The Comparative Anatomy Of The Olofactory Organ And Adaptation Of A Cave-dwelling Genus,Oreonectes

Cavefishes,or hypogean fishes,are a distinctive group of freshwater fishes.Their life histories unseverably bind them to a cave or underground water body.They present some morphological characters adapted to the cave environment for long time evolution known as troglomorphism.China is host to the largest taxonomic diversity make it one of the richest cavefishes countries.Most of them live in Guangxi Zhuang Autonomous Region,and this is particularly as Guangxi is host to the largest taxonomic diversity with more 60%currently described species in China.The genus Oreonectes are one of the big genus in Guangxi except O.platycephalus distributed in Guangxi,Guangdong and Hong Kong in China,as well as in north Vietnam.Due to the darkness all the time,cave is an extreme environment for organisms.To adapt this dark environment,cavefish always has degenerated eyes.On the contrary,other sensory organs including olfactory organ have been developed for satisfying the needs of their normal life.Therefore,we chose on cave-dwelling the genus Oreonectes as a target genus,Traccatichthys pulcher as surface fish outgroup to study the structure of the olfactory epithelium.For getting more knowledge and data on its morphological adaptation to cave dark environment,we observed the olfactory epithelium by applying a scanning electron microscope(SEM)It is obvious that adapt to dark environment by reforming olfactory organ of Oreonectes.make them become typical fish relying on the sense of smell in their life.The main reform in the following aspects.It is an impotant feature of "olfactory sensation" fish that anterior and posterior nostrils separated and with olfactory chamber.It is very in favour of finding chemistry signal along environment with anterior nostril in an elongate tube with a barbel or a short tube at the end,short tube can changes the direction of water.The posterior nostril is nearer the eye than snout with normal of O.platycephalus O.guananensis O.luochengensis O.polystignus O.sp.nov.1 O.sp.nov.2,Conversely,which reduction or loss of eyes their nostrior are very near the snout of O.microphthalmus O.macrolepis and O.translucens.It is supposed to that nostril forward could faster catch chemicals in water for cover the shortage of reduction or loss of eyes.Olfactory sac belongs to G-type,which defined by Yamamoto and always plays an important role in the teleost.And this type can increases olfactory sac interior return circuit,and increasing area of contact water,and overtime the interior water,so that Oreonectes takes full advantageous of the chemistry signal by flow.The olfactory organs of Oreonectes are oval and lie in the olfactory chamber.The olfactory apparatus has primary lamellae arranged on both sides of a narrow raphe.The size of the olfactory lamellae from anterior nostril to posterior nostril become biger,and as the same situation to tongue-like structure.All of the olfactory lamellae of cavefish of Oreonectes are "?"-like sub-pattern,with developed tongue-like structure.The tongue-like structure more developed the more area for sensing chemical singal in water,and the more sensitive olfaction.Each tongue-like structure gathering increases complexity of olfactory sac structural significantly,so that makes flow interior more complexity.The ratio of the olfactory sac length to eye diameter is>1 "olfactory sensation" fish.There is a negative corrlation relationship between olfactory sac and eyes development level.Oreonectes enhanced its olfactory sensitivity cover the visual shortage adaptation to cave dark environment.The surface fish of Oreonectes and Traccatichthys pulcher have few ciliated receptor cells.And most of cavefishes(troglobite and trogophile)have ciliated receptor cells and microvillous receptorcells that correspond with Sinocyclocheilus.We found two pattern sensory cilia bar-like pattern and stick-like pattern on some olfactory epithelium of cavefishes,which were presumed have different functions.there are rich olfactory sensory cells on the olfactory epithelium of cavefishes,which can recognized more types of chemical signals in the water,revealing they reform olfactory organ to adaptation.