1. Bitterlings are small freshwater fish that use long ovipositors to place eggs in host mussels and have morphological adaptations to increase larval survival. The most well-known adaptations are the minute tubercles on the skin surface of larvae, which are developed in early-stage larvae with weak swimming ability and disappear in free-swimming larvae before they leave the host mussel. 2. In the present study, a comprehensive analysis of the developmental stages of Rhodeus pseudosericeus larvae, their morphological and physiological characteristics, their migration inside mussels, and the development of minute tubercle are presented as direct evidence of the morphological function of the minute tubercles. These tubercles began to develop 1 day after hatching (formation stage), grew for 2–5 days (growth stage), reached the peak height after 6–7 days (peak stage), abruptly reduced in height after 8–10 days (abrupt reduction stage), and went through a final gradual reduction (reduction stage) until completely disappearing 27 days after hatching (disappearance stage). 3. The larvae remained in the mussels’ interlamellar space of the gill demibranchs until 10 days after hatching, and began to migrate to mussels’ suprabranchial cavity 11 days after hatching. At this time, the larvae had clear components of heart rate and caudal fin began to develop. At 24 days after hatching, the minute tubercles had almost disappeared, and some individuals were observed swimming out of the mussels. 4. The experiment results herein presented prove that the minute tubercles are a first direct evidence that the bitterling larvae are morphologically adapted to prevent premature ejection from the mussel.