A recurring population of the sea hare Bursatella hirsuta (Gastropoda: Aplysiidae) at Rottnest Island, Western Australia

ABSTRACT

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.

KEYWORDS:

• Aplysia
• intertidal
• Indian Ocean
• boom-and-bust

Acknowledgements

This work was undertaken under permits approved by the Rottnest Island Authority, Western Australian Department of Biodiversity Conservation and Attractions and WA Department of Primary Industry and Regional Development. The authors thank Dr Matt Nimbs of Southern Cross University for confirming the identity of the species and comments on a draft of the manuscript. Similarly, the authors appreciate the comments of Dr Rüdiger Bieler of Field Museum of Natural History, Chicago, USA, on the draft. All authors have read and agreed to the published version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).