A proximate cue for oviposition site choice in the bitterling (Rhodeus sericeus)

1. We investigated two possible proximate cues used for oviposition site choice by females of the bitterling (Rhodeus sericeus), a freshwater fish that spawns on the gills of live unionid mussels. The two cues were the flow velocity and/or oxygen content of water emerging from the exhalant siphon of a mussel. 2. Field observations showed that female bitterling always inspected the exhalant siphons of mussels before they spawned in them. Siphon inspection was not always a prelude to spawning and it may serve as a means of assessing mussel quality. Female skimming behaviour, swimming over a mussel without spawning, may also be used to assess mussel quality, although the mechanism for this is unclear. 3. Measurements of the flow velocity of water emerging from the exhalant siphons of four mussel species (Anodonta anatina, A. cygnea, Unio pictorum and U. tumidus) showed a significant difference among species, with U. tumidus having the highest mean flow velocity and U. pictorum the lowest. 4. Measurements of the change in oxygen concentration of water entering a mussel inhalant siphon and leaving its exhalant siphon in field and laboratory studies showed a significant difference among the four mussel species, with A. cygnea exhibiting a significantly higher change in oxygen concentration than the other species. 5. The presence of bitterling embryos in the gills of a mussel significantly increased its oxygen consumption whereas larval glochidia had no significant effect. We discuss oxygen availability as a possible proximate cue for oviposition site choice in bitterling.