Trophic Niche Comparison Between The Sympatric Species Of Dosidicus Gigas And Sthenoteuthis Oualaniensis In Equatorial Waters Of The Eastern Pacific Ocean

Genetically related Ommastrephidae species Dosidicus gigas and Sthenoteuthis oualaniensis are in a stable sympatric coexistence state in the eastern Pacific equatorial ocean.As highly migratory pelagic cephalopods,D.gigas and S.oualaniensis have important roles in the ecosystem of Eastern Pacific and are important economic species in China's pelagic fishery.In recent years,biologists have preliminarily studied the population structure,age,growth and other basic biology of these two species,but the different behavioral adaptation strategies formed by the pressure of evolutionary selection among homologous species is still limited.In the natural ecosystem,the study of multi-species coexistence in the community is an indispensable and not negligible part,and the exploration of the interspecific relationship between the related Ommastrephidae species in the tropical Pacific Ocean plays an important role in the further understanding of community construction and biodiversity maintenance mechanism.Biogeochemical tracers?e.g.stable isotopes and fatty acids?,as strong complements to the traditional methods of food web research,play important roles in the study of feeding ecology,and can be used to analyze the further information of feeding habits and migration patterns of cephalopods.In this study,the cephalopod species D.gigas and S.oualaniensis in the equatorial waters of the eastern Pacific Ocean were selected as the research objects.By measuring the morphological parameters of the beaks from the D.gigas and S.oualaniensis,the changes of the morphological parameters in the growth and development process of these two species were analyzed.The stable carbon and nitrogen isotope ratios and fatty acid profiles of the mantle muscle of D.gigas and S.oualaniensis were determined.The changes in trophic niche estimated from stable isotope and fatty acid profiles were compared under individual and population levels.Moreover,the feasibility of stable isotope and fatty acid profiles for studies on the feeding relationship and habitat changes between individuals?populations?were also explored.Using the time series data of muscle stable isotope ratio,the changes of trophic patterns of D.gigas and S.oualaniensis under abnormal environmental conditions?El Ni?o?were compared.The results are as follows:?1?Beak is the feeding organ of most cephalopods and has hard and stable structure.This inert tissue grows successively through the whole life span,which is a good carrier containing life history information and can better reflect its growth status.There are feeding information of cephalopods in the carcass soft tissue muscle for a period of time,and the feeding and migration status of cephalopods can be effectively understood by stable isotope analysis.In this study,the beaks of D.gigas and S.oualaniensis of different size groups in the same sea area were chosen,and the differences of individual growth patterns of these two species were investigated by morphological analysis.Principal component analysis?PCA?indicated that the morphological parameters of the upper hood length?UHL?of the beak could be used to study the growth of cephalopods,UHL increased with the growth of mantle length and gonadal maturity.The stable isotopic determination showed that their food source and trophic level are similar.The highly overlapped trophic niche indicated the competition between species was intense.?2?Stable isotope and fatty acid profiles are increasingly employed to understand the survival information of feeding and migration of cephalopods.In this study,the patterns of trophic niche of individuals with the same size of different populations were studied for both species.The results showed that although the carbon and nitrogen stable isotope ratios had no significant difference between the individuals of the same size of different species,their essential fatty acid profiles differed,likely because of their distinct food sources in similar habitats.These results were also verified by the results of trophic niche estimation.The overlap degree of stable isotope trophic niches was higher than that of fatty acids,indicating that the composition of fatty acids could better reflect the dietary differences of homologous species.?3?In order to adapt to the change of habitat environment,relative species often form differentiated feeding strategies.In this study,we used the stable isotope to analyze the trophic niches of two sympatric species during normal climate and El Ni?o period,respectively,combined with sea surface temperature and chlorophyll concentration,to explore the potential impact of environmental changes on the trophic niches and the differences between species.The results showed that?¹³C values of the two species were significantly different,indicating their different food sources.The?¹⁵N value of D.gigas was significantly lower than that S.oualaniensis though the D.gigas individuals were larger,indicating that S.oualaniensis had a higher trophic level,possibly because D.gigas reduced inter-species competition by feeding on smaller prey from lower trophic levels.The trophic niches of the two species overlapped?overlap area>25.8%?during normal climate years,but were narrowed and separated in El Ni?o period,indicating that these two species may have different response to the climate change of increasing of sea surface temperature and changing of primary productivity.Through the above experimental study on the trophic niche of sympatric coexisting D.gigas and S.oualaniensis,not only from the level of individuals and population deepened our understanding of the adaptive mechanisms of feeding behavior in the evolution of oceanic cephalopod homologous species,to reveal the oceanic cephalopods feeding strategies response to environmental changes and its trophic niche reference dynamic pattern.At the same time,these results can provide some references for further research on the coexistence mechanism of sympatric cephalopods.