|MadSci Network: Zoology|
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.
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:
Here's his lookup of the above subkingdoms:
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 Zenillia pullata:
Taking into account female size, the smallest insect eggs are the microtype eggs of Tachinidae, which are usually 0.02 to 0.2 mm long but very rarely as long as 0.4 mm. The eggs of Zenillia pullata are exceptionally minute, only 0.027 by 0.02 mm. The volume of the macrotype egg of Gymnosoma sp., another tachinid, is approximately 2000 times the volume of the microtype egg of Z. pullata."
Below is a web site's information that my friend thought would be helpful in respect to the insect factor:
There are an incredible number of very small insects in the world, far
more than their are giants. Many beetles are less than one millimetre in
length, and the North american Feather-winged Beetle Nanosella fungi at
0.25mm is a serious contender for the title of smallest insect in the
world. Other insect orders which contain extremely small members are the
Diptera (True Flies) and the Collembola (Springtails). Their are many
small Hymenoptera, especially in the Superfamily Chalcidoidea, such as the
Fairy Flies, (of the family Myrmaridae) of which
So, if we only consider adults, the Megaphragma caribea is the smallest
insect. Now, all we need to know is whether the insect truly is the smallest out
of all the other classes.
As you can see, it is surely a very difficult question to answer - not
because of the lack of information regarding the answer, but because the
question itself can be taken in so many ways.
So, if we only consider adults, the Megaphragma caribea is the smallest insect. Now, all we need to know is whether the insect truly is the smallest out of all the other classes.
As you can see, it is surely a very difficult question to answer - not because of the lack of information regarding the answer, but because the question itself can be taken in so many ways.
I hope that the information that I have included above is helpful in giving you an idea as to what the answer you seek may be.
Try the links in the MadSci Library for more information on Zoology.