Intertidal mussel beds from the South-western Atlantic show simple structure and uniform appearance: does environmental harshness explain the community?

ABSTRACT

Communities of the rocky mid-intertidal zone of the South-western Atlantic are uniform in appearance, dominated by dense monocultures of small-size mussels (Brachidontes rodriguezii and Perumytilus purpuratus). To explain this, two hypotheses have been advanced in the literature: environmental harshness due to high potential evaporation and historical contingency after the Last Glacial Maximum. In this study of Uruguayan and Argentine shores, we address the implications and predictions of these two hypotheses from a biogeographic perspective by studying the regional distribution and composition of mid-intertidal mussels. We conducted an extensive latitudinal sampling survey (21 locations, 34–54°S), along with a compilation of available information on mussel bed composition and mussel predators present along the coastline. Then we constructed latitudinal profiles of ecologically significant environmental variables with specific emphasis on potential evaporation, a proxy for desiccation stress. The results show that mussel beds are composed of two species of small mussels, which coexist over a biogeographic transition zone (40–42°S) related to sea surface water temperature. The distribution of mussels along the coastline studied is not consistent with the environmental harshness hypothesis. In addition, in the Central Patagonian zone (44–50°S), two invertebrate predators also inhabit the intertidal rocky shores. However, these localities showed higher environmental harshness (potential evaporation rate) than non-Patagonian localities. We suggest that further attention should be given to historical contingency in order to advance towards a hypothesis consistent with current knowledge on the post-glacial biogeographic history of the South-western Atlantic.

SUBJECT EDITOR:

• Dan Smale

KEYWORDS:

• Historical contingency
• mussel bed
• South-western Atlantic
• stress

Acknowledgements

We acknowledge the late Dr (Lobo) Orensanz, who actively worked on conducting and writing this study. We acknowledge the late Dr S. R. Olivier, who pioneered the ecological study of rocky shores from the South-western Atlantic, for sharing his insights and for making available to us his valuable photographic archive and unpublished notes. M. Bertness was most helpful in attending our requests for clarification and additional information in relation to the important contributions made by him and co-workers to Patagonian rocky-shore ecology. The original manuscript was greatly improved by comments and criticism from R. Paine, C. Bazterrica, F. Hidalgo, G. Palomo, G. Pastorino, L. Arribas, E. Wieters, S. Navarrete, V. Reyna, B. Silliman, S. Gordillo and F. Scarabino. M. Haller, N. Weiler and S. Martínez educated us in various aspects of regional geology. E. Bogazzi and G. Escati-Peñaloza assisted with digital cartography and illustration. L. Bala, A. Gosztonyi, L. Kuba, P. Barón, C. Rios and F. Scarabino provided information on predators of intertidal mussels. V. Savoya, M. Rocha, A. Bortolus and P. Verón helped with field and lab work. MA acknowledges Bianca and Adrián for being supportive during protracted field seasons.

Disclosure Statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Participation of MA was supported by Project CONICET-PIP 02126 to Juan López Gappa, of ES by National Geographic Society 7805-05 and Project FONCYT-PICT 20621, CONICET PIP 089 and 508, and of JMO by Project FONCyT-PICT/Raíces 1839.