Mechanism Of Two Coexisting Balitoridaes Trophic Specialization And Morphological Divergence

Food is the key source of living organisms given that energy intake regulates individual growth,survival and productive fitness.Fish community usually use different food resources to reduce interspecific competition,at the same time,trophic partitioning could trigger morphological changes associated with resource exploitation,which in turn promote trophic segregation.In order to explore the coexistence mechanism of two sympatric Balitoridaes species in Zaodu River,we carried out related research.From November 2016 to March 2018,samples were collected in the Shixi section of Zaodu River in Guizhou Province,China,including Metahomaloptera omeiensis,Sinogastromyzon sichangensis and their potential food:Fixation algae,terrestrial plants,Aquatic insects and Particulate organic matters（POM）.The stable isotope ratios of carbon and nitrogen were determined.SIAR model was used to calculate the food resources,niche width and niche overlap in winter and summer of the fishes to reveal the differences in the utilization of food resource between the two species of fish.To reveal the relationship between external morphology and food resource utilization,calculate the different variation between the two species that presented divergent ecomorphological traits,including relative length of the caudal peduncle（RLP）,relative mouth width（RMW）,relative gut length（RGL）,relative head length（RHL）,relative eye diameter（RED）and relative body high（RBH）,Spearman’s correlation coefficients were estimated between ecomorphological traits andδ¹³C,δ¹⁵N values.To reveal the difference of digestive system between two kinds of fishes under different food resource utilization.80 specimens from both M.omeiensis and S.sichangensis were selected to measure body length,body weight,digestive system weight,calculated digestive system index（DSI）.Live adult fishes were selected to observe the morphology of the mouth and digestive system.The height of mucosal folds,the thickness of muscular layer and the number of mucous cells in each segment of the digestive tract were measured and compared by histological and histochemistry techniques.The main research results were as follows:1.Isotope analysis showed that the distribution values ofδ¹³C andδ¹⁵N of each potential food were all lower than that of the two species,the value ofδ¹³C was similar between the two fishes,however,the value ofδ¹⁵N of S.sichangensis was higher than M.omeiensis.The food composition between them were different.S.sichangensis mainly feeded on aquatic insects,which accounted for 54%,POM for 25%and fixation algae for 15%.However,aquatic insects,POM and fixation algae accounted for a similar proportion in the food composition of the M.omeiensis.The niche width of S.sichangensis was larger than M.omeiensis in the whole year,and the niche width within the species varied with the season.With the increase in temperature and precipitation,their trophic niche width expanded,By contrast,when food was scarce,their trophic width decreased.2.The ecomorphology results showed that there were significant difference between S.sichangensis and M.omeiensis in RLP,RMW,RGL and RBH.The Spearman’s correlation coefficients showed thatδ¹³C andδ¹⁵N were significantly positively correlated with RLP,but negatively correlated with RMW and RGL.3.Anatomical results showed that the morphological difference of the digestive system was obvious.First,the intestine of S.sichangensis was circled into a“φ”shape,with hepatopancreas covered on the ventral part,however,that of M.omeiensis was organized in a complicated ways.Second,the DSI of S.sichangensis were greater than those of M.omeiensis.Besides,the digestive tract of S.sichangensis had a significant higher mucosal fold and thicker muscular layer compared with M.oneiensis,however,there was no significant difference in the number of goblet cells and hepatopancreas（P<0.05）.AB-PAS staining showed that two kinds of similar types of mucous cells in the digestive tract could be divided into four types.TypeⅠfor red,typeⅡfor blue,typeⅢfor purple,Ⅳtype for violet.Oropharyngeal cavity was both typeⅢand typeⅣ,occasional typeⅠand typeⅡmucous cells.The esophagus was typeⅡand typeⅣmucous cells,did not see typeⅠ,typeⅢmucous cells.Stomach was mainly typeⅢand typeⅣ,parts of spreading typeⅠand typeⅡmucous cells.Intestinal tract was given priority to with typeⅡand typeⅣmucous cells,a few typeⅢcells,did not see typeⅠmucous cellsIn conclusion,our results suggest that there is a differentiation in the utilization of food resource between S.sichangensis and M.omeiensis coexisting in ZouDu River,even though they completely overlap in space.The morphological divergence could be associated with niche partitioning,allowing the coexistence of closely related species and reducing interspecific competition.