Willow Moss

F. antipyretica has branched, trailing stems that are triangular in cross-section and may be as long as 60 cm (24 in). The leaves are quite stiff and are arranged in three overlapping rows. Each leaf is lance-shaped or egg-shaped, with a keel and a sharp point, some 4 to 9 mm (0.16 to 0.35 in) long. There are no flowers but minute spores are sometimes produced in smooth sporangia (capsules) between 2 and 2.6 mm (0.08 and 0.10 in) long.

F. antipyretica is found in Europe, Asia and parts of Africa. It grows attached to submerged rocks in fast-flowing water. It also occurs attached to the substrate in lakes and as floating masses in still water, and may be cast up on beaches at the waterside. It thrives in shady positions and prefers acidic water, with a pH of around 8.4 being the maximum tolerated. It is replaced in highly acid mountain and moorland streams in the UK by Fontinalis squamosa.

F. antipyretica grows in large clumps and mats and provides refuge for fish eggs and fry. Numerous invertebrates shelter among the fronds; Chironomid larvae hide in the bases of the leaves and mayfly, caddisfly and stonefly larvae cling to the fronds, and in fast-flowing water black fly larvae are often present. Diatoms and other microscopic algae grow epiphytically on the fronds.

F. antipyretica was traditionally used in cool-water aquaria, but in tropical aquaria the similarly structured "Java moss" ( a Vesicularia species) takes its place.