Comparative Histological Study Of Visual Organs Between Triplophysa Rosa And Triplophysa Bleekeri

Triplophysa rosa and Triplophysa bleekeri are two highland loach species belonging to order Cypriniformes,superfamily Cobitoidea and family Nemacheilidae.The Triplophysa fishes are mainly distributed in Qinghai-Tibet Plateau and adjacent regions.T.rosa is a typical cavefish that can only be found in caves and subterranean river environment at Huolu town,Wulong County,Chongqing in Southwestern China.Cavefish also evolves many distinct troglomorphic characters adaptive to darkness habitat,such as reduction of scales,pigmentation,and translucent body.Conversely,some other sensory organs are enlarged or enhanced such as lateral-line system and appendages(fins).Particularlly,in order to adapt to perpetual darkness,its visual organ is gradually degraded or lost.While T.bleekeri is a river-dwelling sighted fish and widely distributed in open and shallow water with normal eyes.The differences between the two species are mainly due to the different living habitats.In this study,we examined the morphology of visual organs in T.rosa and T.bleekeri in which are dwelling in different environments.We used morphology,histology,immunochemistry and transmission electron microscope techniques to compare the differences in phenotype and visual organs,especially retinal structure and retinal cells in T.rosa and T.bleekeri.We are trying to provide histological references for the ocular degeneration mechanism in cavefishes.The main results are as follows:(1)Based on the morphological analysis,the standard length(SL)of T.rosa and T.bleekeri were significantly different(n = 15).The SL of T.rosa is 101.99 ± 10.57 mm,T.bleekeri just has 80.78 ± 4.75 mm in SL.Moreover,T.rosa shares many typical features with other cave-dwelling blind fishes,such as the reduction of pigmentation,scaleless and translucent body or with severely degraded eyes.The eyes of both fishes are dissected.T.rosa retains small eyes that are buried deeply in adipose tissue and are covered with skin.The diameter of the eyeball is 0.50 ± 0.07 mm,which is significantly smaller than T.bleekeri(2.41 ± 0.20 mm).Beside,in order to adapt to the cave environment,the chemoreceptors and mechanoreceptors(tentacles and the lateral-line system)or appendages(fins)of cave fish are enlarged or enhanced,T.rosa are similar.Significant differences mainly include the following points: dorsal head length,outer rostral barbel length,pectoral-fin length,anal-fin length,caudal peduncle length.(2)The paraffin sections were stained with HE for the eyes of T.rosa and T.bleekeri(n = 6).Histological study shows that the basic structure of the eyeballs of T.rosa and T.bleekeri are similar,such as cornea,sclera,choroid,iris,retina and lens.However,there are different degrees of degeneration in T.rosa,especially the cornea,retina and lens but cornea can not be clearly distinguished to sclera,choroid,retina is uneven throughout the eye,the lens is seriously degrade.These structures are connected together,which means that the T.rosa can not adjust in vision,indicating that the visual degradation of the T.rosa is caused by the structural degradation of the different parts of visual organs.(3)We have further analyzed the retina of T.rosa and T.bleekeri(n = 6).The histological study of the retina shown that it can be divided into 8 layers,from the outside to the inside were the pigment epithelium layer(PEL),the layer of rods and cones(RCL),the outer nuclear layer(ONL),the outer reticular layer(ORL),the inner nuclear layer(INL),the inner reticular layer(IRL),layer of gangion cells(GCL),and layer of optic nerve fibers(LONF).The results show the layers of OPL,ONL,INL,GCL in T.rosa is significantly thicker than that of T.bleekeri,but other layers responsible for the light and the signal conversion,transmission have a different results.We found that the number of retina cells in T.rosa is significantly more but the size was smaller than that in T.bleekeri.(4)The morphological structure and the internal structure of the retina and the cone rod cells were further analyzed by immunohistochemical staining and transmission electron microscope(n = 4).The results show that the morphological structures of the cone and rod cells are abnormal in T.rosa and the extracellular degeneration is serious.The results from the transmission electron microscopy also show that the photoreceptor site disappear,the nucleus is shrunk,and the number of intracellular mitochondrion decreased.We also obtain a significant difference of the rod cells in the T.rosa is lesser than that of T bleekeri,and cone cells have the different results.May be caused by species differences or environment.In summary,we found that T.rosa has a typical morphological characteristics of cave-dwelling fishes,those constructive and regressive changes associate with cave environments.T.rosa lack eyes and body pigmentation,especially visual organs are highly degraded.Compared with T.bleekeri,although the eyes of these two fish are similar in structure,but the various parts of the structures are different.The degeneration visual organ of the T.rosa mainly occurs in the lens and retinal cone rod cells,we got the similar conclusions with morphology,histology,immunochemistry and transmission electron microscope technique.These differences depend mainly on their living environments and habitats,in order to adapt to the cave environment,visual organs is ?non-essential? organ for T.rosa,so it becomes gradually degrade,the corresponding sense of organ adaptive enhancement.T.bleekeri,as a river-dwelling sighted fish,its visual organ plays a major role in its life history,so the vision is more developed than T.rosa.