Choice of oviposition sites on the gills of mussels by bitterlings

Bitterling are small freshwater fishes that have an unusual spawning symbiosis with freshwater mussels. During the spawning season, female bitterling place their eggs onto the gills of freshwater mussels. The embryos develop inside the mussel for approximately a month, eventually leaving the mussel as actively swimming larvae. Previous studies have focused on selection of bitterling for different mussels. Little is known on how bitterling lay their eggs on mussel's gill for better adaptation. In the present paper, field investigations and indoor experiments were conducted to study bitterling eggs' position on mussel's gill, and discuss the adaptation strategy for bitterling. Mussel samples in field investigation were dissected immediately after catching to examine bitterling's eggs on mussel's gill. The results showed that the bitterling mainly choose interior gills, and anterior and middle portions to lay their eggs. These bitterling species were identified as belonging to the Acheilognathus and Paracheilognathus. The indoor experiments were conducted by putting together in a tank 6 - 10 pairs of rose bitterling, Rhodeus ocellatus, and 12 mussels, i. e. 12 for each of the six mussel species, Cuneopsis pisciculus, Unio clouglasiae, Lamprotula tortuosa; Lamprotula caveata, Anodonta woodiana woodiana, and Schistodesmus lampreyanus. Three or six days later, the mussels were dissected to check the bitterling eggs. The result showed that the rose bitterling choose the inner gill of the mussel to lay eggs but without, preference for anterior, middle or posterior portions. These results indicated that different bitterling have different adaptive strategies. With their larvae simple in morphology and lacking the wing-like yolk projection, the Acheilognathus and Paracheilognathus species have longer ovipositors to lay their eggs in deep areas of mussel's gill. Having wing-like projection in the larvae, Rhodeus species have shorter ovipositor and lay their eggs in different portions in mussel's gill