The air fern (Sertularia argentea) is a dead and dried colony of hydrozoans, a species of marine animal in the family Sertulariidae related to corals and jellyfish.

Air ferns are typically dyed green and sold as a curiosity, as a decorative "indoor plant"; the same skeletons of former colonies of hydroids are sold in their natural dried state as the sea fir and Neptune plant as underwater decorations for aquariums.

Despite a superficial resemblance to plants, air ferns are actually animal skeletons or shells of marine hydroids of the class Hydrozoa, phylum Cnidaria.

The fernlike branches of S. argentea are composed of many small, chitinous chambers where individual animals once lived.

"[1] Most commercially sold air ferns are collected as a by-product by trawlers in the North Sea.