Extensive Fouling of Eelgrass (Zostera marina L.) Reproductive Shoots by Invasive Tunicates-A Potential Threat to...

Over the past few decades, several invasive tunicate species have become pervasive pests in many of New England's coastal habitats. Their aggressive fouling can have detrimental effects on photosynthesis and growth of eelgrass, Zostera marina L. particularly during their peak growing season. Here, we document through field observations and photographs the extensive fouling of seed-bearing Z. marina reproductive shoots by invasive tunicates in a Cape Cod meadow during late spring/early summer 2025. Such fouling may block seed release and/or inhibit dispersal, which may reduce meadow reproductive success. The level of fouling observed has not been previously observed at this site. To our knowledge, in New England, colonization of eelgrass reproductive shoots by tunicates has not been previously documented. We conducted preliminary surveys to assess the level of coverage and provide a baseline for future work. Whether or not shifts in the peak abundance of tunicates are occurring merits further study, but could be driven by increasing water temperatures due to climate change. Long-term temperature data in Little Pleasant Bay indicate that water temperatures have increased by approximately 0.084°C since 2006. If aggressive fouling of reproductive shoots negatively affects reproductive output, then earlier peaks in tunicate abundance may significantly reduce meadow reproductive success and threaten long-term persistence.