sedimentary bottoms at first glance appear to be fairly barren areas, as they lack the stability to support many of the spectacular reef based species, and the variety of large organisms is relatively low. the sediment may be moved around by water action, to a greater or lesser degree depending on weather conditions and exposure of the area. this means that sessile organisms must be specifically adapted to areas of relatively loose substrate to thrive in them, and the variety of species found on an unconsolidated sedimentary bottom will depend on all these factors. sedimentary bottoms have one important compensation for their instability, animals can burrow into the sediment and move up and down within its layers, which can provide feeding opportunities and protection from predation. other species can dig themselves holes in which to shelter, or may feed by filtering water drawn through the tunnel, or by extending body parts adapted to this function into the water above the sand.