Identification notes
Fissidens crassipes is quite a common moss of running water and is most often found in lowland eutrophic streams and rivers that are of circumneutral pH. It is more tolerant of polluted waters than our other aquatic Fissidens species. Typically it forms dark patches of small plants on stones that are intermittently submerged or splashed and on bridge masonry at around the normal waterline.
On examination with a hand-lens, one of the most striking things about this plant is its strong-looking leaf border, which is 2-4 stratose and peters out below the leaf apex. This separates it from both Fissidens rivularis and F. bryoides var. caespitans whose borders are contiguous with the nerve. Red pigments are also often present in the border and the nerve although this is not a fully reliable character and one that is shared by Fissidens rufulus, a very similar species. F. rufulus is largely confined to clean, fast-flowing upland watercourses, but microscopical measurement of leaf cells, peristome teeth and archegonium length are usually needed to separate this plant from F. crassipes. Fissidens pusillus also sometimes grows in similar habitats but its perichaetial bracts are conspicuously longer and narrower than its upper stem leaves, unlike F. crassipes, where there is little difference.
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