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 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 surf clam populations ranged between 0.04 and 1.32 g shell-free ash-free dry mass (AFDM) m-2yr-1 (D. hanleyanus) and 0.06 to 0.07 g AFDM m-2yr-1 (M. mactroides). The individual production of D. hanleyanus revealed the highest value at 30 mm apSL (0.16 g AFDM m-2 yr-1), with an individual production of M. mactroides of 0.35 g AFDM m-2yr-1 at 47 mm apSL. Annual production for D. hanleyanus ranged between 0.08 and 0.99 g AFDM m-2yr-1 and for M. mactroides between 0.12 and 0.19 g AFDM m-2 yr-1, resulting in an annual renewal rate (P/B) of 0.82-2.16 and 1.84-2.93, respectively. A review of the renewal rate of several Donax species revealed that the renewal rate increased with decreasing latitude from temperate to tropical regions.Growth estimations based on length-frequency distributions (LFDs) confirmed that both surf clams are fast growing species (D. hanleyanus: L∞ = 44 mm, K = 0.46-0.47 yr-1; M. mactroides: L∞ = 71 mm, K = 0.47 yr-1), and both have a potential for aquaculture. Seasonally oscillating growth with slowest growth rates in austral autumn (WP = 0.45 ~ mid-May) were revealed for D. hanleyanus (C = 0.8) and weak seasonality with slowest growth rates occurring in austral winter (WP = 0.7 ~ mid-August) for M. mactroides (C = 0.1). Compared with growth studies from the 1970s, a slower growth of D. hanleyanus was estimated, but a higher maximum length was recorded. M. mactroides now grow faster but maximum length is lower. Life span of D. hanleyanus was calculated as 4.96-5.18 yrs, and that of M. mactroides was estimated at 6.19 yrs. Overall growth performance (OGP) indices of D. hanleyanus (OGP = 4.54-4.60) and M. mactroides (OGP = 5.23-5.24) resulting from estimated growth parameters showed values comparable with those of other temperate surf clams. The study showed that OGP is inversely correlated with the latitudinal distribution of Argentinean, Uruguayan and Peruvian Mesodesma populations. For growth estimates of both surf clams tagging-recapture experiments using the in situ fluorescent marking (IFM) method and subsequent sizeincrement analyses were a useful alternative to conventional LFD analyses. The fluorescence marker calcein produced clear marks in shells of D. hanleyanus and M. mactroides, emitting a bright green fluorescence band under blue light, which was readily distinguished from naturally occurring autofluorescence, even in low concentrations and short immersion times. Thus calcein is suitable as growth marker of short-term, high-resolution growth studies for both surf clams. Daily growth rates of D. hanleyanus ranged between 8 and 72 μm d-1 and those of M. mactroides between 1 and 70 μm d-1. The relationship between final umbo-shell margin length (umSL2) and daily growth rates was best described by exponential functions. The reproductive biology of M. mactroides was studied at Santa Teresita, and of D. hanleyanus at all three study sites. The gametogenic cycle of the latter correlated significantly with sea surface temperature (SST), relative spermatozoon abundance (RSA), condition index (CI), AFDM and mean size and abundance of oocytes for all three populations. In line the annual reproductive cycle of M. mactroides also correlated with SST. Oocytes of M. mactroides showed highest abundance in winter, indicating a process of gonadal development and sexual maturation. Mean oocyte size decreased significantly in winter and late spring, suggesting spawning events. Annual recruitment patterns in summer-autumn indicate a three-months-long planktonic phase of M. mactroides. The habitat harshness hypothesis was tested, comparing population responses of D. hanleyanus with contrasting habitat morphodynamics. The reproductive phase was more extended in the reflective beach and males and females from the dissipative and intermediate beach were smaller and had lower biomass at maturity. Recruits were more abundant at the dissipative beach, where the recruitment period was also more extended. Spawning events took place twice each year at the dissipative (early spring and spring-summer) and the intermediate beach (winter and summer), whereas continuous gamete releases were noted at the reflective beach. Size and biomass at first maturity were lower at the dissipative beach. In conclusion, although the commercial fishery of M. mactroides is closed since more than 50 years and extractions are still forbidden and D. hanleyanus is not used as a natural living resource yet, the strong decline of surf clam abundance and biomass within the last four decades implies that populations of the Argentinean D. hanleyanus and M. mactroides are in unstable conditions, due certainly to the increasing anthropogenic impact. Future work should integrate sand grain size analyses and mass tourism data (human trampling) in studies of population dynamics of intertidal species, since the abundance of surf clams is principally related to these factors. Furthermore phytoplankton data such as biomass and species composition should be incorporated, as increasing chlorophyll a concentrations trigger the maturation of both surf clams. In order to answer the question where the surf clams migrate to during summer months, sampling in the lower sublittoral is recommended.