Take a close look at the photo above. What kind of animal do you think this is? [The photo comes from here, but don't look there just yet, because that would be cheating.]
Some of the more observant among you may have noticed the five rays visible on the animal, and so you would have correctly decided that this is an echinoderm, seen from the underside. Echinoderms are the phylum of marine animals that includes crinoids (sea lilies and feather stars), asteroids (sea stars or starfish), ophiuroids (brittle stars), echinoids (sea urchins) and holothuroids (sea cucumbers). The five rays are the ambulacra - furrows lined with the tube feet that the echinoderm uses for walking on, or for passing food particles to the central mouth. Before I reveal exactly what kind of echinoderm this is, though, I'll show you another photo of the same specimen (from the same site) seen from the side:
By now, it should be pretty obvious which of the five living classes of echinoderms this is. So if you guessed "starfish"* - you're absolutely right. This specimen is, in fact, the type specimen of Podosphaeraster toyoshiomaruae Fujita & Rowe, 2002. Podosphaeraster is an extremely unusual asteroid known from the western Pacific and north-east Atlantic that has abandoned the typical star-shape of most members of its class, and adopted a near-spherical form much more similar to that of an echinoid. If you were to look closely at the specimen, you would be able to see a difference from a typical echinoid in that the ambulacral furrows only go halfway up the side of the sphere, rather than all the way up as in echinoids.
*Kevin Zelnio is going to kill me for calling it a starfish instead of a sea star. Tough.
The way in which Podosphaeraster has evolved its unusual form is relatively simple. The development of the plates that normally form the dorsal (aboral) surface of the flattened star has been greatly reduced relative to those that form the ventral (oral) surface. The reasons why this unusual morphology has evolved in Podosphaeraster, however, are unknown. Though five species have been described to date, specimens of Podosphaeraster are few and far between. All species are small (the largest specimens are little over a centimetre in diameter) and there is evidence that they live in habitats that are not conducive to easy collecting - among sponges or rocky ground in depths of 85 - 615 m. It may be adapted to living in cracks or crevices in these habitats.
For all its unusualness, Podosphaeraster is not unique. A fossil family of asteroids, the Sphaerasteridae, also developed a similar globose form by the reduction of the aboral surface. Also, Smith (1997) suggested that echinoids could have also evolved from a star-like ancestor in just this way. If true, what might seem an interesting but inconsequential oddity in the asteroid world could actually be very significant in understanding how another of the major modern animal groups came into being.
Fujita, T., & F. W. E. Rowe. 2002. Podosphaerasteridae fam. nov. (Echinodermata: Asteroidea: Valvatida), with a new species, Podosphaeraster toyoshiomaruae, from southern Japan. Species Diversity 7: 317-332.
Smith, A. B. 1997. Echinoderm larvae and phylogeny. Annual Review of Ecology and Systematics 28: 219-241.