With an estimated 3-30 million species of animals, this planet is teeming with biodiversity. Much of this diversity is represented by arthropods which have not even been studied yet.
Out of all the animals we do know about, the range of characteristics are impressive. But it is the extremes in the animal world that grab our imaginations and make us interested in knowing more about the life on our planet. This article will go over a few of the most fascinating extremes in the animal world.
Of all extremes in the animal world, the most obvious is size. The largest living animal — possibly the largest that ever lived — is the Blue Whale, with one individual weighing in at 190 tonnes (209 tons) with a length of 30 m (100 ft). Others have been measured at 33.3 m (110 ft) in length, but not weighed. This makes the longest Blue Whale about as long as a ten-story building.
As for the smallest animal, there are numerous microscopic animals that only consist of a hundred or so cells, including rotifers, gastrotrichs, and the smallest nematodes. These simple animals are so tiny that you can see their individual cells in a micrograph image. Of course, there are numerous unicellular organisms all around us, but because these contain only one cell, they are not classified as animals, which must be multicellular.
Among the phyla that we would consider most familiar, one of the smallest animals is the parasitic wasp Dicopomorpha echmepterygis, measuring just 0.139 mm in length, smaller than some amoeba. Its internal organs are tightly packed in the correct arrangement with barely any space left over for an internal body cavity.
Another of the interesting extremes in the animal world is durability. This record is likely held by the tardigrades, or "water bears," tiny segmented animals that superficially resemble bears because of their fat bodies and tiny grasping appendages. Tardigrades are one of very few animals capable of temporarily suspending their metabolism and going into a state called cryptobiosis. In this state, they can withstand temperatures as low as absolute zero, as high as 151°C (303°F), 1,000 times more radiation than any other animal, more than a decade without water, and pressures as low as a vacuum. Because of these characteristics, it is thought that tardigrades are the only animals that could survive being exposed for extended periods in outer space.
Another of the extremes in the animal world is not size but length. Although the Blue Whale is the largest and heaviest extant animal, it isn't the longest. That title is held by a bootlace worm, Lineus longissimus, that was measured at 55 m (180 ft). These invertebrate predators sneak along the ocean floor, capturing prey with their proboscis. They superficially resemble snakes.