In many, but not all, years a recurring population of the marine aplysiid Bursatella hirsuta occurs on an intertidal limestone platform stretching between Little Armstrong Bay and North Point at Rottnest Island, Western Australia. Using a transect/quadrat method, we measured densities of B. hirsuta during the austral summer of 2020/2021. No individuals were present in December 2020, but a population with a mean density of 16.5 +/- 1.6 (SE) inds m(-2) was present on 17 January 2021. Density was low in bare sand (2.6 +/- 3.2 inds m(-2)) and in the seagrass Amphibolis antarctica (2.0 +/- 1.4 inds m(-2)), high in mixed algae, mostly Phaeophyceae (18.2 +/- 1.8 inds m(-2)) and greatest (27.2 +/- 14.3 inds m(-2)) in a small number of quadrats with a mixture of sand and algae or sand and A. antarctica. The population was estimated at >600,000 individuals. The species was present in February but had disappeared by late March 2021. The population at Little Armstrong Bay and North Point provides a fertile opportunity for developing a better understanding of the biology of B. hirsuta and broader questions of boom-and-bust populations.
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