Temperature and salinity effects on growth, survival, reproduction, and potential distribution of two non-indigenous botryllid ascidians in British Columbia

Two non-indigenous botryllid ascidian species - Botryllus schlosseri (golden star tunicate) and Botrylloides violaceus (violet tunicate) - have become established in British Columbia (BC), Canada. One species, B. schlosseri, is native to Europe while the other, B. violaceus, is native to Asia. Environmental tolerances of both species are poorly understood. We examined the effects of temperature and salinity on growth, survival, and reproduction of these species in the laboratory in order to characterize their environmental tolerances and preferences. Laboratory-raised juvenile colonies were studied using a two-factorial experimental design with five levels of temperature (5, 10, 15, 20, 25 °C) and five levels of salinity (14, 20, 26, 32, 38‰). Both B. schlosseri and B. violaceus possessed broad temperature and salinity tolerances, but B. schlosseri was slightly more euryhalinal than B. violaceus. Generally, B. schlosseri survived environmental conditions of 10-25 °C and 14-38‰, exhibited positive growth at 10-25 °C and 20-38‰, and attained its largest colony sizes at 15-20 °C and 20-38‰. Botrylloides violaceus tolerated environmental conditions between 5-25 °C and 20-38‰, demonstrated positive growth at 15-25 °C and 26-38‰, and attained its largest colony sizes at 20-25 °C and 26-38‰. Results from the laboratory experiment were then used in a modeling exercise to determine the coastal areas of BC that these organisms might be likely to exist in or invade, based on near-surface temperatures and salinities. The model predicted that no areas were totally unsuitable for survival and growth of either species (based solely on temperature and salinity tolerances), with the most suitable locations being along the west coast of Vancouver Island, a region with significant shellfish aquaculture activity.