Daniel Carstensen

Population Structure, Growth, and Production of the Wedge Clam Donax hanleyanus (Bivalvia: Donacidae) from Northern Argentinean Beaches

ABSTRACT Donax hanleyanus Philippi, 1847 (Bivalvia: Donacidae) dominates fine to coarse sandy beach communities of the northern Argentinean Atlantic coast. The population biology of this intertidal wedge clam was studied by determining population structure, growth and production at the three locations Santa Teresita, Mar de las Pampas (both from December 2005 to December 2006) and Faro Querandí (from March 2005 to December 2006). Von Bertalanffy growth functions were established from length-frequency distributions using an asymptotic length (L∞) of 44 mm and the growth constants (K) of 0.46 and 0.47 y-1 respectively of Mar de las Pampas and Faro querandí. Compared with growth studies four decades ago, D. hanleyanus today is growing more slowly, but is reaching a higher maximum length. Longevity is estimated to be approximately five years. The present study confirms that the overall growth performance index is habitat-specific, grouping Donacidae into tropical/subtropical, temperate and upwelling species. The intertidal biomass of D. hanleyanus ranged between 0.04 and 1.32 g ash-free dry mass (AFDM) m-2yr-1. Individual production revealed the highest value at 30 mm length (0.16 g AFDM m-2yr-1) and annual production ranged between 0.08 and 0.99 g AFDM m-2yr-1, resulting in renewal rate values (P/) between 0.82 and 2.16. The P/ ratios of D. hanleyanus populations increased with decreasing latitude from temperate to tropical regions. Only at Santa Teresita D. hanleyanus was found living with the sympatric yellow clam Mesodesma mactroides. A significant negative correlation between abundances of both surf clams suggests that abundance peaks of D. hanleyanus are related with population crashes of M. mactroides. Spatial differences in abundance are significantly related to sand texture as confirmed by nonmetrical multidimensional scaling, but not to sea surface temperature. However, the decrease of D. hanleyanus seems to be principally related to human activities.

Recurrent, thermally-induced shifts in species distribution range in the Humboldt current upwelling system

El Niño-Southern Oscillation (ENSO) is a global climate variablility, which fundamentally influences environmental patterns of the Humboldt Current System (HCS) off Chile and Peru. The surf clams Donax obesulus and Mesodesma donacium are dominant and highly productive bivalves of exposed sandy beaches of the HCS. Existing knowledge indicates that El Niño (EN, warm phase of ENSO) and La Niña (LN, cold phase of ENSO) affect populations of both species in a different way, although understanding of the mechanisms underlying these effects is still lacking. The aim of this study was to test hypotheses attempting to explain field observations on the effect of strong EN or LN events by using controlled experimental conditions. Growth and mortality rates of both species were registered during a four-week experiment under EN temperature conditions, normal temperature conditions and LN temperature conditions. While D. obesulus exhibited reduced growth and higher mortality under LN conditions, M. donacium showed reduced growth and higher mortality under EN conditions. The results clearly indicate different temperature tolerance windows for each species, possibly reflecting the evolutionary origins of the Donacidae and Mesodesmatidae in regions with contrasting temperature regimes. These results provide experimental support for previous hypotheses suggesting that thermal tolerance is the driving factor behind observed changes in the species distributions of D. obesulus and M. donacium during the extreme phases of ENSO.