The ecology of macrofauna on the sandy bottom of oceanic islands in the tropics has rarely been studied. The present study was conducted at Isla del Coco, Costa Rica, a tropical oceanic island, to determine how the soft-bottom macrofauna assemblages vary along different levels of ocean exposure and their associated environmental conditions. The macrofauna was collected from five grabs at each of the 27 stations (between 3 and 75 m deep) in April 2010. The broad-scale pattern consists of a gradual increase in the complexity of assemblages (taxonomical or feeding guilds in the sediment) from the inner part to the outside of the bays. The leading determinant of this pattern was the interplay between ocean exposure and sediment variation with depth that produces habitat heterogeneity. The main taxa responsible for differences between ocean exposure zones were the polychaetes Westheidesyllis heterocirrata and Magelona californica, the filter-feeding bivalve Gouldia californica, and the predatory isopod Eurydice caudata. Coarse bottoms had more mobile predators and suspension feeders, whereas passive suspension feeders or subsurface deposit feeders dominated fine sands. The deeper stations were dominated by organisms feeding actively on particles from the water or on the sediment surface. Moreover, more feeding guilds were found in deeper waters. Additionally, at the small-scale view (grabs within each station), macrofauna composition evidences a mosaic pattern, mainly at the inner and middle of the bays, caused by biological or environmental disturbances.
