The population biology and production of the stout razor clam Tagelus plebeius Lightfoot, 1786 were investigated on an intertidal sandflat on the southeast coast of Brazil (Enseada Beach, São Sebastião, state of São Paulo) between April 1997 and April 1998. Two rectangular sites of 50 X 10 m parallel to the waterline were established, site A (upper intertidal level) and site B (middle intertidal level), where the samples were taken in an 0.5 x 0.5 m quadrat. High abundances were recorded in winter and spring, with no significant differences between the sites. The high bivalve abundances were related to the presence of very fine homogeneous sediment with low salinities. Tagelus plebeius had negative allometric growth, characteristic of deep burrowers for the relationships DM/SL and AFDM/SL. Parameters of the modified von Bertalanffy growth function were: L∞ = 67.01 mm, K = 1.73 year-1, t0 = -0.11 year, C = 0.43, WP = 0.96. The instantaneous mortality (Z) was 3.12 year-1, relatively high in comparison to other tropical bivalve populations. Secondary production was 1.53 g AFDM m-2 year-1, with a P/B ratio reaching 1.37 year-1. This high turnover ratio (P/B) was related to a rapid population replacement, connected with the short life span and high mortality of the species.

The spatial distribution of mollusks in the intertidal zone was examined monthly from August 1995 through July 1997, in Enseada, Barra Velha and Araçá beaches in southeastern of Brazil. One study sector was selected in Enseada and Barra Velha, and two sectors in Araçá (Araçá I and Araçá II). The sectors were 10 m wide and equivalent in length to the width of the intertidal zone. Each sector was divided into three horizontal levels: lower, middle and upper, where the samples were taken with a cylinder corer with a base area of 0.16 m². In order to characterize the intertidal environment in these areas, some environmental variables were analyzed. In general, the mollusks were distributed in the sectors as follows: Enseada - Olivella minuta (Link, 1807) in the lower level and Tagelus plebeius (Lightfoot, 1786) in the upper level; Araçá I - O. minuta in the lower level, Tellina lineata Turton, 1819 and Anomalocardia brasiliana (Gmelin, 1791) in the middle levels; Araçá II - Cerithium atratum (Born, 1778) in the lower level, O. minuta in the lower and middle levels, and A. brasiliana and Corbula caribaea Orbigny, 1842 in the middle level; Barra Velha - Tagelus divisus (Spengler, 1794), Lucina pectinata (Gmelin, 1791) and Tellina versicolor De Kay, 1843 in the lower level, and A. brasiliana and Macoma constricta (Brugüìere, 1792) in the upper level. The intertidal zone of the study sectors could be divided into two biological zones: the upper zone, where T. plebeius, A. brasiliana and M. constricta were more abundant; and the lower zone, where O. minuta, C. atratum, T. lineata, T. versicolor, C. caribaea, T. divisus and L. pectinata were abundant.

This work constitutes a systematic survey of the Annelida Polychaeta from the sublitoral of the São Paulo State coast (Ubatuba), southeast Brazil. From 105 identified species, 20 are new records for the brazilian coast. An analysis of affinity degree between the fauna of the studied region and of the adjacents biogeographic provinces shows that 25,5% of the species are endemic for the Paulista Province, 19,0% amphi-american, 17,0% cosmopolite and 16,8% amphi-atlantic. An affinity with species from the Patagonian (7,4%) and Caribbean (2,0%) Provinces is smaller than that of other authors, for the southeast Brazilian coast.