With more than 1,800 species of starfish, only one can claim the title of smallest: the paddle-spined seastar (Patiriella parvivipara). It shares the essential characteristics of other starfish, including arms radiating from a central body and locomotion using tube feet, just on a tinier scale.
The paddle-spined seastar, when fully mature, is about the size of one of your fingernails. They measure about 5 millimeters, which is less than 1/2 inch. If you're interested in checking out their tiny tube feet, you'll need a microscope to watch them ripple as the starfish moves water through its vascular system. In comparison, the world's largest starfish is the Midgardia xandros species, which can reach 4 1/2 feet across.
The classic starfish has five arms, but the paddle-spined seastar is a bit different: She normally has six. Although this isn't completely unheard of in the starfish world, it is unusual in the smaller starfish varieties. According to Australia's Museum Victoria, the first paddle-spined seastar photographed had only five arms, which occurs when there's a problem during reproduction.
Although starfish in general are able to regenerate detached arms and some detached arms can grow into new starfish, most species reproduce by spreading and fertilizing eggs. The paddle-spined seastar is different -- it appears to reproduce solely by splitting itself in half. A mature paddle-spined seastar will split, typically right down the middle, and each half will grow three more arms until both have six fully functioning arms. Those two seastars can then split to create two more.
The paddle-spined seastar has been found in only one area along the coast of southern Australia. Researchers at the Museum Victoria first found the species off the Port Phillip Bay in 2007, tucked in with the algae and sponges. Since the initial discovery, researches have found the paddle-spined seastar in other areas along Australia's Victorian coastline.