The Morphological Differentiation and Its Signification of Sucking Disc of The Genus Discogobio

The morphological differentiation of Discogobio is expressed in macro-structures and micro-structures.In the macro-structures,the differentiation showes the changes of sucking disc size that can be divided into two types.One type is the large sucking disc,whose posterior edge extend to or beyond anterior margin of eye vertically and width of central pad is equal to or larger than the diameter of eye;another is the small sucking disc,whose posterior edge does not extend to anterior margin of eye vertically and width of central pad is smaller than the diameter of eye.Although the macro-morphology differentiation of sucking disc of Discogobio seems related to environment where every species lives,actually it is only a superficial phenomenon.Because D.yunnanensis with large sucking disc and D.macrophysallidos with small sucking disc can be found simultaneously in rivers as well as lakes or pools.So the authors consider that types of sucking disc may be a clue of recording the differentiating history of Discogobio rather than a clue of recording their adaptation for varied niches.The two groups with different types of sucking disc are two natural groups who were derived from an ancestor.In the micro-structures,the differentiation is expressed in changes of papillae distribution and tentacle numbers at the top of papillae.Based on the results observed by scanning electron microscopy and ecological information of Discogobio,it is inferred that the distribution of papillae on disc and the tentacles at top of papillae are more connected with habitats of the fishes.When they evolved continually for adapting various environment,the two groups shown different ways in their micro-morphology.The development of large sucking disc group evolved mainly towards expanding distributive area of papillae on the central pad and increasing tentacle numbers at the top of papillae;in this group D.elongatus developed perfectly and more adapted swift current niches.The development of small sucking disc group evolved in two aspects respectively.One was of expanding distributive area of papillae on the central pad and increasing tentacle numbers at the top of papillae,so the adhensive ability of the sucking disc can be strengthened,such as rivers populations of D.macrophysallidos;another was not of strengthening the adhensive ability of the sucking disc,so the papillae on the sucking disc was scattered and with less tentacles and the ability of the fishes movement were developed by enlarging the diameter of eye and expanding the length of barbel,and D.longibarbatus is a representative.Therefore,the sucking disc morphological differentiation of Discogobio is a complex phenomena,which records the evolutionary history of Discogobio and adaptation of every species in the genus.