Vertical distributions of the epifauna on manganese nodules: Implications for settlement and feeding

Over 65% of the sessile organisms living on manganese nodules from the tropical North Pacific belong to taxa whose abundances are related to vertical position on the nodule. The nodule surface texture and boundary layer flow environment, two factors that also vary vertically, were investigated to determine whether they could account for the observed faunal distributions. The surface texture of nodules from the tropical Pacific is generally rough and knobby at the nodule base and smooth near the summit. Twenty‐five taxa, including 68% of the total individuals, were found in significantly different abundances between the rough and smooth surfaces of 34 nodules. These distributions may be due to larval responses to surface texture and can account for much of the observed vertical zonation of the fauna. Boundary layer flows over nodules were characterized from laboratory flume studies and field observations. In flows at the field site, it is expected that mean boundary shear stress and horizontal flux of particles (food or larvae) will increase from the nodule base to the summit, while particle contact rate and deposition will decrease. The observations that suspension feeders persist near the nodule summit and deposit feeders concentrate near the base suggest that vertical distributions of some taxa may be determined by adult feeding requirements. Individuals of most taxa are relatively scarce at the nodule base, indicating that their larvae are not colonizing where they accumulate passively. High abundances of matlike agglutinated foraminifers (of unknown feeding type) at the nodule summit may be due to larval responses to the relatively higher shear stresses.