Sponge and mollusk associations in a benthic filter-feeding assemblage in the middle and lower Xingu River, Brazil

During a pilot survey, sponges and mollusks were sampled from the bottom of the middle Xingu River (rapids) and lower Xingu River (ria) via hand-picking in shallows and trawling and surface-supplied dives in deeper waters. The survey revealed a benthic community composed of eight sponge species in four genera and three families, and added six species to the Xingu Basin fauna. Sponges exhibited distinct variation in body shape corresponding to local environmental conditions such as water depth, current velocity and available substrates. Sponges inhabiting rocky bottoms in swift currents (rapids) typically formed crusts. Those in the deeper, calmer waters of the ria attained massive and elaborate forms while attached to infaunal bivalves. Oncosclera navicella and Drulia cristata exhibited crusts and massive forms as adaptations to rapids and ria, respectively. In the middle to lower Xingu, sponges encrusted the shells of eleven species of bivalves (seven infaunal, three epifaunal, and one infaunal/epifaunal) and one infaunal/epifaunal gastropod. Bivalves provide key substrates for supporting sponges above the sand, mud and detritus of the Xingu ria. Potential impacts of the Belo Monte Dam complex on the sponge fauna of the middle Xingu rapids are discussed.