Platyhelminth fauna of the Island of Sylt: a meta-analysis of distributional patterns and description of 19 new species

Abstract

Over the past 70 years, numerous authors studied the platyhelminth fauna near the island of Sylt in the eastern North Sea, each with a specific focus on varying aspects of ecology, morphology or systematics, and most of them dealing with a single habitat type. These data are combined with new data to get a comprehensive view of species richness, the distribution of individual species across tidal levels and sediment types and the resulting communities. So far, 398 platyhelminth species have been recorded around Sylt island, plus a still growing number of unidentified or undescribed species, in particular from subtidal waters. The distribution over habitat types (as defined by sediment types and tidal level) is described for the known species. Neighbouring habitat types overlap in species composition, and faunal composition changes continuously over environmental gradients. The number of species recorded per habitat type mirrors the sampling intensity and varied between some 20 species in poorly studied habitats and 150 in the well-studied intertidal sand flats. Corrected for variations in sampling intensity, platyhelminth species richness showed no significant differences between sediment types and only moderate variation over tidal levels. On a larger spatial scale, three faunal assemblages can be differentiated: the supratidal harbours brackish-water species (mesohaline in the upper and polyhaline in the lower supratidal), the intertidal polyhaline-marine species with a wide tolerance of variations in physical factors and the subtidal marine (± stenohaline) species. With respect to sediment type, mud and sand dwellers are well separated in the supra- and subtidal belt but less in the intertidal. Provided these rules are general, I conclude platyhelminth species richness in a given section of coastline mainly depends on the ranges of environmental factors covered. Nineteen new species encountered during this study are described.