Where does chitin occur naturally?

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Chitin is one of the most abundant biopolymers in the biosphere. An estimated 1011 tons of chitin exist, and the annual natural production of chitin is estimated at 1010 tons. Chitin, thus, is one of the richest renewable resources. However, there are no reliable estimates so that the often read statement of chitin being "the second most abundant biopolymer after cellulose" is nothing but a fairy tale told about chitin and chitosan. Chitin is present in fungi and throughout the invertebrates all the way down to the sponges, mostly in the form of α-chitin, except for in the endoskeleton of mollusks such as squid where it occurs as β-chitin. It is absent in bacteria and archaea, and also does not occur in plants and vertebrates such as humans. However, some algae are also known to produce α- or β-chitin. And recently, chitin has also been reported to occur in the scales of certain marine fish species where it seems to be complexed with collagen.