Some like it hot: temperature stress and juvenile development in the invasive colonial tunicate Botrylloides violaceus

Abstract

The Salish Sea, a vital region for marine biodiversity in the Pacific Northwest, is predicted to increase ~1.5°C over the next century. Rising sea temperatures can change species distributions and may support the range expansion of invasive species with broader temperature tolerances or of tropical/subtropical origin. Botrylloides violaceus is a colonial ascidian native to the subtropical waters of the West Pacific and has successfully invaded the Salish Sea. In this study, acute temperature challenge experiments were conducted to understand how rising temperatures impact colony proliferation. Juvenile colonies were cultured in unfiltered seawater over three temperature ranges: 10-12°C, 14-16°C, and 18-20°C. Growth was assessed using counts of healthy, regressing, and budding zooids, while stress was inferred based on colony heartbeat and a qualitative health assessment. Production of healthy zooids and primary buds significantly increased with temperature and yielded larger colonies within 10 days. Heart rate significantly decreased in colonies that were maintained at 20°C, however, more baseline data are needed to relate this metric to overall colony health. These results indicate that rising temperatures in the Salish Sea are expected to increase the proliferation of B. violaceus, intensifying resource competition in benthic communities and potentially spelling danger for native populations.