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.
Description
Despite a superficial resemblance to plants, air ferns are actually animal skeletons or shells of marinehydroids of the classHydrozoa, phylumCnidaria. The dried colonies are often dyed green, but the coloring will dissolve when soaked in water. Undyed ferns are sometimes labeled as "Neptune plants".
Colony of Sertularia argentea used in an aquarium
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.
Sometimes dried bryozoa are sold as "air ferns."[1]
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