Orientation and position of substrata have large effects on epibiotic assemblages

The orientation and position of hard substrata used to test ecological hypo theses about sessile marine plants and animals have often been based on log istical convenience. Much of our understanding of the ecology of epibiota i s based on artificial habitats (particularly the undersides of floating pon toons), despite epibiotic organisms being an important and conspicuous comp onent of natural hard substrata (e.g, vertical surfaces of rocky reefs). We assessed the model that pontoons act as inherently different habitats from rocky reefs, independent of the size, shape, age and composition of the su bstratum, by comparing the development of epibiota on settlement panels in the 2 habitats. Panels orientated the same way on pontoons and rocky reefs were found to support different epibiotic assemblages, and panels orientate d differently (vertical vs horizontal undersides) also supported different assemblages. Position (reef vs pontoon) affected a broader range of taxa th an did orientation, although effects of each were generally inconsistent am ong sites. Covers of spirorbid polychaetes, encrusting bryozoans, mussels, colonial ascidians and red filamentous algae were affected greatly by posit ion, Tubiculous polychaetes, barnacles and species of brown and green filam entous algae were influenced by orientation. This study provides experiment al evidence that pontoons are fundamentally different habitats from natural rocky reefs. The results highlight the need for caution in the use and int erpretation of studies using artificial habitats when testing hypotheses ab out naturally occurring assemblages.