Respiratory and acid–base consequences of zebra mussel infestation on the valves of unionids

Abstract

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 respiratory gas state in a sample of the unionid bivalve Elliptio complanata, both with zebra mussels attached to the valves and with zebra mussels removed. Both sets of clams were exposed for 24 h to the following three environmental gas treatments: normoxia–normocapnia ([Formula: see text] 153 torr, [Formula: see text] 0.3 torr), moderate hypoxia–hypocapnia ([Formula: see text] 35 torr, [Formula: see text] 0 torr), and severe hypoxia–hypercapnia ([Formula: see text] 8 torr, [Formula: see text] 2.5 torr). Blood pH, [Formula: see text], [Formula: see text], total CO2, and osmolality were determined. Although the effects of gas treatment were profound, zebra mussel infestation had no effects on any of the variables measured. Zebra mussel infestation increased with size of the unionid host, but we detected no relationship between the degree of infestation and any blood variable. This suggests that competition for oxygen in the medium or reduction in ventilatory capability due to mechanical interference and consequent disruption in gas exchange by infesting zebra mussels does not apply at this level of infestation (mean 36.4 and maximum 80 zebra mussels per unionid host). The possibility of zebra mussels outcompeting unionids for suspended food particles and a resultant decline in unionid tissue energy stores are more likely consequences of infestation.