Orientation and position of substrata have large effects on epibiotic assemblages

The orientation and position of hard substrata used to test ecological hypotheses about sessile marine plants and animals have often been based on logistical convenience. Much of our understanding of the ecology of epibiota is based on artificial habitats (particularly the undersides of floating pontoons), despite epibiotic organisms being an important and conspicuous component 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 substratum, 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 orientated differently (vertical vs horizontal undersides) also supported different assemblages. Position (reef vs pontoon) affected a broader range of taxa than did orientation, although effects of each were generally inconsistent among sites. Covers of spirorbid polychaetes, encrusting bryozoans, mussels, colonial ascidians and red filamentous algae were affected greatly by position, Tubiculous polychaetes, barnacles and species of brown and green filamentous algae were influenced by orientation. This study provides experimental evidence that pontoons are fundamentally different habitats from natural rocky reefs. The results highlight the need for caution in the use and interpretation of studies using artificial habitats when testing hypotheses about naturally occurring assemblages.