Sea Stars, Sea Urchins, and Relatives (Phylum Echinodermata)
All members of this large (over 6,500 species) phylum live in marine waters. As with nearly all groups of invertebrates, there is, among the echinoderms, a great deal of variation in terms of appearance and lifestyle.
Echinoderms are unique among all animals in possessing a highly specialized water vascularization system. Canals originating from the many hundreds of tube feet transport water through a series of valves that, when opened or closed, allow the animal to move. The pressure that can be achieved using this system can be surprisingly high, allowing sea stars for example, to pry open the shells of the clams and oysters upon which they feed.
What purposes do tube feet serve?
The tube feet of the echinoderms are unique in the animal world. In addition to locomotion, they function as respiratory organs and assist in feeding and in sensory perception.
Among the other unique organs possessed by echinoderms such as sea stars and sea urchins are the tiny, pincherlike pedicellariae. Thousands of these “living forceps” are to be found all over the exoskeleton. In some species they are thought to remove sand and other debris from the animal's surface, while in others they appear to be connected to poison glands. Echinoderms are bottom dwellers that do not actually swim but are, nevertheless, fairly active in many cases. Various species obtain their food by predation, scavenging, or by filtering it from the water. The eggs and sperm are simultaneously shed into the surrounding ocean, and the wormlike larvae swim or float about for a time before maturing into the adult form.
Sea stars are among the most popular of the echinoderms in the aquarium trade. Many species are extremely colorful and grow to quite large sizes. Most sea stars are adapted to opening bivalves such as clams and oysters via the water pressure built up through the vascularization system. Their actual method of feeding is equally unique, with the stomach being everted through the mouth and into the open clam or oyster shell.
Sea urchins are also quite popular with marine aquarists, despite the fact that they possess spines that can inflict painful wounds. Sea urchins move about on tube feet in much the same manner as sea stars.
Brittle and basket stars are extremely unusual. The brittle stars have long, sinuous arms and lie about motionless until they detect the presence of food in the water. At that time the arms begin to snake about rapidly and the animals unerringly head toward the origin of the food's scent. These interesting scavengers do quite well in the aquarium when fed bits of clam, shrimp, and fish. The equally strange basket stars hold their many-branched arms out to form a type of net to snare small food items from the surrounding water.
Sea cucumbers are sessile echinoderms that are occasionally offered for sale or collected from tide pools by hobbyists. When disturbed, they have the unsettling habit of discharging their stomach through the anus. Amazingly, the stomach can be regenerated, but handle these creatures very carefully so as to avoid eliciting this bizarre form of defense in the first place.
Sea lilies look just the way their common name suggests. These filter feeders are rarely kept in aquariums. They have existed in their present form for nearly 500 million years.