Immortality

====Biologically immortal species====
{{see also|List of longest-living organisms}}

Life defined as biologically immortal is still susceptible to causes of death besides aging, including disease and trauma, as defined above. Notable immortal species include:
* ''Bacteria'' – Bacteria reproduce through [[Fission (biology)|binary fission]]. A parent bacterium splits itself into two identical daughter cells which eventually then split themselves in half. This process repeats, thus making the bacterium essentially immortal. A 2005 [[PLoS Biology]] paper<ref>{{cite web|url= http://hal.archives-ouvertes.fr/docs/00/08/01/54/PDF/stewart_Plos.pdf|title= Aging and Death in an Organism That Reproduces by Morphologically Symmetric Division}}</ref> suggests that after each division the daughter cells can be identified as the older and the younger, and the older is slightly smaller, weaker, and more likely to die than the younger.<ref>{{cite web |url=https://www.newscientist.com/channel/health/mg18524855.800-bacteria-death-reduces-human-hopes-of-immortality.html |title=Bacteria Death Reduces Human Hopes of Immortality |date=5 February 2005 |website=New Scientist magazine, issue 2485| page= 19 |access-date=2 April 2007}}</ref>
* ''[[Turritopsis dohrnii]]'', a jellyfish (phylum [[Cnidaria]], class [[Hydrozoa]], order [[Anthoathecata]]), after becoming a sexually mature adult, can transform itself back into a [[polyp (zoology)|polyp]] using the cell conversion process of [[transdifferentiation]].<ref name="CheatingDeath">{{cite web|url=http://10e.devbio.com/article.php?ch=2&id=6|title=Cheating Death: The Immortal Life Cycle of Turritopsis|last=Gilbert|first=Scott F.|date=2006|access-date=14 June 2009|url-status=dead|archive-url=https://web.archive.org/web/20151121052612/http://10e.devbio.com/article.php?ch=2&id=6|archive-date=21 November 2015}}</ref> ''Turritopsis dohrnii'' repeats this cycle, meaning that it may have an [[indefinite lifespan]].<ref name="CheatingDeath" /> Its immortal adaptation has allowed it to spread from its original habitat in the Caribbean to "all over the world".<ref name="Telegraph-immortal-jellyfish">{{cite news |url=https://www.telegraph.co.uk/earth/wildlife/4357829/Immortal-jellyfish-swarming-across-the-world.html |archive-url=https://web.archive.org/web/20090130115250/http://www.telegraph.co.uk/earth/wildlife/4357829/Immortal-jellyfish-swarming-across-the-world.html |url-status=dead |archive-date=30 January 2009 |title={{-'}}Immortal' jellyfish swarming across the world |date=30 January 2009 |publisher=[[Telegraph Media Group]] |access-date= 14 June 2009 |location=London}}</ref><ref>{{cite web|url=http://news.nationalgeographic.com/news/2009/01/090130-immortal-jellyfish-swarm.html|archive-url=https://web.archive.org/web/20090202063305/http://news.nationalgeographic.com/news/2009/01/090130-immortal-jellyfish-swarm.html|url-status=dead|archive-date=2 February 2009|title="Immortal" Jellyfish Swarm World's Oceans|website=news.nationalgeographic.com|date=29 January 2009|access-date=19 August 2017}}</ref>
* ''[[Hydra (genus)|Hydra]]'' is a [[genus]] belonging to the phylum [[Cnidaria]], the class [[Hydrozoa]] and the order [[Anthomedusae]]. They are simple fresh-water [[predatory]] animals possessing [[symmetry (biology)#Radial symmetry|radial symmetry]].<ref>Gilberson, Lance, ''Zoology Lab Manual'', 4th edition. Primis Custom Publishing. 1999.</ref><ref>{{Cite news|url=https://www.livescience.com/53178-hydra-may-live-forever.html|title=Hail the Hydra, an Animal That May Be Immortal|work=Live Science|access-date=19 August 2017}}</ref>