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Anna McCarthy - Benthic Ecology Lab

Whitman College, Walla Walla, WA

Effects of Temperature on the Growth Rate of Didemnum lahlei with Comparisons to Botryllus schlosseri and Botrylloides violaceous

Abstract: Unusually extensive hard surface area occupation and rapid spread of a previously undescribed species of colonial ascidian tunicate, provisionally identified as Didemnum lahlei, has recently been observed in areas around Georges Bank (Massachusetts), Whangamata Harbour (New Zealand), and covering large areas of rocky bottom in eastern Long Island Sound.  Preliminary observations suggested that it was most abundant at deeper sites with cooler summer temperatures.  In this study, I examined how growth rates of D. lahlei varied with temperature and contrasted its growth to that of two other colonial ascidians from Long Island Sound, Botryllus schlosseri and Botrylloides violaceous.  Temperature data collected in the field were used to set temperature conditions for laboratory experiments.  The three species were exposed to four temperature treatments: ambient, roughly 4-5º C below ambient, 2º C above ambient, and 4-5º C above ambient.  Findings suggest that D. lahlei has a higher growth rate in cooler conditions (ambient or below), and that the growth rate exceeds that of both Botryllus and Botrylloides.  The species' ability to rapidly spread may be the result of its comparatively fast growth rate and ability to grow in cooler, deeper waters.