The invasive ascidian, Didemnum vexillum, has recently spread into south shore bays of Long Island, New York including Great South Bay and Shinnecock Bay. In order to determine the impact this invasive species will have on these bays, colonies from Shinnecock Bay were studied, due to easiest accessibility. D. vexillum currently forms extensive mats on the pillars of Ponquogue Bridge located in Shinnecock Bay. These mats have the potential to drastically alter the structure of the ecosystem; an effect which has been observed in other waters that the species has invaded. Thus, in order to predict the impact D. vexillum may have on predator-prey relationships in Shinnecock Bay, clearance rates (L gdw-1 h-1) on total chlorophyll-a</italic) and picoplankton were measured, both in the laboratory and in the field, from May, 2009 through October, 2009. In addition, during the clearance rate experiments conducted in the laboratory, oxygen consumption was measured. Field growth rates of D. vexillum colonies were also estimated. Using the growth and oxygen consumption rates, as well as literature values of ammonia excretion and absorption efficiency, I calculated how much energy (gdw-1 h-1) the species needed to consume to support the observed growth. Clearance rates from both laboratory and field experiments were always very low, and most were not statistically significant from zero, suggesting D. vexillum did not feed during the experiments. Five out of the seven oxygen consumption data were statistically significant from zero; however the values were lower than most other ascidians, suggesting the colonies of D. vexillum were limiting their energy expended by not feeding on suspended particles in the water column. Estimated clearance rates needed to support the growth rates were much higher than the measured clearance rates and were within the range of values for other colonial ascidians. However, these estimates may be lower than what actual clearance rates of D. vexillum are due to low growth and oxygen consumption rates.