Sandra M. Fiori

Mass mortality of the yellow clam, Mesodesma mactroides (Bivalvia: Mactracea) in Monte Hermoso beach, Argentina

Yellow clam was the most abundant species in the sandy mesolittoral community of Monte Hermoso (Buenos Aires Province, Argentina). In November 1995, a sudden massive mortality almost decimated its population. The number of dead individuals was estimated in near 63 million, with a total biomass of 428.03 ton (fresh weight with shells). Simultaneous samples of sediments, seawater and clams did not allow detecting the probable cause of this phenomenon. No abnormality was found in the physicochemical parameters that could justify the mortality. Several specific mortality factors were discarded, as the analysis for heavy metal contamination, phytotoxins, abnormal phytoplankton composition, and protozoan tissue-parasites gave negative results. The meteorological conditions were rather stringent during a period immediately previous to the mortality, this accounting for a probable switch condition. This sole factor can not however account for the progressive decimation of the species observed from 1993 on: in the term of 3 years mass mortalities of Mesodesma mactroides occurred in Brazil, Uruguay and Argentina, covering the entire area of distribution of the species. The yellow clam is now in a risk condition as for its conservation status.

Potential impacts of petroleum exploration and exploitation on biodiversity in a Patagonian Nature Reserve, Argentina

Petroleum exploration and extraction are common on the Patagonian steppe, but their impacts on the native biodiversity have not been properly evaluated. We describe both activities in a Patagonian nature reserve and consider their potential impacts on biodiversity. More than 2025 km of seismic lines inside the reserve resulted in 87.21 m2/ha (0.9%) of directly affected land, and 793 fragments of native habitats were defined with a mean area of 1.26 ± 0.74 km2. Vegetation recovery on seismic lines is extremely poor. We discuss the role of seismic lines as barriers to native species, and their significance in encouraging poaching and the expansion of exotic invasive plants. There is a high degree of overlap between current petroleum activities and areas of special conservation concern (high erosion risk, vegetation diversity, abundance of endemic plant species, and habitat quality for native vertebrates). All these have a significant impact on the efficiency of the conservation area and highlight the urgent need to implement appropriate mitigating actions.

Burrowing Crab ( Neohelice granulata ) as a Potential Bioindicator of Heavy Metals in the Bahía Blanca Estuary, Argentina

Bioindicators can be used to determine spatial and/or temporal variations of heavy metals in a certain coastal or marine environments. This study investigated the potential use of the burrowing crab Neohelice granulata from two different locations in the Bahía Blanca estuary, a moderately polluted ecosystem. Concentrations of zinc (Zn), nickel (Ni), and lead (Pb) in soft tissues of male and female crabs were measured. In addition, concentrations of the three metals in eggs were compared with concentrations in female crabs. No geographical differences were found for any of the three metals, whereas sexual and seasonal differences were obtained for Zn and Ni, with the winter season posing lower concentrations. Moreover, the three metals were detectable in eggs and were lower than concentrations in female crabs (except for Zn). Finally, the usefulness of this species as a potential bioindicator of heavy-metal pollution within this estuarine ecosystem is discussed.

Winter distribution, density and size of Mesodesma mactroides (Bivalvia, Mactracea) in Monte Hermoso Beach (Argentina)

The yellow clam Mesodesma mactroides (Deshayes, 1854) is a seasonal migrant that moves in spring to the sandy upper intertidal level. In this paper we analyze the spatial distribution of density and mean shell size of the yellow clam population in Monte Hermoso beach (Argentina) in winter 1995, i.e., three months before the mass mortality occurred in November 1995. Sampling covered 32 km of beach, with a regular design of 22 transects. The major environmental gradient in the beach was determined using principal component analysis (PCA) on the correlation matrix of the environmental data (beach morphology, slope, and sand granulometry). Correlation analysis was used to assess the relationship between the score of a site (transect) on the first and second principal component, and clam mean density and mean shell size. Most of the beach seems to be habitable for clams, their spatial heterogeneity not having been explained by the measured variables since, although the first axis of the PCA has demonstrated an E-W physical gradient, clam density was not in correlation with it. Density was maximum near the piers, even though these are points with high tourist activity. It seems that non-extractive touristic activities do not affect population density but rather mean shell size, probably due to reduction of growth rates. The abundance of the winter population, as compared with the assessment done after the mass mortality of November, strongly suggests that a great part of the population was overwintering in the intertidal fringe.

Influence of Grain Size on Burrowing and Alongshore Distribution of the Yellow Clam (Amarilladesma mactroides)

ABSTRACT The yellow clam Amarilladesma mactroides (Reeve, 1854), is an intertidal species that prospers mainly on dissipative sandy beaches along the temperate Atlantic coast of South America, from Brazil to Argentina (24–41° S). This large clam is considered a fast burrower, which lives buried in the sediment, migrating seasonally into the intertidal zone. The present study explores the effect of sediment grain size on the burrowing performance of this species, to elucidate the influence of granulometry on the alongshore distribution of the A. mactroides population. Laboratory trials were performed with clams of different sizes, to study the influence of grain size on the burial rate. Clam distribution was analyzed along a 32 km coastal fringe whose granulometric composition varies from very fine to coarse sand. The values of the mean burrowing rate index, a measurement of clam mobility, suggest that burrowing is fast to very fast in fine and medium sand and becomes slower toward areas with extreme particle size (very fine and coarse sand). The burial time of A. mactroides was positively correlated with shell length: small animals can burrow into substrates that may exclude larger animals. Adults clams burrowed in a very limited range of sand grain sizes. They displayed fastest burial times in grain sizes typical of dissipative beaches, i.e., fine to medium sand. Patchy distribution and density variation of A. mactroides alongshore reflect the relation between grain size and burrowing performance: the population is absent in sites with the highest proportion of coarse sands, its density increases in patches with the highest proportion of fine and medium sand and peaks at a site with the highest proportion of fine sand. Results indicate that the discontinuous distribution of A. mactroides along its range could be due to a postsettlement process. Clams can potentially recruit on a wide morphodynamic range, but only may thrive in beaches which sand grain size allows them a rapid reburial during migratory and local movements.

Mass Mortalities Affecting Populations of the Yellow Clam Amarilladesma mactroides Along Its Geographic Range

ABSTRACT The yellow clam Amarilladesma mactroides (Deshayes, 1854) is considered a vulnerable species since the mid-1990s. Populations have experienced mass mortalities throughout their entire range of distribution (23–41°S) along exposed sandy beaches of Atlantic South America. Detrimental anthropogenic impacts have further contributed to failure of populations to make a recovery. To determine the factors involved in these events, density prior to a mortality event was calculated and live yellow clams encompassingmost of its geographic range distribution were analyzed histologically to describe parasites and pathologies. Moreover, moribund specimens were analyzed by molecular techniques to test for the presence of the virus OsHV-1. A mortality event was recorded after a maximum density of 127 clams/m2 was attained. No clear pattern was found between the prevalence and intensity of infection and localities, mortality events, or sampling season. Although OsHV-1 was not observed in any of the yellow clams tested, the possibility that another viral agent was implicated cannot be ruled out. The presence of bacteria of the genus Vibrio in combination with stress caused by a relatively high population density is suggested as the likely cause of these episodic mass mortalities.