RESPIRATORY AND ACID-BASE CONSEQUENCES OF ZEBRA MUSSEL INFESTATION ONTHE VALVES OF UNIONIDS

Infestations of the zebra mussel (Dreissena polymorpha) on the valves of unionids purport to cause the eventual death of the host, although the means is uncertain. We examined the blood acid-base and respirator y gas state in a sample of the unionid bivalve Elliptio complanata, bo th with zebra mussels attached to the valves and with zebra mussels re moved. Both sets of clams were exposed for 24 h to the following three environmental gas treatments: normoxia-normocapnia (PO2 153 torr, PCO 2 0.3 torr), moderate hypoxia-hypocapnia (PO2 35 torr, PCO2 0 torr), a nd severe hypoxia-hypercapnia (PO2 8 torr, PCO2 2.5 torr). Blood pH, P O2, PCO2, total CO2 and osmolality were determined. Although the effec ts of gas treatment were profound, zebra mussel infestation had no eff ects on any of the variables measured. Zebra mussel infestation increa sed with size of the unionid host, but we detected no relationship bet ween the degree of infestation and any blood variable. This suggests t hat competition for oxygen in the medium or reduction in ventilatory c apability due to mechanical interference and consequent disruption in gas exchange by infesting zebra mussels does not apply at this level o f infestation (mean 36.4 and maximum 80 zebra mussels per unionid host ). The possibility of zebra mussels outcompeting unionids for suspende d food particles and a resultant decline in unionid tissue energy stor es are more likely consequences of infestation.