What is the smallest animal in the world?

Answer 1

Your question has a number of difficult aspects to it, but I have asked a friend of mine to help me do some research into the answer for you and I think he has included a great number of pieces of information that you may find interesting.

Here is what I eventually decided was the most productive answer to give you. I hope it helps you out! (after all, it is 99% his answer I have included below. He generally gives me such good feedback that I try to quote him directly as much as possible.)

Here we go: An animal is:
An organized living being endowed with sensation and the power of voluntary motion, and also characterized by taking its food into an internal cavity or stomach for digestion; by giving carbonic acid to the air and taking oxygen in the process of respiration; and by increasing in motive power or active aggressive force with progress to maturity.

OR:

Any of a kingdom (Animalia) of living things including many-celled organisms and often many of the single-celled ones (as protozoans) that typically differ from plants in having cells without cellulose walls, in lacking chlorophyll and the capacity for photosynthesis, in requiring more complex food materials (as proteins), in being organized to a greater degree of complexity, and in having the capacity for spontaneous movement and rapid motor responses to stimulation" These are the animal subkingdoms, and the principal classes under them:

* Vertebrata, including Mammalia or Mammals, Aves or Birds, Reptilia, Amphibia, Pisces or Fishes, Marsipobranchiata (Craniota); and Leptocardia (Acrania).
* Tunicata, including the Thaliacea, and Ascidioidea or Ascidians.
* Articulata or Annulosa, including Insecta, Myriapoda, Malacapoda, Arachnida, Pycnogonida, Merostomata, Crustacea (Arthropoda); and Annelida, Gehyrea (Anarthropoda).
* Helminthes or Vermes, including Rotifera, Ch�tognatha, Nematoidea, Acanthocephala, Nemertina, Turbellaria, Trematoda, Cestoidea, Mesozea.
* Molluscoidea, including Brachiopoda and Bryozoa.
* Mollusca, including Cephalopoda, Gastropoda, Pteropoda, Scaphopoda, Lamellibranchiata or Acephala.
* Echinodermata, including Holothurioidea, Echinoidea, Asterioidea, Ophiuroidea, and Crinoidea.
* Coelenterata, including Anthozoa or Polyps, Ctenophora, and Hydrozoa or Acalephs.
* Spongiozoa or Porifera, including the sponges.
* Protozoa, including Infusoria and Rhizopoda.

Answer 2

Protozoa and amoebas are NOT animals, because they are unicellular. Animals are considered as being muticellular ( http://www.madsci.org/posts/archives/dec97/880417327.Zo.r.html , From Brittanica's free section, "Animals are: (kingdom Animalia), any of a group of multicellular organisms that are thought to have evolved independently from the unicellular eukaryotes." ). And also see the phylogenetic tree at http://tolweb.org/Eukaryotes/3 .

Plankton refers to living things that are unable to swim against the ocean current. The group MAY contain animals, but consist mostly of algae & protozoa ( http://dictionary.reference.com/search?q=plankton ), which are unicellular.

http://www.madsci.org/posts/archives/dec97/880417327.Zo.r.html speculates on what might be the smallest animal.

Like vicky_b said, it's harder to pinpoint the smallest animal than the largest animal. There is enormous biodiversity among the very small creatures, and more are being discovered daily. Apparently, the US Department of Energy's Newton Ask a Scientist website ( http://www.newton.dep.anl.gov/askasci/gen01/gen01561.htm ) concurs.

Placozoa may be the simplest animals ( http://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html ), so it's possible they can be the smallest.

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Answer 3

I heard there are some little mites that crawl up and down your eyelashes, but I've never
seen them.
They are smaller than half the thickness of a piece of paper.

There are more different kinds of ultra-tiny animals than there are normal big animals.
It's much easier to find the biggest animal in the world, than the smallest.

Answer 4

The smallest organism on the planet is the Potato Spindle Virus (2 X 10^ -8 meters in diameter).

Answer 5

a dust mite is the smallest animal in the world.

Answer 6

Plankton or krill are the smallest animals. Interestingly, they are the main food of the largest animal-the blue whale.

Answer 7

Ameoba is the smallest animal in the world.

Answer 8

Dust mites are 420 micrometres long. A micrometre is one one thousandth of a millimeter and there are one million micrometres in a meter. It is not to be confused with a micrometer which is used in astronomy and something different. They are arachnids.

Scientists believe they have only identified about 5% of mites and they have identified 45,000 species so there must be about 225,000 species of mites estimated to exist.

Answer 9

the bumblebee bat is the smallest mammal in the world

Answer 10

Planktons