Seasonal reproduction of the non-native vase tunicate Ciona intestinalis (Linnaeus, 1767) in Nova Scotia, Canada, in relation to water temperature

Intra-annual and inter-annual reproductive periodicity for in situ populations of Ciona intestinalis were documented from October 2013 to August 2015 in the small vessel marina at the Bedford Institute of Oceanography in Dartmouth, NS. Three metrics of reproduction were monitored: (i) larval settlement, (ii) gonad development and (iii) gamete viability. In situ settlement was observed between June and November 2014. Gonad development during spring consisted of a sharp increase in the proportion of males followed by development into hermaphrodites, which resulted in a near entirely-hermaphroditic population throughout the summer and fall. The proportion of males and hermaphrodites began to decline by late fall; hermaphrodites were absent by early winter (i.e., late December or January), whereas males persisted at low abundance throughout the winter. In-vitro fertilization assays demonstrated that gametes became non-viable by early December. The seasonal changes in development stages were compared with respect to ambient seawater temperature and growing degree days. The spring maturation in 2014 occurred 3 weeks earlier than observed in 2015 (i.e., May 2014 and June 2015), likely due to a 2.0 °C higher mean seawater temperature than the same period in 2015. The effect of temperature on development rate was confirmed in a mesocosm experiment where the 3 °C elevated temperature treatment resulted in earlier sexual maturation by ten days for males and eight days for hermaphrodites compared to those in the unheated treatment. These results demonstrate the potential for an extended reproductive window, and perhaps extended geographic range, in response to predicted increases in ambient sea surface temperatures in Atlantic Canada in the next few decades.