Space size relative to prey width (Sp/Py) influences macrofaunal colonization of artificial structures

Abstract

We tested an index of habitat complexity, Sp/Py (inter-structural space size/prey body width), by deploying artificial structures on a coral reef near Dibba, United Arab Emirates. The struc- tures were polyethylene tube bundles with tube diameters of 3, 12 and 24 mm and a mixed treatment composed of all 3 tube sizes. We counted and identified mobile macrofauna that colonized the inte- rior of the tubes and divided them into 3 body width categories: small ( 12 mm). Medium and large fauna were excluded from 3 mm tubes, and large fauna were excluded from 12 mm tubes. Small fauna were significantly more abundant in 24 mm tubes compared with 3 mm tubes, with intermediate abundance in 12 mm tubes. A second experiment determined that small fauna probably responded positively to tube interior volume, or responded to differences in hydrodynamic properties between the treatments. Medium fauna were more likely to be found in 12 mm tubes that matched their own body width than in 24 mm tubes. Species richness was signifi- cantly lower in the 3 mm tubes compared with the other treatments, in part because errant poly- chaetes, gastropods, caprellid amphipods and crabs were less likely to be found in the 3 mm tubes. These results indicate that habitats with very narrow inter-structural space sizes may be 'too com- plex' and may exclude certain fauna, resulting in decreased species richness. Also, some fauna may prefer habitats with intermediate space sizes that match their own body width.