Marko Herrmann

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.

Aplicación de calceína para la estimación del crecimiento de la almeja amarilla Mesodesma mactroides Reeve, 1854

Growth rates of Mesodesma mactroides were estimated comparing two different methods at the intertidal of the exposed sandy beach Santa Teresita, Argentina. Results of a short tagging-recapture experiment using the in situ fluorescent marking (MFI) method and subsequent sizeincrement analysis were compared with results from lengthfrequency distributions (DFT) analysis from a 25 month quantitative sampling. Residuals, derived from MFI method and DFT analysis, were of similar magnitude and distribution, suggesting that both methods are equally appropriate to estimate growth of M. mactroides. Calcein was useful as non-lethal growth marker for M. mactroides, emitting a bright green fluorescence band under blue light.

Estimativas de crescimento de bivalves tropicais e subtropicais: recomendação para um método padronizado

Resumo A sequencia de uma coorte especial atraves da analise de distribuicoes de frequencias de comprimento (LFD), proveniente de uma temporalmente extensa amostragem quantitativa, e uma tecnica comum entre ecologistas para estimar o crescimento populacional de uma unica especie. Neste metodo, uma dada classe anual e seguida e a alteracao na dimensao media do modo e equivalente a um crescimento medio. No entanto, a analise LFD so funciona bem em especies de bivalves com um periodo de reproducao ou recrutamento muito estreito, o que e normalmente o caso de bivalves de regioes temperadas e polares. Pelo contrario, uma vez que em bivalves tropicais o periodo de recrutamento normalmente e prolongado ou taxas de crescimento sao variaveis, a analise LFD tem aplicacao limitada e nao pode ser util para estimacao de crescimento em especies tropicais e subtropicais. Por consequencia, o presente estudo resume as informacoes disponiveis sobre as estimativas de crescimento dos bivalves tropicais e subtropicais e recomenda, principalmente para pesquisadores que nao dominam o idioma ingles, experimentos de marcacao-recaptura em tempo curtos, usando o metodo de marcacao fluorescente in situ (IFM), para medir o crescimento absoluto como um metodo padrao apropriado para futuros estudos de bivalves de importância economica e ecologica, que habitam areas climaticas tropicais e subtropicais. Abstract - Following a particular cohort by analyzing length-frequency distributions (LFD), originating from a time consuming quantitative sampling, is a common technique among ecologists to estimate the population growth of a single species. In this method, a given year class is followed, and the change in the average size of the mode is equivalent to average growth. Nevertheless, LFD analysis only works well in bivalve species with a very narrow reproductive or recruitment period, which is normally the case of temperate and polar bivalves. On the contrary, since in tropical bivalves the recruitment period is usually extended or growth rates are variable, LFD analysis has limited application and may not be useful for tropical and subtropical species to estimate growth. On this account, the present study summarizes the available information on growth estimations of tropical and subtropical bivalves and recommends, mainly for researchers who do not dominate the English language, short time tagging-recapture experiments, using the in situ fluorescent marking (IFM) method, for measuring absolute growth as an appropriate standard method for future studies of economically and ecologically important bivalves, inhabiting tropical and subtropical climate areas.