Larval exposure to low salinity compromises metamorphosis and growth in the colonial ascidian Botrylloides violaceus

Abstract

AbstractCoastal and estuarine ecosystems exhibit extreme fluctuations in abiotic factors that influence the success and distribution of many introduced marine invertebrates. The frequency of these fluctuations may worsen under the scenario of climate change due to more intense storm events or greater precipitation. While many studies have focused on the response of adult ascidians to salinity, few have examined the response of larvae to low salinity events or post‐metamorphic asexual reproduction after larval exposure to low salinity. Here we tested the effect of low salinity on larval metamorphosis and post‐metamorphic growth of the introduced ascidian Botrylloides violaceus. We exposed tadpole larvae to salinities ranging 0–30 ppt. Tadpoles metamorphosed in salinities as low as 16 ppt and percent metamorphosis of larvae was not significantly different at salinities >20 ppt. A few tadpoles metamorphosed within 2–4 hr after release, and larval metamorphosis continued for 3–4 days, suggesting that larvae could disperse considerable distances. We then tested post‐metamorphic survival, growth, and asexual reproduction of larvae exposed (5–7 days) to varying salinities (18, 20, 25, and 30 ppt), and placed the resulting colonies in full‐salinity natural seawater; colony growth was monitored for 9 weeks post‐metamorphosis. Results indicated that colonies whose larvae were exposed to low salinity (<25 ppt) had fewer zooids than colonies from larvae exposed to seawater at 30 ppt, but total area occupied by each colony was similar among salinity treatments. Because each individual zooid within a colony filter‐feeds and reproduces through asexual budding, fewer feeding individuals within a colony will likely reduce future growth and reproduction. Due to the likelihood of more frequent heavy rain events and low‐salinity conditions for the northwest Atlantic Ocean, there is need for continued research to assess the response of ascidians to factors like salinity and to evaluate the spread of ascidians and their adaptation to estuarine and coastal habitats.