Byrne, R. A., and T. H. Dietz, 1996.  Water and Ion Balance in the Zebra Mussel, Dreissena polymorpha, SUNY College at Fredonia, Louisiana State University

Abstract from The Sixth International Zebra Mussel and Other Aquatic Nuisance Species Conference, Dearborn, Michigan, March 1996

The body fluid composition of Dreissena polymorpha is characterized by being extremely dilute (35-45 mOsm.kg^-1) even for a freshwater bivalve. Osmotic influx of water from the dilute environment is counteracted by highly active epithelial ion transporting mechanisms. However, we have recently shown that the epithelium of Dreissena polymorpha is also highly leaky, so that there is a rapid turnover of ions within the extracellular compartment. We do not have any information on how these ions are partitioned between the intracellular fluid (ICF) and extracellular fluid (ECF) spaces. We also have no information on the rate at which ECF is turned over (clearance rate) in response to changes in external ionic and osmotic content.

Using a radiolabeled extracellular marker we calculated the rate of inulin clearance of zebra mussels in pondwater as 3.27 ± 0.24 mL.g dry tissue^-1.h^-1 (mean ± SEM). Dextran (10 kD) clearance was similar at 3.65 ± 0.39 mL.g^-1.h^-1. These values are 6-10 times higher than clearance rates for unionid bivalves. Clearance rates using both inulin and dextran as markers declined to 0.26 ± 0.11 and 0.34 ± 0.09 mL.g^-1.h^-1, respectively when animals were incubated in 10% seawater (approx. 100 mOsm.kg^-1) for over 24h, suggesting a reduction in renal flow in response to the hyperosmotic medium. However, incubation in hyperosmotic 45 mmol.L^-1 NaCl for 12h resulted in inulin clearance rates increasing to 27.19 ± 7.32 mL.g^-1.h^-1, whereas dextran clearance remained at 0.53 ± 0.13 mL.g^-1.h^-1. The absence of potassium in the medium (NaCl solution) interferes with the ability of surface epithelial cells to regulate volume, thereby causing a shrinkage resulting in paracellular leakage. A small marker, such as inulin passes through the leaky barrier, whereas a large molecule (dextran) is retained in the extracellular fluid. The presence of K (in 10% seawater) allows cells to volume regulate and no leakage occurs.

Calculation of ECF and ICF volumes was difficult due to problems measuring total water content accurately. We used a bathing medium marker to account for mantle cavity and adsorbed water. The mean total water volume was 17.25 ± 0.57 mL.g^-1; ECF volume was 11.40 ± 0.57 mL.g^-1 and ICF volume was 6.44 ± 0.36 mL.g^-1 (n=28). These values are approximately double those of a unionid clam. The intracellular ion concentrations (in mmol.L-1 of cell water) were Na: 9.8 ± 1.2; K: 11.4 ± 1.0; Cl: 3.4 ± 0.9; Mg: 5.6 ± 0.3, demonstrating the expected preponderance of potassium as the intracellular cation. After 6 days incubation in magnesium-free pondwater, total body magnesium declined from 45.2 ± 1.9 µmol.g dry tissue^-1 to 31.0 ± 1.4 µmol.g^-1. Although ECF [Mg] fell from around 1 mmol.L^-1 to 0.05 mmol.L^-1, however, ICF [Mg] at 5.4 ± 1.1 mmol.L^-1 was not different from pondwater controls suggesting zebra mussels defend intracellular stores of magnesium at the expense of ECF content.

These studies further demonstrate the relatively poor adaptation to freshwater by Dreissena polymorpha. Leaky external epithelia allow ions to exit and water to enter the animal. In order to combat this, zebra mussels possess highly active ion transporting mechanisms and a rapid fluid clearance capacity. Unionid bivalves, however, maintain lower overall ion transporting rates and a lower fluid clearance due, in part, to a more rigorous control of epithelial permeability.

Keywords: Zebra_mussel, Basic_biology
Product Type: Publication, Proceedings
User Type: General