A morphologically variant natural population of cyprinids without dorsal fin possibly derived from Mystacoleucus marginatus (Osteichthyes: Teleostes) from the Lancang Jiang River, Yunnan, China

In July 2001, 15 grotesque cyprinid specimens were collected in the Lancangjiang River (of the upper reaches of the Mekong River) in Menglun Town, Xishuanbanna Prefecture, Yunnan Province, PR China. These specimens are characterized by surprising characters that have not been observed in natural populations of cyprinids previously, such as an absence of a dorsal fin and of some supraneurals. Other characters, such as the number of branched rays of the anal fin, the formula of pharyngeal teeth, the condition of barbels, and the number of scales are not stable and vary greatly. The more stable characters of these variants, such as the formula of pharyngeal teeth, the pattern of striations on the scales, the number of vertebrae, the length of the gut, etc., have many similarities with those of Mystacoleucus marginatus (Cuvier & Valenciennes, 1842). These observations indicate that these variants may have originated from M. marginatus.