Abstract
The yellow clam Mesodesma mactroides (Bivalvia: Mesodesmatidae) was once the most abundant intertidal species on the Atlantic coast of northern Argentina and an important commercial resource in South America. This study of a population inhabiting the intertidal zone of the sheltered-dissipative sandy beach Santa Teresita documents the species’ population biology, including demographic structure, growth and production during December 2004 and December 2006, and adumbrates the critical state of M. mactroides at present. A total of 3,015 M. mactroides were collected and measured, whereas individuals were found with an anterior–posterior shell length between 2 and 64 mm. A von Bertalanffy growth function with an asymptotic length (L ∞) of 85 mm and a growth constant (K) of 0.47 year−1 was established from length–frequency distributions. The longevity of the species is estimated at approximately 6 years, and instantaneous mortality rate was about three times higher than 40 years ago. Besides, this study confirmed that the overall growth performance index (OGP) is habitat-specific and can be used to group M. mactroides and M. donacium from different areas into temperate and upwelling species. Furthermore, OGP is inversely correlated with the latitudinal distribution of Mesodesma populations. The intertidal biomass ranged between 0.06 and 0.07 g AFDM m−2 year−1. Individual production was observed to be highest at 47 mm length (0.35 g AFDM m−2 year−1), and annual production ranged between 0.12 and 0.19 g AFDM m−2 year−1, resulting in productivity values (P/B) between 1.84 and 2.93. The comparison of the results of the present study with those of growth studies conducted on M. mactroides 40 years ago revealed the following considerable differences in the population structure of M. mactroides, indicating the conservation status of this intertidal bivalve as endangered: (1) present growth rates are faster, but that the maximum length attained has decreased, (2) the numbers of individuals per square metre were many times higher in the past than in the present, (3) bivalves from the present work never reached the ‘commercial size’ of 60 mm and (4) 40 years ago, the population of M. mactroides was composed of up to three cohorts, whereas in this study, there was only one single cohort visible.
Highlights
The world’s open coastlines are dominated by almost 70% of sandy beaches (Bascom 1964; McLachlan and Brown 2006), at which 39% of the global human population are Helgol Mar Res (2011) 65:285–297 living
Greatest abundance was recorded in May 2005, with 110 ± 25 ind. m-2 and absolute abundance peaked at 543 ind. m-2 in April 2005 (Fig. 2)
Comparison of historical data (Olivier et al 1971) with present situation, both diagrammed in Fig. 3, showed considerable differences in the population structure of M. mactroides
Summary
The world’s open coastlines are dominated by almost 70% of sandy beaches (Bascom 1964; McLachlan and Brown 2006), at which 39% of the global human population are Helgol Mar Res (2011) 65:285–297 living (within 100 km from the coast, CIESIN 2000). Exposed sandy beaches are one of the most dynamic environments, where sediment is being constantly moved by waves, winds and tides (McLachlan and Brown 2006). Due to the ability to absorb wave energy, the sandy beach ecosystem is one of the most debitable types of dynamic coastlines. The sandy beach ecosystem is characterized by sediment grains, which define the pore space between sediment particles. This socalled interstitial system is important as a habitat for organisms and for the filtration of seawater. The interstitial environment of sandy beaches is mainly controlled by the factors grain size, sorting, shape, packing, porosity, pore size and permeability (McLachlan and Turner 1994). Surf clams are commonly the main primary consumers in soft bottom communities and can contribute up to 95% of the total biomass (McLachlan et al 1981; Arntz and Fahrbach 1991; Ieno and Bastida 1998), sandy beaches are inhabited by most phyla of invertebrates as interstitial forms or as members of the macrofauna, or both
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