No sepals or petals, just the reproductive parts. The mosquito fern does other amazing things besides turn red. These two organisms have a symbiotic relationship where both partners benefit. The alga changes atmospheric nitrogen to a form that can be used by other plants, especially mosquito ferns. In return, Azolla supplies the alga with some nutrients and a protected cavity at the surface of the water in the sunlight.
Enough nitrogen is produced to share, however.
The mosquito fern, six species world-wide, has tiny overlapping leaves along a slightly elongated stem with small roots that hang below the water's surface. Its dangling roots and those of rooted duckweeds make the plant popular with those of us who raise live bearing tropical fish like guppies. The dangling roots make places for young fish to hide from hungry parents, uncles, aunts and cousins. We think the mosquito fern gets its common name because the plants can grow so densely on the waters surface that mosquitos cannot penetrate the mass of fronds to lay their eggs nor can their larvae reach the air to breathe. To find water ferns in the Rogue Valley try Railroad Slough north of Tolo just before you reach Gold Ray Dam, or the farm pond just before you reach Crowson Road as you head south of Ashland on Old US 99.