THE EFFECT OF SALINITY AND TEMPERATURE ON SPAWNING AND FERTILIZATION IN THE ZEBRA MUSSEL DREISSENA-POLYMORPHA (PALLAS) FROM NORTH-AMERICA

Zebra mussels have dispersed from their original site of introduction in the Great Lakes into the Mississippi River, Hudson River, and other watersheds in which they will encroach upon brackish water estuaries. To investigate their likelihood of reproductive success in such estua ries, we investigated the conditions of temperature, salinity, and acc limation under which spawning and fertilization could occur. Reproduct ive function of mussels that were acclimated to salinities up to 7.0 p arts per thousand (ppt) at 12 degrees, 20 degrees, and 27''degrees C f or 1 to 21 days was tested. Reproductive function of non-acclimated mu ssels that had been maintained in fresh-water aquaria was also tested in various salinities, Spawning was induced by serotonin, previously d emonstrated to elicit spawning of fertile gametes in fresh water. Succ essful fertilization was indicated by oocyte cleavage after adding spe rm. Nonacclimated mussels spawned in salinities of 1.75 and 3.5 ppt at 12 degrees, 20 degrees, and 27 degrees C, but not at 7.0 ppt. Fertili zation using gametes from non-acclimated mussels occurred only in fres h water and at 1.75 ppt. Acclimation for as little as 2 days enhanced spawning. Fertilization rate in a salinity of 3.5 ppt improved within 4 days of acclimation and continued at a high level for as long as 21 days of acclimation. Although animals acclimated for 4 days in 3.5 ppt spawned readily when tested in salinities as high as 7.0 ppt, almost no fertilization occurred in 7.0 ppt. The reduction in fertilization a t increasing salinities may be due in part to reduced sperm motility. Unfertilized oocytes remain intact for hours in fresh water; however, in salinities as low as 0.7 ppt, unfertilized oocytes tended to ruptur e within 2 hours. These data show that although sudden increases in sa linity produce an immediate decrease in the reproductive capacity of z ebra mussels, acclimation to brackish water can occur, and zebra musse ls may be able to reproduce in brackish water below 7.0 ppt.