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Population structure of Donax hanleyanus (Bivalvia: Donacidae) on two sandy beaches on the Argentinean coast

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Herrmann, M. , Laudien, J. , Arntz, W. and Penchaszadeh, P. E. (2007): Population structure of Donax hanleyanus (Bivalvia: Donacidae) on two sandy beaches on the Argentinean coast , XX Encontro Brasileiro de Malacologia (EBRAM), Rio de Janeiro.-10.08.2007. .
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Abstract:

The population structure of the intertidal surf clam Donax hanleyanus (Philippi, 1845) (Bivalvia: Donacidae) was studied monthly at the dissipative, anthropogenically affected sandy beach Santa Teresita (36°32S, 56°41W) from December 2004 to December 2006 and at an exposed, nature protected sandy beach Faro Querandí (37°29S, 57°07W) from March 2005 until December 2006. Both sites are located in the province of Buenos Aires, Argentina. Quantitative samples to determine abundances and growth parameters were taken from a series of stations (4 m intervals) along a transect perpendicular to the shoreline from the spring tide high water mark to the low tide water mark. At each station, three replicates were taken to 35 cm depth with a 0.16 m2 corer. The sand was sieved on a 1 mm mesh. Maximum anterior-posterior length shell of each individual was measured to the lower 0.1 mm with vernier callipers to obtain monthly length-frequency distributions. A distribution subdivided in two belts was observed: recruits were found mainly during autumn in the middle swash zone, while juveniles and adults occurred across the tidal gradient up to the retention zone. Population density increased to 2475 individuals m-2 in December 2004 at Santa Teresita and to 606 individuals m-2 in June 2006 at Faro Querandí. Length frequency distributions indicated constant growth of the at least three co-occurring size classes. A von Bertalanffy growth function with a growth constant (K) of 0.75 yr-1 at Santa Teresita and 0.59 yr-1 at Faro Querandí and an asymptotic length (L∞) of 42 mm (observed) corresponding to an age of four years was established. The overall growth performance (OGP) index P was calculated by P = log (K [L∞]3). OGP of Argentinean D. hanleyanus (P = 4.7 for Santa Teresita and P = 4.6 for Faro Querandí) corresponds well with values calculated from data set of Defeo (1996) for the Uruguayan populations (P = 4.5). As was to be expected, our values fits with the pattern of the temperate species group in the auximetric grid (according to Laudien et al. 2003).

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