PATTERNS OF ABUNDANCE AND SEASONALITY OF POLYCHAETES IN A SW ATLANTIC ESTUARINE EPIBENTHIC SHELL ASSEMBLAGE
Gutiérrez, J. L.; Palomo, G. & Iribarne, O.
Departamento de Biología (FCEyN), Universidad Nacional de Mar del Plata. Funes 3250 3° piso, (7600) Mar del Plata, Argentina
Shell assemblages of the stout razor clam, Tagelus plebeius, are common in intertidal areas of SW Atlantic estuaries (36° to 39° S). In the Mar Chiquita coastal lagoon, Argentina (37° 45' S, 57° 19' W), the habitat structure provided by these shells support a rich benthic community and many organisms dwell in the cavities of empty shells. In this study, we investigated the seasonal pattern of abundance and intertidal distributions of polychaete species that inhabit empty shell cavities. Samples were taken bimonthly between July 1996 and September 1997. Each sample was obtained according to the following scheme: three 20 m transects were demarcated and 10 shells were randomly taken at the mean lower low water (MLLW) level, and 0.5 m and 0.8 m above MLLW. Shell cavity sediments were sieved through a 500 mm screen to obtain adult polychaetes. Five additional shells were taken from each transect and internal sediments sieved through a 100 mm screen to collect juvenile polychaetes. Five polychaete species were collected from shell cavities: Laeonereis acuta, Neanthes succinea, Nephtys fluviatilis, Polydora ligni, and Heteromastus similis, with mean densities of 37.7 (SD = 9.18), 2.07 (SD = 1.52), 0.05 (SD = 0.11), 1.37 (SD + 0.64), and 15.66 (SD = 15.63) individuals/100 shells, respectively. The abundance of L. acuta was nearly constant during the year, showing greatest numbers in the summer, similar to the occurrence of juveniles. In contrast, H. similis was more abundant in winter, while juveniles attained maximum densities during late autumn and winter. Neanthes succinea was absent during the first part of the sampling period, first appearing in March 1997, with densities increasing to the end of the sampling period. Polydora ligni and N. fluviatilis were only present in samples collected in January and March 1997. All species were most abundant at the MLLW except H. similis, which was equally abundant at the MLLW and 0.5 m above, and P. ligni which occurs only 0.8 m above MLLW. The role of physical and biological factors in determining these patterns are discussed.