Evaluating the role of freshwater bivalve shells as anti-predator refuges for fish.

The larvae of unionid mussels are obligate parasites. Their survival and metamorphosis into juvenile mussels depend on attaching to physically suitable hosts. Although unionid mussels are important ecosystem engineers, the effects of their shells on the host fish species remain poorly understood. The unionid species, Pronodularia japanensis, uses freshwater gobies (Rhinogobius) as hosts for its larvae. In this study, we conducted microcosm experiments under predation risk from a carnivorous fish, the Korean perch (Coreoperca herzi), to assess whether empty shells of unionid mussels could provide anti-predator refuges for gobies. Additionally, we examined whether shells of the Asian clam (Corbicula) could serve as alternative refuges to the unionid shells. The presence of unionid shells substantially enhanced goby survival after a 48-h predation period, whereas Asian clam shells had no such effect. Video recordings revealed that gobies hid beneath unionid shells but not beneath Asian clam shells. Our findings indicate that the shells of deceased unionids can positively influence potential host fish, suggesting a mutualistic relationship between unionids and host fish mediated through shell-derived refuge effects. Furthermore, our results suggest that replacing unionids with non-native Asian clams in natural habitats may negatively affect host fish populations.