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.
