Effects of a hypersalinity gradient on epiphytic Corallinaceae (Rhodophyta) in Shark Bay, Western Australia

Abstract

The extent to which a natural hypersalinity gradient (37-55.permill.) in Shark Bay, Western Australia, affects those Corainaceae (Rhodophyta) epiphytic on the leaves of the seagrass Amphibolis antarctica has been examined. Four species of Fosliella and one each of Pneophyllum and Melobesia were identified. The species composition of the epiphyte community did not change along the gradient, and both the basic germination disc patterns and position of trichocytes within each taxon remained constant. With the exception of stations at 47.2 and 49.3.permill. salinity, the overall density of epiphytic coralline algae on Amphibolis leaves decreased markedly as salinity levels rose. At all stations, most individuals on the oldest Amphibolis leaves lacked germination discs. For Pneopyllum the proportion of plants without germination discs was at least 88% at salinities up to 49.3.permill. but dropped at the highest salinities. For Fosliella the proportion ranged from 74 to 95% with no relationship to salinity level. Fertility decreased with increasing salinity. The ratio of gametangial to sporangial plants rose from 1:1 at 37.3.permill. to 4:1 at 55.permill.. Development of the epiphytic coralline community within leaf clusters occurred in a consistent sequence at all stations. On the young leaves, spores settled and divided to produce germination discs and intact thalli. On older leaves, the plants became fragmented but remained sterile. Subsequently, conceptacles were produced, almost exclusively on portions lacking germination discs. With increasing salinity, percent cover of both leaves and stems decreased. Increasing salinity slows the rate of growth and development of both the host and the coralline community. This observation is consistent with mechanisms known to operate in other algae and in angiosperms.