How many phyla in a kingdom?

As a naturalist - a time-honored if self-appointed title - I am most interested in animals, and this blog often focuses on new or rediscovered species. If we zoom out past the species to the big picture (past the classical names of genus, family, order, and class, not to mention newer additions like "tribe" and "species complex") the animal kingdom is divided into phyla.  How many phyla? 

Right now, the generally accepted answer is 36. Most are grouped in Mollusca, Arthopoda, and Nematoda, with us chordates coming in later at about 60,000 species.  Finding a new phylum is a career-making event for a scientist.

Not surprisingly, the recent additions tend to be small animals. The last one proposed for a large animal - Vestimentifara for the giant tube worms in the genus Riftia - was eventually joined with other species from the old phylum Pogonophora to make up the family called the Siboglinidae in the phylum Annelida (the annelid worms, which include, among other things, the earthworms). 

Little ones, though, have turned up, and some of their stories are interesting.

In 1995, Cycliophora was added.  The species involved in this case, Symbion pandora, passed this imposing test without much difficulty.  Symbion is about the size of the dot on the letter i.  It was discovered three decades ago on the mouthparts of a lobster, but the initial catalogers placed it in a new genus and left it at that.  A reexamination by Peter Funch and Reinhardt Kristensen of Copenhagen University showed Symbion resembled nothing else on Earth. Dr. Simon Morris of the University of Cambridge called Symbion “the zoological highlight of the decade.”

The phylum name is Greek for “carrying a small wheel” and refers to the animal’s round, cilia-fringed mouth.  Most of the press and public fascination has centered around the animal’s strange reproductive habits.  At different stages of its life, Symbion reproduces sexually by “budding off” male and female offspring, or asexually, in which case its digestive tract metamorphoses into a larva.  Even American humorist Dave Barry paid attention to this story, writing, “Zoologists, who don’t get out much, are excited over an animal that basically reproduces by pooping.”

Kristensen said there were probably countless discoveries still to be made concerning tiny marine life.  “This is only the beginning,” he said. “When we have finished, the zoological system will be turned upside down.”

Kristensen had his reasons for making such a bold statement. In 1983, he had named another new phylum, Loricifera, for miniature animals which look sort of like animated pineapples with snouts.  Loriciferans were first identified in 1974 from seafloor samples taken off the coast of France.  They burrow through gravel or sand on the floor of shallow areas of the ocean. The phylum name means "girdle wearer," in reference to the ring of scale-like structures which encircle and protect the animal.  The snouts are mouthparts which can be retracted into the body: indeed, a Loriciferan can retract its entire head.  At least ten species have been collected from depths of fifty feet to 1,500 feet off Europe and North America. If it seems odd that a phylum is erected for a single discovery - kind of like addining a new borough onto New York City because one new house was built - every phylum has to start somewhere, with something, the way a millimeter-wide flat animal that might be ancestral to sponges was cited in 1971 to create the phylum Placozoa, which in English means, guess what, "flat animal." A phylum can also be created by splitting an existing one: Acoelomorpha was split off from Platyhelminthes in 2004, I suppose for being too weird even for that collection of animals, which eat and extcete from the same hole and include the goofy-looking Platyhelminthes, the flatworms  you experimented on in school.

In October 2000, Kristensen and Funch proposed yet another phylum.  Micrognathozoa was named to house Limnognathia maerski, a miniscule creature found in the frigid fresh water of wells in Greenland.  Only 1/250th of an inch long, it nevertheless has unusual, complex jaws used to scrape algae from rocks.  The nominate species appeared to consist entirely of parthenogenic females. Must be boring. Meanwhile, scientists are still arguing over the phylum Xenacoelomorpha, another flatworm-related group proposed in 2016.

I'd go on, but my head is starting to hurt. The point of all this is that animal classification, like animals themselves, continues to evolve, and we probably have not reached the end of splitting existing phyla or discovering new one in the wild, if you call sea-bottom ooze "the wild” – which I presume it is to any animal a millimeter long.   

The world's only known placozoan, Trichoplax adhaerens (Photo credit unknown)

Some further reading

Angier, Natalie.  1995.  “Flyspeck on Lobster Lips Turns Biology on its Ear,” New York Times, December 14.

Anonymous.  1996.  “Life on Lobster Lips,” Discover, March.

Associated Press. 1995.  “Tiny animal in a class by itself,” Colorado Springs Gazette-Telegraph, December 17, p.A20.

Reuters.  2000.  “Scientists Find Completely New Animal in Greenland,” October 12.

Walker, Dave. 1996.  “A Lobster's Microscopic Friend: Symbion pandora - a new life form and a new phylum,” Microscopy UK, http://www.microscopy-uk.org.uk/mag/articles/pandora.html.