1 INTRODUCTION The abyssal plains, the deep ocean seafloor between 3000 and 6000 m water depth, is the largest habitat in the world, accounting for some 50% of the Earth's surface (see Bruun, 1956; Gage & Tyler, 1991; Glover & Smith, 2003; Radziejewska et al., 2003; Ramirez-Llodra et al., 2010; Zenkevich & Birstein, 1957).