Trophic Niche Differentiation And Coexistence Mechanism Of Different-Sized Groups Of Dosidicus Gigas Off Peru

Dosidicus gigas belongs to Ommastrephidae and is a cephalopod species distributed in the eastern Pacific Ocean from the Gulf of Alaska to southern Chile.D.gigas is considered a trophic carrier linking spatially relatively independent marine ecosystems and plays an important role in the marine ecosystems in the eastern Pacific Ocean.Three groups of length-at-maturity were distinguished on the basis of the mantle length（ML）of adult males and females.Intraspecific feeding variability maximizes the utilization of available food resources by D.gigas.However,the feeding strategies and coexistence mechanisms of three-sized groups remain unclear.In this study,we evaluated the feeding strategies of three-sized groups by analyzing muscle fatty acid composition,stable carbon and nitrogen isotopes,and quantifying the morphological characteristics of beak in a sample off Peru,Eastern Pacific Ocean,so as to explore the coexistence mechanism among the three-sized of D.gigas.In order to provide useful reference for the assessment and management of the fisheries resources of the eastern Pacific and the sustainable development and utilization of the resources.The results are as follows:（1）Fatty Acid Profile of Jumbo Squid（Dosidicus gigas）off the Peruvian Exclusive Economic Zone:Revealing the Variability of Feeding Strategies.Results of fatty acid analysis showed that the three group of fatty acids are dominated by polyunsaturated fatty acids（50.75%～51.64%）,followed by saturated fatty acids（34.72%～36.73%）,and the content of monounsaturated fatty acids was the least（11.64%～14.53%）.C16:0,C18:1n9,C20:1n9,C20:5n3,C22:6n3 were the main fatty acids that caused the difference in fatty acid composition of small,medium and large-sized group.The higher content of C20:4n6and（C18:2n6+C18:3n3）indicates that the large-sized group may feed more frequently in nearshore and deep waters.In addition,the large-sized group occupies the largest niche area,followed by the medium-sized group,and the small-sized group occupies the smallest niche area.Furthermore,there is no niche overlap between large and medium populations using fatty acid standard ellipses;the niche overlap between the large and small populations was obvious;the standard ellipses of the small and medium populations overlap significant,and the overlap area percentage was 32.16%.For the gender variability,the fatty acid profiles significantly differed between females and males for both small and medium groups（small-sized group:F=6.20,p<0.05;medium-sized group:F=6.57,p<0.05）and there was less competition between genders.The fatty acid composition of females and males for large populations was similar（F=1.44,p>0.05）,and competition between genders was more intense.Studies have shown that feeding competition was more intense in small and medium-sized group,and males tended to food on higher trophic levels of prey than females to reduce competition and better coexist in the same waters.Moreover,large-sized group,on the other hand,distinguish themselves from small and medium-sized group by broadening their living space,but it exhibited obvious internal gender competition.（2）Feeding strategies and trophic niche divergence of three groups of Dosidicus gigas off Peru:based on stable carbon and nitrogen isotopes.Results of stable carbon and nitrogen isotopes analysis showed that D.gigas had a wide range ofδ¹³C andδ¹⁵N values in muscle tissue,variable feeding behaviors and wide food sources.Theδ¹³C orδ¹⁵N values showed no significant difference between the small-and medium-sized groups(δ¹³C:p=0.68;δ¹⁵N:p=0.95),which shared the same habitats and fed on preys with the similar trophic level.Compared to small-and medium-sized groups,the large-sized group had a smaller range of habitats and consumed more nearshore foods.Both isotopes and feeding apparatus morphology indicated a high degree of niche overlap between the small-and medium-sized groups,whereas the large-sized group differed significantly from other groups.In addition,the niche width of the female was larger than that of the male in all three groups.We inferred that the sex differences in body length and reproductive behavior led to the difference in niche width.The isotopic niche overlap between female and male samples was the most significant in large-sized group and the least significant in the small-sized group,indicating that different feeding strategies were adopted by the three groups.These findings proved that the three groups of D.gigas off Peruvian waters adopted a feeding strategy with inter-and intra-group regulation.This feeding strategy maximizes the use of food and habitat resources and ensures that different size groups can coexist in the same waters.（3）Feeding strategies and trophic niche divergence of three groups of Dosidicus gigas off Peru:based on morphology of feeding apparatuses.Results of feeding apparatus morphometric metric showed that three-sized groups differed significantly in all the morphometric measurement traits.Post hoc Tukey’s HSD tests indicated that 12morphometric metrics of the large-sized group were significantly higher than those of the small-and medium-sized groups.Four morphometric metrics（upper hood length（UHL）,upper crest length（UCL）,upper rostrum length（URL）,upper rostrum width（URW））were significantly different between the small and medium-sized groups（ANOVA,p<0.05）.There were no significant difference between the small and medium-sized groups in eight morphometric metrics（upper lateral wall length（ULWL）,upper wing length（UWL）,lower hood length（LHL）,lower crest length（LCL）,lower rostrum length（LRL）,lower rostrum width（LRW）,lower lateral wall length（LLWL）,and lower wing length（LWL））.In the small-sized group,URL showed no significant sex difference.However,the other morphometric metrics（UHL,UCL,UWL,URW,ULWL,LHL,LCL,LRL,LWL,LRW,and LLWL）showed significant differences between male and female（ANOVA,p>0.05）.In addition,12 morphometric metrics showed significant sex differences in the medium-sized group（ANOVA,p<0.05）.In contrast,12 morphometric metrics showed no significant sex differences in the large-sized group（ANOVA,p>0.05）.In addition,SEAc was the largest in the large-sized group,followed by medium-and small-sized group.No overlap was observed in morphometric niche metrics between large-and medium-sized groups.Nevertheless,the niche overlap between large-and medium-sized group was small（20.13%）.The overlap rate between male and female was low in small-sized groups（18.23%）.However,the overlap between male and female niches was significant in the medium-（79.34%）and large-sized groups（78.56%）.