1 7. The polychaete Reefs of Jeram, Selangor Ribero L. and Polgar G. Institute of Biological Sciences, Institute of Ocean and Earth Sciences, University of Malaya, 50603 Kuala Lumpur, Malaysia 7.1 Introduction Biogenic reefs are defined as “solid, massive structures which are created by accumulations of organisms, usually rising from the seabed, or at least clearly forming a substantial, discrete community or habitat which is very different from the surrounding seabed” (Brown et al., 1997). Reef builder organisms include primary frame-builders, which build the reef framework (ecosystem engineers; Jones et al., 1994); and secondary frame-builders, which encrust and strengthen the main structure (Scoffin and Garrett, 1974). The former ones include scleractinians, cyanobacteria, coralligenous algae, bivalves, vermetid gastropods, and polychaetes (Wood, 1999). The reefs’ three-dimensional structure creates habitats and ecological niches for a variety of organisms, thereby considerably increasing the biodiversity and ecological complexity of soft-bottom coastal communities (Cusson and Bourget, 1997). Tubes are either formed by calcium carbonate (e.g. Serpulidae), or cemented grains of sand and shell fragments (e.g. Sabellariidae, Terebellidae). In particular, tube-building sabellariid polychaetes can form clumps of cemented vertical tubes on intertidal flats, building formations spreading over several hectares, and with a linear extension along coast of several kilometres (Dubois et al., 2002). Polychaete reefs have a worldwide distribution (Dubois et al., 2002), including North American coasts (e.g. Posey et al., 1984); Chile (Zamorano et al., 1995); western Europe (e.g. Chapman et al., 2007); Mediterranean Sea (e.g. Fornós et al., 1997); India (e.g. Achary, 1974); New Zealand (Smith and Witman, 1999); Australia (Minchinton, 1997); and even Antarctica (Ramos and San Martìn, 1999). The early literature described the polychaete reefs’ geomorphology, distribution, and their effects on sedimentological processes (e.g. Gram, 1968). More recently, biological and ecological aspects were also investigated at individual (e.g. Pawlik, 1990), community and ecosystem levels (e.g. Poloczanska et al., 2004). Aggregations of the endobenthic Lanice conchilega (Pallas, 1766), which have been referred to as polychaete reefs (Van Hoey et al., 2008),oxygenate the sediment surrounding the tubes, pumping water into and out of the bottom sediment (Forster and Graf, 1995); polychaete reefs influence sea- bottom hydrodynamics, affecting bottom roughness by increasing the heterogeneity of incoherent