MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published July 12 Species richness, spatial distribution and colonisation pattern of algal and invertebrate epiphytes on the seagrass Amphibolis griffithii Michael A. Borowitzka, Roger C. Lethbridge, Lorna Charlton School of Biological and Environmental Sciences, Murdoch University, Murdoch 6150, Western Australia ABSTRACT: The distribution of epiphytic algae and sessile invertebrates on the seagrass Amphibolis griffithji is not random. The number of epiphyte species increases with increasing seagrass height and there are approximately twice as many epiphytic algal species as Invertebrate species. Epiphytes growing on the stems have a clear apico-basal distribution, with algae such as Haliptilon roseum, Laurencja filiforrnis and Hypnea spp. most abundant on the upper 30 '10 of the seagrass stem, while the bryozoan Celleporjna sp and the hydrozoan Thyroscyphus margjnatus are most abundant near the base of the stem. Other species of algae and invertebrates have intermediate distributions. Eplphyte biomass increases with seagrass height and the bulk occurs on the uppermost 20 cm of the tallest plants. This is mainly due to 1 or 2 algal species. Recruitment of epiphytes also follows a distinct pattern related to plant size (age), with species such as the crustose coralline algae rapidly colonising the leaves and stems of new seagrass plants. Other very early colonisers are the bryozoans Pyripora potita and Electra flagellurn on the stems, and the hydrozoan Plumularia compressa and the bryozoan Thairopora mamjllaris on the leaves. INTRODUCTION Stands of the seagrass Amphibolis antarctica (Labil- lardiere) Sonder et Ascherson ex Ascherson and A. Griffithii (J. M. Black) Den Hartog (Potamogetonales, Cymodoceaceae) are very common in the shallow waters of southern and southwestern Australia (Kirk- man & Walker 1989). A. antarctica occurs from Carnar- von, Western Australia (24"55'S) to Wilson's Promon- tory, Victoria (39"001S), whereas A. griffithii has a more restricted distribution from Kalbarri, Western Australia (27"401S) to Victor Harbour, South Australia (35"301S) (Ducker et al. 1977, Borowitzka pers. obs.). Unlike most other seagrasses, Amphibolis species have lignified, long-lived, upright stems and these stems, as well as the terminal leaf clusters, provide excellent substrata for many large epiphytic algae and invertebrates. As first reported by Hawey (1847, 1855), the epiphy- tic biota of Amphibolis spp. are very diverse. Ducker et al. (1977) recorded 116 epiphytic algae and 20 species of invertebrates epiphytic on Amphibolis, and noted that their species list was undoubtedly not comprehen- sive. Amphibolis is a major habitat for many of these species, but most are also found on adjacent algae or IQ Inter-Research/Printed in F. R. Germany grow epilithically; only a few algae (Dicranema revolu- tum (C. Ag.) J. Ag., D. cincinnalis Kraft and Metago- niolithon stelliferum (Lamarck) Weber van Bosse), and invertebrates (Stenochiton cymodocealis and Cam- panularia tincta) grow exclusively, or almost exclu- sively, on Amphibolis. The turnover and biomass pro- duction of the epiphytic biota contributes significantly to the primary production of the Amphibolis ecosystem (Ducker et al. 1977), and thus to the highly productive shallow coastal ecosystems of southern and southwest- ern Australia. The epiphytes are also an important food resource and habitat for many fish, the western rock- lobster Panulirus cygnus and many other invertebrates (Joll & Phillips 1984, Nichols et al. 1985, Howard et al. 1989, Klumpp et al. 1989). Aside from their importance as food and shelter, Amphibolis and its epiphytes also provide a 'natural' model for the study of settlement, recruitment and competition between sessile marine algae and inver- tebrates. The rather constant morphology of Amphi- bolis stems and leaves over its geographical range make it particularly suitable for studies of this type. This paper describes the spatial distribution of epiphytes on the seagrass, and the influence of plant