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

1. We investigated two possible proximate cues used for oviposition site ch oice by females of the bitterling (Rhodeus sericeus), a freshwater fish tha t spawns on the gills of live unionid mussels. The two cues were the flow v elocity and/or oxygen content of water emerging from the exhalant siphon of a mussel. 2. Field observations showed that female bitterling always inspected the ex halant siphons of mussels before they spawned in them. Siphon inspection wa s 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 si phons of four mussel species (Anodonta anatina, A. cygnea, Unio pictorum an d U. tumidus) showed a significant difference among species, with U. tumidu s having the highest mean flow velocity and U. pictorum the lowest. 4. Measurements of the change in oxygen concentration of water entering a m ussel inhalant siphon and leaving its exhalant siphon in field and laborato ry studies showed a significant difference among the four mussel species, w ith A. cygnea exhibiting a significantly higher change in oxygen concentrat ion than the other species. 5. The presence of bitterling embryos in the gills of a mussel significantl y increased its oxygen consumption whereas larval glochidia had no signific ant effect. We discuss oxygen availability as a possible proximate cue for oviposition site choice in bitterling.