Although the Indo-Pacific is the global centre of diversity for the heterobranch sea slugs, their distribution remains, in many places, largely unknown. On the Australian east coast, their diversity decreases from approximately 1000 species in the northern Great Barrier Reef to fewer than 400 in Bass Strait. While occurrence records for some of the more populated sections of the coast are well known, data are patchy for more remote areas. Many species have very short lifecycles, so they can respond rapidly to changes in environmental conditions. The New South Wales coast is a recognised climate change hot-spot and southward shifts in distribution have already been documented for several species. However, thorough documentation of present distributions is an essential prerequisite for identifying further range extensions. While distribution data are available in the public realm, much is also held privately as photographic collections, diaries and logs. This paper consolidates the current occurrence data from both private and public sources as part of a broader study of sea slug distribution in south-eastern Australia and provides an inventory by region. A total of 382 species, 155 genera and 54 families is reported from the mainland coast of New South Wales.

Kelp forests are declining in many regions globally with climatic perturbations causing shifts to alternate communities and significant ecological and economic loss. Range edge populations are often at most risk and are often only sustained through localised areas of upwelling or on deeper reefs. Here we document the loss of kelp forests (Ecklonia radiata) from the Sultanate of Oman, the only confirmed northern hemisphere population of this species. Contemporary surveys failed to find any kelp in its only known historical northern hemisphere location, Sadah on the Dhofar coast. Genetic analyses of historical herbarium specimens from Oman confirmed the species to be E. radiata and revealed the lost population contained a common CO1 haplotype found across South Africa, Australia and New Zealand suggesting it once established through rapid colonisation throughout its range. However, the Omani population also contained a haplotype that is found nowhere else in the extant southern hemisphere distribution of E. radiata. The loss of the Oman population could be due to significant increases in the Arabian Sea temperature over the past 40 years punctuated by suppression of coastal upwelling. Climate-mediated warming is threatening the persistence of temperate species and precipitating loss of unique genetic diversity at lower latitudes.

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.

There is increasing evidence of poleward migration of a broad range of taxa under the influence of a warming ocean. However, patchy research effort, the lack of pre-existing baseline data, and taxonomic uncertainty for some taxa means that unambiguous interpretation of observations is often difficult. Here, we propose that heterobranch sea slugs provide a useful target group for monitoring shifts in distribution. As many sea slugs are highly colourful, popular with underwater photographers and rock-pool ramblers, and found in accessible habitats, they provide an ideal target for citizen scientist programs, such as the Sea Slug Census. This maximises our ability to rapidly gain usable diversity and distributional data. Here, we review records of recent range extensions by tropical species into the subtropical and temperate waters of eastern Australia and document, for the first time in Australian waters, observations of three tropical species of sea slug as well as range extensions for a further six to various locations in the Tasman Sea.

Southern range extensions of the tropical Australian heterobranch sea slug fauna may be symptomatic of climate change, particularly in south-eastern Australia, a recognized climate change hot-spot. In the Solitary Islands Marine Park, northern New South Wales, the presence of biogeographic mixing means that the southern coastal distribution limit of several tropical marine species and the northern limit of some temperate species overlap. During a year-long series of monthly surveys of sea slug communities on intertidal rocky shores, we observed four species beyond their previously recorded range –Colpodaspis thompsoni, Elysia pusilla, Phidiana bourailliandHerviella claror. The southern migration of species is an anticipated response to warming seas and we note an apparent extension of the southern limit for these taxa.