585 2009 Population dynamics of the surf clams Donax hanleyanus and Mesodesma mactroides from open-Atlantic beaches off Argentina Populationsdynamik der Brandungsmuscheln Donax hanleyanus und Mesodesma mactroides an offenen Atlantikstränden Argentiniens Marko Herrmann ALFRED-WEGENER-INSTITUT FÜR POLAR- UND MEERESFORSCHUNG In der Helmholtz-Gemeinschaft D-27570 BREMERHAVEN Bundesrepublik Deutschland ISSN 1866-3192 Hinweis Notice Die Berichte zur Polar- und Meeresforschung The Reports on Polar and Marine Research are issued werden vom Alfred-Wegener-Institut für Polar-und by the Alfred Wegener Institute for Polar and Marine Meeresforschung in Bremerhaven* in Research in Bremerhaven*, Federal Republic of unregelmäßiger Abfolge herausgegeben. Germany. They appear in irregular intervals. 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Population dynamics of the surf clams Donax hanleyanus and Mesodesma mactroides from open-Atlantic beaches off Argentina Populationsdynamik der Brandungsmuscheln Donax hanleyanus und Mesodesma mactroides an offenen Atlantikstränden Argentiniens _______________________________________________ Marko Herrmann Ber. Polarforsch. Meeresforsch. 585 (2009) ISSN 1866-3192 Dedicated to my family Marko Herrmann Alfred Wegener Institute for Polar and Marine Research (AWI) Section of Marine Animal Ecology P.O. Box 120161 D-27515 Bremerhaven (Germany) [email protected] Die vorliegende Arbeit ist die kaum veränderte Fassung einer kumulativen Dissertation, die in der Sektion „Sektion Ökologie Mariner Tiere“ bei Prof. Dr. W. E. Arntz angefertigt und dem Fachbereich 2 (Biologie/Chemie) der Universität Bremen im Jahr 2008 vorgelegt wurde. Contents 1 Extended Abstracts ................................................................................... 3 1.1 Summary ..................................................................................................... 3 1.2 Zusammenfassung ...................................................................................... 7 1.3 Resumen ................................................................................................... 11 2 Introduction ............................................................................................. 15 2.1 Sandy Beach Ecosystems ......................................................................... 15 2.2 The Wedge Clam ...................................................................................... 16 2.3 The Yellow Clam ....................................................................................... 21 2.4 Aims .......................................................................................................... 25 3 Materials and Methods ............................................................................ 26 3.1 Study Sites ................................................................................................ 26 3.2 Population Structure .................................................................................. 31 3.3 Growth Estimation ..................................................................................... 33 3.4 Reproduction ............................................................................................. 36 4 Summary of Results ................................................................................ 39 4.1 Population Structure .................................................................................. 39 4.2 Growth Estimation ..................................................................................... 46 4.3 Reproduction ............................................................................................. 51 5 General Discussion and Conclusions ................................................... 56 5.1 Population Structure .................................................................................. 56 5.2 Growth Estimation ..................................................................................... 58 5.3 Reproduction ............................................................................................. 69 6 Publications ............................................................................................. 75 6.1 Publication I ............................................................................................... 75 6.2 Publication II ............................................................................................ 105 6.3 Publication III ........................................................................................... 127 6.4 Publication IV .......................................................................................... 155 6.5 Publication V ........................................................................................... 175 7 Appendix ................................................................................................ 194 7.1 Acknowledgments ................................................................................... 194 7.2 List of Abbreviations ................................................................................ 197 7.3 References .............................................................................................. 200 Extended Abstracts – Summary 3 1 Extended Abstracts 1.1 Summary The surf clams Mesodesma mactroides Reeve, 1854 and Donax hanleyanus Philippi, 1847 are the two dominating species in macrobenthic communities of sandy beaches off northern Argentina, with the latter now surpassing M. mactroides populations in abundance and biomass. Before stock decimation caused by exploitation (during the 1940s and 1950s) and mass mortality events (1995, 1999 and 2007) M. mactroides was the prominent primary consumer in the intertidal ecosystem and an important economic resource in Argentina. Since D. hanleyanus was not commercially fished and not affected by mass mortality events, it took over as the dominant species, but did never reach the former abundance of M. mactroides. Currently abundance and biomass of both surf clams are a multiple smaller than those of forty years ago, indicating the conservation status of D. hanleyanus and M. mactroides as endangered. Therefore the aim of this study is to analyse the population dynamics (population structure, growth and reproductive biology) of D. hanleyanus and M. mactroides, and to compare the results with historical data in order to detect possible differences within surf clam populations forty years ago and at present. Monthly quantitative sampling was carried out between December 2004 and December 2006 in the intertidal zone of Santa Teresita, Mar de las Pampas and Faro Querandí. All three sandy beaches display different exposures with contrasting morphodynamics and can be classified as sheltered-dissipative, exposed-intermediate and exposed-reflective, respectively. While D. hanleyanus inhabits all three beaches, M. mactroides was almost exclusively present at Santa Teresita. Whereas the latter is an endemic species to Argentina, living specimens of D. hanleyanus were first reported on Argentinean beaches in 1960, although fossils have been found from the Querandinense period in the early Holocene. Live individuals of D. hanleyanus measured between 3 and 40 mm and those of M. mactroides showed an anterior-posterior shell length (apSL) between 2 and 64 mm. Both surf clams were found associated with very few species and low abundance of other macrozoobenthic fauna. The gastropod Olivancillaria vesica auricularia was recorded as a 4 Extended Abstracts – Summary potential predator of both surf clam species especially in Santa Teresita. At Faro Querandí the American oystercatcher Haematopus ostralegus was observed feeding on D. hanleyanus. Mean abundance of M. mactroides was recorded as 110 ind. m-2 and those of D. hanleyanus as 50 to 267 ind. m-2. The abundance of the latter was linked to sand grain size. Highest abundance was recorded at the reflective beach Faro Querandí, which is characterised by coarse sediment. The decrease of D. hanleyanus in summer periods appears to be principally related to human activities. The intertidal biomass of the