The Early Development And Temperature Effect On Discogobio Yunnanensis (Regan) Larvae And Juveniles

Discogobio yunnanensis has a cylindrical body shape with a flattening tail,slightly flat in head and abdomen complex mouth structure with a large and U-shape folded sucking disc.It is a small cyprinid fish with special adaptability to flowing water and it is also named"Stone Fish"because it likes to scrape the surrounding clumps attached to the stone surface.Mainly distributed in Jinsha River,Yuanjiang River,Nanpanjiang River and Yangtze River.However,some habitats have been destroyed with the establishment of cascade hydropower stations,coupled with overfishing and other reasons in these years,the conservation of wild D.yunnanensis is worth serious consideration.In this study,healthy males and females at their mature stage were chose to produce fertilized eggs by artificial insemination after temporary raise in lab system.The hatching process were in running water,and then,well-developed newly hatched larvae were selected as experimental materials for the measurement and observation of growth,development and survival of larvae and juveniles using morphological and histological methods.Furthermore,this study also explored the effect of temperature on the growth and survival rate of D.yunnanensis larvae and juveniles.The main results and conclusions are as follows:?1?The newly hatched larvae have the body weight of?0.012±0.001?g and the total length of?7.16±0.03?mm,the average length and height of yolk sac is?4.04±0.02?mm and?1.35±0.02?mm when cultivated at 20±0.5?.Larvae could be fed from 9^th day post hatching?dph?and the yolk sac is absorbed thoroughly at 28^thh dph,the stage of mixed nutrition last 19days long and the early larvae stage last 28 days long.We suggested that the existence of this stage is to reduce the death rate at their early life stage which is beneficial to establish nutritional relationships between larvae itself and the outside world.The scales begin to appear at 45^thh dph and entirely completed at 80^thh dph.Late stage larva last 17 days from 28^thh dph to 45^thh dph.In addition,we made a little variation for determining the juvenile stage which last 120 days from the scales appeared to 165^thh dph when the sucking disc completely formed according to the specific characteristics of D.yunnanensis.?2?The evolution of the sucking disc was classified into 3 developmental stages:the pre-forming stage,the forming stage and the perfecting stage.In the pre-forming stage,the primordium of sucking disc has not yet formed;while in the forming stage,the sucking disc primordium appears;the lower lip flap,granulation,lateral projection and sucking disc form to a connected whole but there is still gaps;the prototype of the sucking disc is formed.In the perfecting stage,the gaps disappeared and the sucking disc is fully formed,the sucking disc are covered with various sizes of mastoids except for the horseshoe-shaped ridge and the leading edge of central pad.Taste buds are mainly distributed in the rostral fold,maxillary barbel,rostral barbel,protuberance of lips,central pad and corner of the mouth.?3?Our results also showed that temperature significantly affect the growth and survival rate at larvae and juveniles stages of D.yunnanensis,the developmental process was significantly accelerated with the increasing acclimated temperature.In these 20?,23?and26?groups,the earliest fully formed sucking disc occurs in the 23?group compared with other groups,while the highest survival rate was at 20?.There was no difference in total length and body weight among 20?,23?and 26?groups?P>0.05?,which indicate that the effects of these temperatures on the growth of total length and body weight of D.yunnanensis were similar.In conclusion,we suggested that the most breeding temperature is20²6?for D.yunnanensis in mass production.