One of the organisms I had the opportunity to observe in my Embryology class was the small sea star Leptasterias hexactis. As the species’ name suggests it has six arms, rather than five, as is common for asteroids. Even more interesting is how Leptasterias reproduces. Typically, sea stars spawn by raising themselves up on their arms and releasing hundreds of thousands of small eggs into the surrounding water from the gonopores located on the aboral surface, which is the surface you would see if you were looking at a sea star clinging to a rock. Leptasterias, on the other hand, have their gonopores on the oral surface, which is the side that contains the mouth, and what most people would consider the “bottom” of the sea star. The female Leptasterias produces a relatively small number of eggs (a few hundred) and broods her eggs, which is to say instead of releasing her eggs into the water, she holds them under the oral surface.
The top picture shows the oral view of an individual with a brood (the large yellowish mass in the center). The eggs are large (almost a millimeter in diameter) and yolky and the development is lecithotrophic (i.e. non-feeding, or yolk dependent). After about 40 days the juveniles have tube feet, and the entire brooding period is about two months (Chia 1966). The mother does not feed during brooding. The two bottom pictures show an advanced juvenile of Leptasterias, which has been living independently for a week or two. The middle picture shows the oral side (mouth in middle) and the bottom picture shows the aboral side. The protrusions ending with little bulb-like suckers seen under the central disk and arms are the tube feet, which the juvenile uses for locomotion.
Chia, F.S. 1966. Brooding behavior of a six-rayed starfish, Leptasterias hexactis. Biological Bulletin 130:304-315.