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Finding the sweet spot: drilling precision on shelled molluscs by Octopus vulgaris type III in False Bay, South Africa

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Abstract

Octopus vulgaris type III preys upon molluscs by drilling a hole in the shell, through which it injects venom, weakening the muscles and allowing the octopus to remove and eat the mollusc flesh. In False Bay, South Africa, the abalone Haliotis midae, the kelp limpet Cymbula compressa and the helmet snail Semicassis zeylanica, are all common octopus prey, but each has a very different shell morphology and underlying anatomy. It is hypothesized that O. vulgaris type III targets the muscle attachment site of all three species and that the precision of drilling should correlate with the relative size of these muscle attachment sites. Measurements of the locations of drill holes from collected shells of each species showed that drilling locations were significantly non-random. In the abalone and limpet, the muscle attachment site was targeted two and 4.5 times more frequently than expected, respectively. Octopus vulgaris type III drilled helmet snails with extreme precision, with over half the holes drilled on the spire at an angle of 45° and 90° from the lip. Kernel density heat maps demonstrated that octopus drilled abalone with low precision, limpets with medium precision and helmet snails with high precision. The reason for the high precision in the helmet snail remains speculative, as the main drilling location did not align with the columellar muscle attachment and no target could be defined. This study is the first to assess octopus drilling in temperate southern Africa and to compare drilling precision across different prey species.

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Data availability

The datasets analysed in this study are available from the corresponding author on request.

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Acknowledgements

For continued support we would like to acknowledge the Sea Change Project of the Sea Change Trust. We thank AbagoldTM for donating cultivated Haliotis midae specimens to use for this study; R. Nkuna from Amanzi Biosecurity for help with the dissection of organisms; R. Dickson for creating the specimen drawings; C. Foster for thought-provoking discussions and allowing us access to his treasured shell collection; and the friends and colleagues who assisted with sampling efforts.

Funding

This study was supported through the Keystone Grant 542-1001 Seaforest Species—from the Save Our Seas Foundation given to JL of the Sea Change Trust. A postgraduate scholarship from the National Research Foundation (NRF) of South Africa to GNF; a University Research Committee Core Grant to CLG.

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Contributions

All authors contributed to the study conception and design. Data collection and analysis were performed by GNF. The first draft of the manuscript was written by GNF and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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Correspondence to Gareth N. Fee.

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Conflict of interest

The authors have no relevant financial or non-financial interests to disclose.

Ethics approval

This research was authorised by scientific research permit number RES/2021/72 issued to the Department of Biological Sciences, University of Cape Town by the South African Department of Environment, Forestry and Fisheries (now Forestry, Fisheries and the Environment). No ethical clearance was required as only gastropod molluscs were collected from the field.

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Responsible Editor: R. Villanueva.

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Fee, G.N., Mather, J., Landschoff, J. et al. Finding the sweet spot: drilling precision on shelled molluscs by Octopus vulgaris type III in False Bay, South Africa. Mar Biol 170, 22 (2023). https://doi.org/10.1007/s00227-022-04152-6

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  • DOI: https://doi.org/10.1007/s00227-022-04152-6

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