Vertical distribution, segregation by size and recruitment of the yellow clam Mesodesma mactroides Deshayes, 1854 (Mollusca, Bivalvia, Mesodesmatidae) in exposed sandy beaches of the Rio Grande do Sul state, Brazil.

The vertical distribution and the segregation by size of the yellow clam Mesodesma mactroides Deshayes, 1854 were investigated in the intertidal zone and its limits with the lower and upper shores at exposed sandy beaches of Rio Grande do Sul state, Brazil. The gathering was made throughout 12 months; from the Pinhal beach towards the south of the State, in six 15 km equidistant transects, determined through random selection. In these transects, the calculation of 30 consecutive waves was made in order to determine its average amplitude point (P0), from which two points were marked towards the lower shore (P-1 and P-2) and three towards the upper shore (P1, P2 and P3). A 30 by 50 cm cylinder was buried down to the depth of 40 cm, the material was separated with a 0.25 cm mesh and the specimens were quantified and measured in length. The yellow clam presented segregation by size, especially between recruit and adult individuals, with recruits occupying preferably the zones above P0 and adults from this point towards the sea. The young specimens are distributed through all zones, mixed with adult and recruit specimens, which dismisses the hypothesis of segregation by size in function of competition for space and food, once the burying depth is directly proportional to their length. However, the segregation between recruits and adults might be related to the filtering mechanism of the adults, which could ingest the larva, as well as the fact that the recruits, being small and light, are easily transported to the regions above P0. Seasonal migration was observed for adult individuals during winter and spring, probably associated to the reproduction period of the species, being that the peak of recruitment was greater in the end of the winter and the beginning of spring.

Vertical distribution, segregation by size and recruitment of the yellow clam Mesodesma mactroides Deshayes, 1854 (Mollusca, Bivalvia, Mesodesmatidae) in exposed sandy beaches of the Rio Grande do Sul state, Brazil / Distribuição vertical, segregação por tamanho e recrutamento do marisco branco Mesodesma mactroides Deshayes, 1854 (Mollusca, Bivalvia, Mesodesmatidae) em praia arenosa exposta do Rio Grande do Sul, Brasil

The vertical distribution and the segregation by size of the yellow clam Mesodesma mactroides Deshayes, 1854 were investigated in the intertidal zone and its limits with the lower and upper shores at exposed sandy beaches of Rio Grande do Sul state, Brazil. The gathering was made throughout 12 months; from the Pinhal beach towards the south of the State, in six 15 km equidistant transects, determined through random selection. In these transects, the calculation of 30 consecutive waves was made in order to determine its average amplitude point (P0), from which two points were marked towards the lower shore (P-1 and P-2) and three towards the upper shore (P1, P2 and P3). A 30 by 50 cm cylinder was buried down to the depth of 40 cm, the material was separated with a 0.25 cm mesh and the specimens were quantified and measured in length. The yellow clam presented segregation by size, especially between recruit and adult individuals, with recruits occupying preferably the zones above P0 and adults from this point towards the sea. The young specimens are distributed through all zones, mixed with adult and recruit specimens, which dismisses the hypothesis of segregation by size in function of competition for space and food, once the burying depth is directly proportional to their length. However, the segregation between recruits and adults might be related to the filtering mechanism of the adults, which could ingest the larva, as well as the fact that the recruits, being small and light, are easily transported to the regions above P0. Seasonal migration was observed for adult individuals during winter and spring, probably associated to the reproduction period of the species, being that the peak of recruitment was greater in the end of the winter and the beginning of spring.

Foram investigadas a distribuição vertical e a segregação por tamanho no marisco branco Mesodesma mactroides Deshayes, 1854 na zona intermareal e seu limite com o infra e o supralitoral nas praias arenosas expostas do Rio Grande do Sul, Brasil. Ao longo de 12 meses, foram realizadas as coletas a partir da praia de Pinhal em direção ao sul do Estado, em seis transectos eqüidistantes 15 km, determinados mediante sorteio. Nestes, procedeu-se ao cálculo de 30 ondas consecutivas para avaliar o seu ponto médio de amplitude (P0), a partir do qual, foram assinalados dois pontos em direção ao infralitoral (P-1 e P-2) e três em direção ao supralitoral (P1, P2 e P3). Enterrou-se um cilindro de 30 x 50 cm até 40 cm de profundidade, sendo o material triado em malha de 0,25 cm com os exemplares quantificados e mensurados em comprimento. O marisco branco apresentou segregação por tamanho, em especial entre os indivíduos recrutas e os adultos, com os recrutas ocupando preferencialmente as zonas acima de P0 e os adultos, deste ponto em direção ao mar. Já os jovens estão distribuídos por todas as zonas, mesclados com os espécimes adultos e com os recrutas, o que descarta a hipótese de segregação por tamanho em função da competição por espaço e por alimento, uma vez que a profundidade em que se enterram é diretamente proporcional ao seu comprimento. Já a segregação entre os indivíduos recrutas e adultos, pode estar relacionada ao mecanismo de filtração dos adultos, que poderiam ingerir as larvas, como também, ao fato de que os recrutas, por serem pequenos e leves, são facilmente transportados para as regiões acima de P0. Foi constatada a migração sazonal por parte dos indivíduos adultos no inverno e na primavera, provavelmente associada, ao período de reprodução da espécie, pois o pico de recrutamento mais pronunciado foi no fim do inverno e no início da primavera.