Changes in the macrobenthos of a sandy beach during "El Niño" 1997-98 in the Malaga Bay, Colombian Pacific

 Changes in the macrobenthic community of a sandy-beach were studied during the "El Niño" 1997-98 event at the Malaga Bay, in the central Colombian Pacific. Species composition was recorded monthly from October 1997 to May 1998 at four tidal levels, and species diversity (Shannon-Wienner, H') and richness were calculated. In order to analyze community structure changes, multivariate ordinations were performed (non-metric multidimensional scaling) and species responsible for the observed temporal changes were discriminated using a dissimilarity index. Differences in species diversity and richness were related to a seriation pattern in vertical species distribution . A common pattern of structure change was found at four tidal levels on the beach. Population reduction or increase of some species and vertical modifications of the spatial distribution patterns were the main source of temporal community change

Reproductive cycle and gonad development of the Northern Argentinean Mesodesma mactroides (Bivalvia: Mesodesmatidae)

The reproductive cycle and gonad development of the yellow clam Mesodesma mactroides was studied over a period of 24 months (January 2005December 2006) at the Argentinean sandy beach Santa Teresita. Histological examination of gonadal tissue revealed that sex ratios did not significantly deviate from the proportion of 1:1 and no case of hermaphroditism was found. The reproductive cycle of M. mactroides followed an annual cyclicality, which was significantly correlated to monthly mean sea surface temperatures (SST). Oocytes showed highest abundance in winter, indicating a process of gonadal development and sexual maturation. The mean oocyte size decreased significantly during spring. Modal oocyte sizes decreased significantly during winter and late spring of each year, suggesting spawning events. The condition index was not useful in describing the annual reproductive cycle of M. mactroides. Ash-free, shell-free dry mass was chosen to detect the condition of the specimens, and this significantly correlated with monthly mean SST and the gametogenic cycle. Annual recruitment patterns during summerautumn indicated a 3-month-long planktonic phase of M. mactroides. The reproduction cycle and gonad development of M. mactroides showed only weak differences between data from the present study and those collected 40 years ago

Population dynamics of the tropical bivalve Cardita affinis from Málaga Bay, Colombian Pacific related to La Niña 1999–2000

The population dynamics of the “cholga” Cardita affinis (Sowerby 1833) from Málaga Bay, Colombia, was studied from December 1999 to February 2002, which included the 1999–2000 La Niña event (LN) and the post-LN period 2001–2002. This climatic deviation caused precipitation anomalies in Málaga Bay. Salinity, precipitation, and sea surface temperature anomalies were highly correlated with the bivalve’s body mass cycle. Irregular spawning events were observed during LN by comparison with the regular period. Individual growth and mortality were found significantly higher during LN than during the post-LN period while longevity was almost twofold lower during LN. Increased mortality was probably related to environmental stress. Individual production and productivity were higher during LN, although the annual biomass was lower than during the post-LN period. These results may be related to higher food availability during LN, which agrees well with the results on growth performance. The observed changes provide a base line for future studies regarding effects of El Niño/LN events on population dynamics of tropical bivalves