Effects of temperature on growth rates of colonial ascidians: A comparison of Didemnum sp. to Botryllus schlosseri and Botrylloides violaceus

An invasive colonial ascidian, Didemnum sp. [described as Didemnum lahillei by Lambert (2004) and Didemnum vestum by Kott (2004) and ecologically similar to Didemnum vexillum (Kott, 2002)], has occupied unusually large areas of seafloor and spread rapidly throughout the New England region (Valentine et al., 2007-this issue; Bullard et al., 2007-this issue). Its extreme abundance at depthsN30 m suggests that Didemnum sp. may be more successful in cooler, deeper waters than other recent ascidian invaders into the New England region. These other invaders are often restricted to shallow warmer-water embayments (e.g., Whitlatch and Osman, 2001) and have greatly increased recruitment in years with warm winter temperatures (Stachowicz et al., 2002). Because the distribution of Didemnum sp. suggested that it may be affected by temperature differently than the other invaders, we examined whether small differences in ambient water temperature that occur between different depths or