The Ajuruteua Peninsula is located in the municipality of Bragança in the state of Pará, northern Brazil. The region and its habitats are described in detail in Barletta et al. (1998), Wolff et al. (2000) and Cohen & Lara (2003). Avicennia trees are the tallest vegetation
and found on the landward side of the mangrove whereas Rhizophora occurs on the seaward side. In certain areas, a fringe of Spartina brasiliana is found as a pioneer species at the shoreline. Large intertidal areas are submerged daily and are also subject to high levels of precipitation during the rainy season (Lacerda 1999). Bivalve species (except shipworms) from the sandy and muddy-sandy beaches were surveyed monthly at Canela Island (00°47’ S, 46°43’ W) between December 2001 and November 2002 using a 1-m2 quadrat within which the substrate was dug to a depth of 30 cm and sieved through a 1-mm mesh. A random stratified sampling strategy was used in which a first stage sampling area of 100 m2 within
the habitat was chosen randomly, and within this first
stage area 10 random positions were chosen for placing
the quadrats (second stage sampling units) on each
sampling occasion. The survey of Mytella guyanensis
took place in the mangrove at Canela Island between
September 2001 and August 2002. Random stratified
sampling of the mussel bed, which has a total area of
7500 m2, was carried out using a 1-m2 quadrat. Without
disturbing the sediment, a total of 50 quadrats
were visually inspected for the presence of M. guyanensis
each month. The survey of shipworm species
took place in an area measuring 100 m by 50 m delimited
at the beach and mangrove at Canela Island,
and at the Furo Grande mangrove (00°50’ S, 46°38’
W) between August and October 2002. Each area was
divided into 10 adjacent, parallel transects measuring
10 by 50 m. On each sampling occasion, a single
transect within each area was chosen randomly and
searched for logs. All logs measuring between 18 and
23 cm in circumference were marked and numbered
in situ and of these 10 were randomly selected for
examination. The selected logs were cut so as to measure
30 cm in length. The logs were carefully opened
and shipworms removed with a forceps. Density was
expressed as the number of individuals per cm3.