Changes in the height of minute tubercles on the skin of Korean bitterling embryos (Acheilognathus signifer) and embryo movement in the host mussels.

Bitterlings are freshwater fish that have developed morphological adaptations to improve the survival of their embryos in host mussels. The most well-known adaptation is the development of minute tubercles, which develop in the early embryonic stage when the embryos have poor swimming ability, and disappear when the embryos reach the free-swimming stage and leave the host mussels. In this study, the embryonic developmental stages of Acheilognathus signifer were analysed to elucidate the relationship between the changes in the height of the minute tubercles and their movement. The height changes in the minute tubercles in the embryos can be divided into five stages, i.e., formation, growth, peak, reduction and disappearance. The authors found that the embryos lived in the gill demibranch of the host mussel until day 6 after hatching. The movement of embryos to the suprabranchial cavity in the gill demibranch was firstly observed on day 7. At this point, the embryos showed a heartbeat and movement. From day 13, the minute tubercles had almost disappeared, and the hatchlings started swimming outside the host mussels from day 16. These observations highlight the different adaptations of minute tubercles among bitterling groups without wing-like projections.