What is the smallest animnal in the world.... would an animnal be considered a bug?

Question

i am doing some research on animnals and that is one of my questions...please help..

Answer 1

Animals are defined as multi-cellular, and attaining sustenance through eating other organisms (rather than producing their own nutrition through photosynthesis like plants).

Thus, amoebae and bacteria are not animals (they are not multi-cellular), and while the tiny fish are indeed animals, they are not the smallest animals - there are many insects and other critters that are multicellular animals that are much smaller than them.

As far as the smallest multi-cellular animals, it's sort of a tie between Tardigrades (sometimes known as water bears) and Rotifers (also known as wheel animals).

Both groups are definitely animals, and can be as small as 0.1 mm long. In fact, many of them are actually smaller than amoebae (which are single-celled).

Answer 2

Here is what I eventually decided was the most productive answer to give you. I hope it helps you out!

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, Chætognatha, 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.

Here's his lookup of the above subkingdoms:

Vertebrates: The big animals

Tunicata: Small, marine, invertibrate filter feeding creatures, and potato-shaped doohickeys that attach to boat hulls. (barnacles?)

Articulata: Insects, spiders, crustaceans, extinct stuff etc.

Helminthes/Vermes: Worms and smaller marine things

Molluscoidea: Moss animals and extinct lamp shells

Mollusca: Snails, slugs, clams, squid, etc.

Echinodermata: Starfish and sea urchins

Coelenterata: Flatworms, jellyfish

Spongiozoa: Sponges, considered the most "primative" of the multi-celled animals, but yet still animals.

Protozoa: Algae, fungi stuff, but see below.

As you can see, "animal" can define a very diverse number of living beings.

From Brittanica's free section:

Until the 1970s the general view was that the protozoans were animals, and as such they were placed in the animal kingdom as the phylum Protozoa. Under this system of classification, zoologists placed the coloured flagellates in the phylum Protozoa despite their obvious plant affinities. Botanists, on the other hand, classified the same organisms as algae, which were regarded as plants. Protozoans are now regarded as a phylum or subkingdom of the kingdom Protista (sometimes called the Protoctista), which also includes the algae. The Society of Protozoologists, which periodically reviews the systematics of the group, favours the subkingdom level for the Protozoa." ... "The protozoa are a collection of single-celled eukaryotic (i.e., possessing a well-defined nucleus) organisms. As such, they are among the simplest of all living organisms. Although they comprise a subkingdom in the kingdom Protista, protozoans are not necessarily related to one another. In biological terms, they are not a natural group but simply a collection of organisms. There are more than 65,000 described species, of which over half are fossil.

Animals are: (kingdom Animalia), any of a group of multicellular organisms that are thought to have evolved independently from the unicellular eukaryotes.

Animals differ from members of the two other multicellular kingdoms, the plants (Plantae) and the fungi (Mycota), in fundamental variations in morphology and physiology.

Here is what my friend's opinion on which animal group would have to be considered: "I think protozoans would be considered the smallest animal, EXCEPT that it appears they are no longer considered animalia. I'd guess the next smallest would be from the insect kingdom (which has so many uncharted species finding the smallest would be difficult), or perhaps the sponge. That's my guess."

If I were to define "animal" I would simply and generically define it as "any living being not directly associated with the plant, mold, fungi, or algae sorta-species" and thus consider protozoans the smallest of all animals but... If you don't consider them animals, I would tend to agree that insects do have a very distinct chance of being the next best choice.

In contrast, the largest animal (ever) is believed to be the blue whale. Blue whales - ironically, eat some of the smallest living organisms in the world known as plankton.

More information on the small guys:

All animals are members of the Kingdom Animalia, also called Metazoa. This Kingdom does not contain the prokaryotes (Kingdom Mondera, includes bacteria, blue-green algae) or the protists (Kingdom Protista, includes unicellular eukaryotic organisms). All members of the Animalia are multicellular, and all are heterotrophs (that is, they rely directly or indirectly on other organisms for their nourishment). Most ingest food and digest it is an internal cavity.

Animal cells lack the rigid cell walls that characterize plant cells. The bodies of most animals (all except sponges) are made up of cells organized into tissues, each tissue specialized to some degree to perform specific functions.

In most, tissues are organized into even more specialized organs. Most animals are capable of complex and relatively rapid movement compared to plants and other organisms. Most reproduce sexually, by means of differentiated eggs and sperm. Most animals are diploid, meaning that the cells of adults contain two copies of the genetic material. The development of most animals is characterized by distinctive stages, including a zygote, formed by the product of the first few division of cells following fertilization; a blastula, which is a hollow ball of cells formed by the developing zygote; and a gastrula, which is formed when the blastula folds in on itself to form a double-walled structure with an opening to the outside, the blastopore.

Somewhere around 9 or 10 million species of animals inhabit the earth; the exact number is not known and even our estimates are very rough. Animals range in size from no more than a few cells to organisms weighing many tons, such as blue whales and giant squid. Most animals inhabit the seas, with fewer in fresh water and even fewer on land.

In my opinion, this is because water plays such a basic, yet important role in our lives, and salt water, rather than fresh water, carries many more of the needed "ingredients" for life. In the ocean, for example, a trace amount of every element can be found due to runoff from land and erosion. Someone once said that there is "gold in them there hills!" There sure is, and rain runoff and erosion are certainly taking a lot of it to our oceans since there is a measurable amount in every bit of sea water tested.

Research continues on the evolutionary relationships of the major groups of animals. For the sake of convenience, we shall follow the system outlined in Hickman (1994), but for some groups we shall incorporate the results of current research in our classification and discussion.

Mesozoa

     Phylum Mesozoa

Parazoa

     Phylum Porifera

     Phylum Placozoa

Eumetazoa

Radiata

          Phylum Cnidaria

          Phylum Ctenophora

Bilateria

          Protostomia

               Acoelomates

                    Phylum Platyhelminthes

                    Phylum Nemertea

               Pseudocoelomates

                    Phylum Rotifera

                    Phylum Gastrotricha

                    Phylum Kinorhyncha

                    Phylum Gnathostomulida

                    Phylum Nematoda

                    Phylum Priapulida  

                    Phylum Nematomorpha

                    Phylum Acanthocephala

                    Phylum Entoprocta

                    Phylum Loricifera

               Eucoelomates

                    Phylum Mollusca

                    Phylum Annelida

                    Phylum Arthropoda  ***

                    Phylum Echiurida

                    Phylum Sipuncula

                    Phylum Tardigrada

                    Phylum Pentastomida

                    Phylum Onychophora

                    Phylum Pogonophora

          Deuterostomia

               Phylum Phoronida

               Phylum Ectoprocta

               Phylum Brachiopoda

               Phylum Echinodermata

               Phylum Chaetognatha

               Phylum Hemichordata

               Phylum Chordata

*** Spiders and insects. I still tend to think the insect is a good shot, although perhaps something from the flatworm family is smaller. Mites, and ticks are also part of Anthropoda, subphylum Chelicerata, class Arachnida (along with spiders).

Interested in seeing the Guiness Book of Insect Records?: Guiness Book of Insect Records

No smallest size, though. However, they have the smallest EGGS, and if an egg is considered a "whole" animal (since it contains an entire animal at it's most advanced stage... A stretch, but possible if you want to really generalize the issue), then the winner is: