Effects of Temperature, pH, and CO<sub>2</sub> on Transformation of the Glochidia of Anodonta suborbiculata on Fish Hosts and in Vitro

Abstract

Low temperature is known to suppress immune function in ectothermic vertebrates, including fish. Therefore, we hypothesized that low temperature might facilitate successful encystment and transformation of the parasitic glochidia larvae of unionid mussels on fish hosts. Glochidia of the flat floater, Anodonta suborbiculata, are present on hosts from January through March, when water temperature is low. The % transformation success of attached glochidia in laboratory infections on fish hosts (golden shiner, Notemigonus crysoleucas) was significantly higher at 10°C (67%) and 15°C (62%) than at 21°C (42%). In contrast, transformation success of glochidia in vitro was significantly lower at 10°C (39%) than at 15°C (89%) or 20°C (93%). Thus, the improved survival at low temperature on fish hosts does not appear to involve direct effects of temperature on the glochidia. These results support the hypothesis that immune suppression of fish hosts by low temperature enhances transformation success of this winter-breeding species. We also tested the effects of CO₂, pH, and HEPES buffer on transformation of flat floater glochidia in vitro at 15°C. Transformation success was highest at the lowest pH tested (7.6) and the highest level of CO₂ tested (5% CO₂). This result is consistent with previous studies of other species, but is puzzling because the physiological pH of fish body fluids is higher and the level of CO₂ is much lower. Although most workers have incubated glochidia in 5% CO₂, we found that transformation success was fairly high (68%) without elevated CO₂, provided that pH was kept low. HEPES buffer reduced transformation success.