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In recent decades, the transplantation of corals has been a primary focus of restoration strategies in areas where coral populations have declined [...]

Biodiversity surveys of marine species are largely conducted during the day, and often over relatively short timeframes, due to practical, operational, and budgetary constraints. As a consequence, surveys can underestimate biodiversity for some groups, such as heterobranch (formerly opisthobranch) molluscs, where abundances vary seasonally by orders of magnitude and different species are active at different times of the day. Here, we quantitatively assess day/night (diel) and seasonal variation in heterobranch sea slug assemblages using monthly, diel scuba surveys for 13 months at three sites in Port Stephens, New South Wales (NSW). All heterobranch sea slugs encountered in 50 × 5-m belt transects were recorded using identical survey methods both during the day and at night. Significant differences were detected between day and night assemblages, with species richness and abundance being consistently higher at night. Significant cyclical patterns were also detected in sea slug assemblages over the duration of the study, for both day and night assemblages, with species assemblages changing throughout the study period. The results demonstrate that marine diversity studies conducted only during the day, and those which do not account for all seasons, are likely to underestimate diversity and abundance of molluscs, particularly heterobranch sea slugs. It is, therefore, important that studies which aim to provide a comprehensive catalogue of molluscan biodiversity include not only day-time surveys, but also those conducted at night. They should also include temporal replication in order to capture ephemeral species.

The distribution of heterobranch sea slugs is generally poorly documented at a regional scale. Thus, it is currently difficult to quantify biodiversity, identify endemic and invasive species, and track range shifts at scales relevant to conservation management. For Lord Howe Island, which lies ~600 km east of the New South Wales (NSW) mid-north coast, data from a range of taxa indicate high biodiversity and endemism, but this has not been examined for heterobranch sea slugs. To address this deficit, we collated occurrence data on sea slugs from both private and public sources, including museum records, scientific literature, field guides and citizen science activities. A total of 186 nominal (formally described) species in 82 genera and 31 families were identified from intertidal and subtidal habitats. Of these, two species are endemic to Lord Howe Island, two have not been recorded elsewhere in Australia, and 28 have not been recorded on the mainland coast of NSW. These results support studies of other taxa suggesting that the relative isolation of the island has facilitated the development of diverse and unique assemblages. However, this isolation is moderated by larval transport from surrounding regions, resulting in considerable overlap of the species pool with the mainland coast of NSW and tropical areas to the north.

Port Stephens, on the central New South Wales coast, provides ideal oceanographic and benthic conditions for the settlement and growth of larvae of tropical species delivered from the north by the East Australian Current. The popularity of the bay for recreational and scientific diving has facilitated extensive documentation of the biota over several decades, confirming its high biodiversity. Of the 313 species of heterobranch sea slugs recorded from Port Stephens to date, 30 are not known to occur further south. Our observations increase the number of taxa with a southern distribution limit at Port Stephens by 12 species and add to a growing list of marine taxa that are progressively extending their southern range, potentially as a result of climate change.