MEPS 547:107-119 (2016)  -  DOI: https://doi.org/10.3354/meps11642

Separation anxiety: mussels self-organize into similar power-law clusters regardless of predation threat cues

John A. Commito1,2,*, Natasha J. Gownaris1,4, Danielle E. Haulsee1,5, Sara E. Coleman1,6, Brian F. Beal3

1Environmental Studies Department, Gettysburg College, Gettysburg, PA 17325, USA
2Unità di Biologia Marina e Ecologia, Dipartimento di Biologia, Università di Pisa, Via Derna 1, 56126 Pisa, Italy
3Division of Environmental and Biological Sciences, University of Maine at Machias, Machias, ME 04654, USA
4Present address: Institute for Ocean Conservation Science, School of Marine and Atmospheric Sciences, Stony Brook University, 100 Nicholls Road, Stony Brook, NY 11794-5000, USA
5Present address: College of Earth, Ocean and Environment, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA
6Present address: The Nature Conservancy, Department of Ocean and Coastal Conservation, Rhode Island Chapter, 159 Waterman Street, Providence, RI 02906, USA
*Corresponding author:

ABSTRACT: Mussels have myriad effects on population, community, and ecosystem processes. Their aggregation behavior is an inducible defense that links non-consumptive effects of predators to benthic spatial pattern formation. Aggregation increases intraspecific competition but can be beneficial due to lower perimeter-related predation and other risks. Mytilus edulis aggregation responses to predation threats have not been investigated outside of Europe. We studied the effects of chemical cues from heterospecifics (predators Carcinus maenas, Nucella lapillus; herbivore Littorina littorea) and conspecifics (injured and intact M. edulis) on M. edulis aggregation behavior in Maine, USA. Mussels self-organized into fractal power-law spatial patterns like those in the field. Aggregations had lower perimeter:area (P:A) ratios than singletons, despite having more complex, irregular shapes with higher fractal dimensions (D). However, with one exception, no significant differences in aggregation rate, P:A ratio, and D were observed for any chemical cue treatment when compared to no-cue controls. Our experiment revealed higher aggregation rates than reported from similar experiments, leaving little scope for additional aggregation when exposed to chemical cues. We suggest that increased aggregation in response to predation threat is context-dependent: costs outweigh benefits beyond some optimal aggregation size, and mussels in our experiment were at the upper aggregation limit beyond which more aggregation could have negative consequences. Bet-hedging with a power-law distribution of aggregation shapes and sizes may be the optimal spatial strategy, especially if predation and other risks are variable in space and time.


KEY WORDS: Chemical cue · Fractal power-law · Inducible defense · Maine · Mussel aggregation · Mytilus edulis · Non-consumptive effect · Predation threat


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Cite this article as: Commito JA, Gownaris NJ, Haulsee DE, Coleman SE, Beal BF (2016) Separation anxiety: mussels self-organize into similar power-law clusters regardless of predation threat cues. Mar Ecol Prog Ser 547:107-119. https://doi.org/10.3354/meps11642

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