Abstract
Nudibranchs are a charismatic and widely distributed group of marine invertebrates that can be useful biological indicators of ocean change, thanks to their responsiveness and dispersal ability. However, these same traits mean nudibranchs are also often detected outside their native ranges. In recent years, two non-native nudibranch species, Polycera capensis and Polycera hedgpethi, have been increasingly observed in south-eastern Tasmanian waters, indicating poleward dispersal of ~ 600 km from the Australian mainland, where both species were introduced decades earlier. We conducted dive surveys and reviewed records from citizen science and long-term monitoring databases to describe the history, distribution, and status of these overlooked introductions within Tasmania, and Australia more broadly. We report persistent populations of both species at the southern edge of their global distribution, with evidence that at least P. capensis is likely breeding in Tasmania. Although first detected in Tasmania 10–20 years ago, 80% of all P. hedgpethi records from the region have come since 2021, and 97% of P. capensis records since 2022. Citizen scientists contributed 43% of all P. capensis records in Tasmania. Observations were concentrated around ports and other anthropogenic structures, consistent with known dispersal via vessel fouling. This case study likely represents a climate-driven secondary invasion from mainland Australia, with the human-mediated introduction and subsequent spread being facilitated by ocean warming. It also highlights both the effectiveness and challenges of citizen science for monitoring cryptic species, and raises questions around species prioritisation and the triaging of science and management responses to our rapidly changing oceans.