Distribution of the epiphytic organisms on Posidonia australis and P. sinuosa, two seagrasses with differing leaf morphology

ABSTRACT: The distribution of epiphytic algae and sessile invertebrates on the leaves of the seagrasses Posidonia australis Hooker f. and P. sinuosa Cambridge and Kuo is not random. Epiphyte load on the leaves of both species increases with increasing distance away from the basal meristem. There are approximately 3 times as many epiphytic algal species as invertebrate species, and many of these epiphytes grow at distinct locations on the leaves. Epiphytic invertebrates were found primarily on the basal sections of the leaves, whereas algae were most abundant near the leaf apex. Distribution of epiphyte load across the leaf surface was also non-random, with initial settlement of epiphyte propagules occurring at the margins of the leaves. The structure of the epiphytic community is strongly correlated with leaf age, with a greater abundance of epiphytic species occurring on the older leaves. It is clear that leaf morphology also plays a significant role in the distribution of the epiphytes. There is no apparent difference in the epiphytic community between the sides of the flat P. australis leaf whereas, in P. sinuosa, the concave side of the curved leaf supports a more diverse epiphytic community than the convex side. Similar patterns in the distribution of epiphytic organisms were observed on artificial seagrass leaves, indicating that, although a temporal component is involved, epiphyte distribution is influenced mainly by the relative position upon the leaf surface as well as leaf morphology, which affects the water flow pattern over the leaf