Spawning preference and female's morphological adaptation of symbiotic freshwater bitterling fishes to the host-mussel body size

Background/Question/Methods

Symbiosis involves an interaction between individuals of two different species. In parasitism, while parasites evolve exploiting particular hosts, the hosts counter-adapt to eliminate or minimize fitness loss burdened by parasites. Bitterling fishes (subfamily Acheilognathinae) are obligatory to spawn in the gills of freshwater mussels. Bitterlings and mussels thus form a symbiotic relationship, although whether the interaction is one side or both sides positive remains unclear. The size of host mussels may be an important factor in shaping ecological and evolutionary processes of bitterlings, as it has direct or indirect effects on the fitness of bitterlings. Seventeen localities from the Han River in South Korea were investigated and bitterling fishes and host mussels were collected at each locality during April-May in 2019. To test for the host size preference for spawning and female bitterling’s morphological adaptation to the host size, we examined whether the collected mussels were spawned by bitterlings, measured the host body size, and estimated female’s reproductive phenotypes (e.g. ovipositor ratio and egg shape). Eggs and/or larvae laid by bitterlings were further analyzed for the species identification using RFLP (restriction fragment length polymorphism) to determine the spawning frequency and preference of each species.

Results/Conclusions

Bitterling fishes preferred to use larger mussels as their spawning grounds, perhaps because of the fitness advantage in relation to the offspring survival. The mean length of spawned mussels was significantly larger than that of unspawned mussels. Our reciprocal transplant experiments provided evidence supporting the hypothesis that bitterlings prefer to spawn in larger mussels, even unfamiliar hosts far away from home. Moreover, there was a positive correlation between host size and number of eggs/larvae, suggesting that bitterlings spawned more eggs (i.e. higher fecundity) and/or higher survivorship in larger hosts. Interspecific competition appears to occur inside the mussels, as the use of inner and outer gills as spawning slots differed among species. In response to the host size, positive relationships between mean length of spawned mussels and female’s ovipositor ratio and egg ratio indicate morphological adaptation. Bitterlings might have evolved exploiting large-sized mussels for spawning through the ecological benefits of higher survival for their offspring because of higher oxygenation. We also found that reproductive strategy evolved in a different way for different species. The results of our study will provide an important insight into the ecological symbiotic relationship between bitterlings and mussels.