Air fern

Sertularia argentea
Colony of Sertularia argentea
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Hydrozoa
Order:	Leptomedusae
Family:	Sertulariidae
Genus:	Sertularia
Species:	S. argentea
Binomial name
Sertularia argentea (Linnaeus, 1758)

Neptune plant or Air fern is a name given to a product that is composed of a species of marine animal called Sertularia argentea, also known as the "sea fir".

These so-called "ferns" are dead and dried colonies of hydrozoans, colonies of marine hydroids, class Hydrozoa, phylum Cnidaria. Hydroids are related to corals and jellyfish.

These dried hydroid colonies are commonly sold as a curiosity, as a supposedly decorative "indoor plant", or as underwater decorations for aquaria in stores. They are sometimes labeled as "Neptune plants". Despite a superficial resemblance to plants, they are actually animal skeletons or shells. The dried colonies are often dyed green, but, when soaked in water, the coloring will dissolve.

The fernlike branches of S. argentea are composed of many small, chitinous chambers where individual animals once lived. When the colony was alive, a polyp with numerous tentacles occupied each of the chambers, called hydrotheca.

Also note that sometimes dried bryozoa are sold as "air ferns."

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

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Wikipedia