreasing,as well as the lipid content per unit mass,lipid energy and its proportion of each tissue.We evaluated the correlation between shifting diet and energy demand for reproduction for I.argentinus by using stable isotope and tissue energy density analyses.The methodologies of fatty acid biochemical markers and tissue energy density technique were also applied to study the possible pathway of reproductive energy in terms of food intake.The main results and conclusions were as follows:(1)Lipid accumulation and contribution to reproductive energy.Results indicate that the energy density of all tissues decreased significantly after lipid extraction.During the maturation,female I.argentinus maintained a relative stable level of the lipid content and percentage of lipid energy in mantle soma and oviduct eggs.However,there was a significant decreasing trend in the lipid content and percentage of lipid energy in the digestive gland but an increasing trend in the ovary(Maximum in Phase IV)along with maturation.Furthermore,Pearson’s r analysis indicates that there was a significantly negative relationship in the lipid content,lipid energy and percentage of lipid energy between gonadal tissues(Combination of ovary and oviduct eggs)and digestive gland,but without a significant relationship between gonadal tis-sues and mantle soma.In summary,female I.argentinus exhibits tissue-specific lipid content and lipid energy accumulation.The lipid content has a significant contribution to the reproductive energy,and lipids in the gonadal tissues are significantly correlated with lipids in the digestive gland,which further proves that energy allocation to reproduction in I.argentinus mainly depends on the income resources.(2)Shifts in diet are related to increasing energy demand for reproduction.We found that reproductive energy increased significantly after the onset of maturation,while soma energy remained stable and digestive gland energy decreased.Consequently,the total energy accumulation did not change with maturation,but the proportion of total energy allocated to reproduction increased significantly.The stable isotopes δ13C and δ15N in the digestive gland,which reflect recent food intake,enriched significantly and the isotopic niche increased with maturation.Bayesian mixed effects models revealed that δ13C and δ15N were positively correlated with the percentage of energy allocated to reproduction and not with mantle length nor chlorophyll a concentration.Cumulatively,the results indicate that the diet shift in I.argentinus after the onset of maturation is attributed to an increasing energy demand for reproduction.These findings indicate that I.argentinus may feed on different prey items to meet the reproductive energy demand during sexual maturation.(3)Food preferences with maturation.During physiologically mature stage to spawning period,we detected that the overall fatty acids of digestive gland varied significantly.During the maturation,there was a significant decreasing trend in DHA and 22:1n9 and there was a significant increasing trend in 18:1n9c.The lower relative levels of DHA and 22:1n9 are possibly indicative of diet poor in crustaceans and cephalopods,while the higher relative levels of 18:1n9c are indicative of a diet rich in mesopelagic fish.In addition,there was a significant and negative relationship between the relative amounts of DHA and reproductive energy accumulation.These lines of evidence indicate that the squid shifts diets from crustaceans to fish and cephalopods as they grow,and such shift is closely related to the increased reproductive energy.In conclusion,energy allocation to reproduction in I.argentinus mainly depends on the income resources.Meanwhile,shifting diet in I.argentinus after the onset of maturation is attributed to an increasing energy demand for reproduction.And the squid shifts diets from crustaceans to fish and cephalopods as they grow,and such shift is potentially related to optimizing energy acquisition through the increase of high energy prey intake.The results put forward our understanding of the foraging strategy of wild animals in terms of energy acquisition for reproduction.Such findings would also provide insights into the species’ life history and support the resource exploitation in a manner of sustainability. |
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