Development of the sucking disc of Discogobio yunnanensis larvae and juveniles

Abstract:

Discogobio yunnanensis is a small Cyprinid fish, well adapted to flowing water and mainly distributed in Jinsha River, Yuanjiang River, Nanpanjiang River, and Yangtze River. Due to the long-term adaptation to the lotic environment, the lips and related structures have changed accordingly. The lower lip gradually stretches backwards, and the chin uplift is specialized into a mouth-sucker structure with horseshoe folds. However, D. yunnanensis is also named "stone fish" because it likes to scrape the surrounding clumps attached to the stone surface. At water temperature of (20.0±0.5)℃, the unique characteristics in the morphology and histology of the sucking disc of D. yunnanensis larvae and juveniles were observed using light and scanning electron microscopy to understand the characteristics of the sucking disc. The evolution of the sucking disc was classified into three development stages: the pre-forming stage, the forming stage, and the perfecting stage. The primordium of the sucking disc was not formed in the pre-forming stage. In the forming stage, the sucking disc primordium appeared; the lower lip flap, granulation, lateral projection, and the main body of sucking disc were connected as a whole with few gaps, and the prototype of the sucking disc was formed. In the perfecting stage, the gaps disappeared, and the sucking disc was fully developed. There were no mastoids at either side of horseshoe swollen and the leading edge of central pad; however, the other areas of the sucking disc were covered with mastoids of different sizes. Taste buds were mainly distributed in the rostral fold, maxillary barbel, rostral barbel, protuberance of lips, central pad, and corner of the mouth.