Actinia tenebrosa is a species of the genus Actinia belonging to the family Actiniidae under the order Actiniaria. They belong to the same order as corals, Anthozoa under the phylum Cnidaria. It is a mid intertidal species found along the tropical coasts of Australia and New Zealand. They are solitary polyps that lack the medusa stage as seen in some other Cnidarians. They are viviparous and practice both sexual and asexual reproduction where the former is thought to be the process where the first poly arose, followed by asexual reproduction as the species expands its range. They are opportunistic feeders that display intra specific aggressive behaviors during contest for space. Consistent with other cnidarians, A. tenebrosa has similar morphology and anatomy consistent with other cnidarians with the exception of acrorhagi that are only present in some genus such as the Actiniaria. The acrorhagi are specialised offense structures used solely during aggressive interactions with other conspecifics in a bid for territory. The structure of the nematocysts in A. tenebrosa is examined in detail because of its presence unique only to the phylum Cnidaria as well as its apparent importance in prey capture, defense and offense in the species. The results of a phylogenetic study were included in an attempt to elucidate the evolutionary relatedness amongst other genus of the family Actiniidae.
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