Marine invertebrates at various trophic levels point at an important role in many ecosystems. Echinoderms (Greek for "spiny skin") is a phylum which contains only marine invertebrates. The adults are recognizable by their radial symmetry (usually five-point) and include starfish, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles). Echinoderms are unique among
Spiny lobster lobster's antennae against a "file". The noise is produced by frictional vibrations - sticking and slipping, similar to rubber materials sliding against hard surfaces. While a number of insects use frictional vibration mechanisms to generate sound, this particular acoustic mechanism is unique in the animal kingdom. Significantly, the system does not rely on the hardness of the exoskeleton, as many other arthropod sounds do, meaning that the spiny lobsters can continue to produce the deterrent noises even in the period following a moult when they are most vulnerable. The stridulating organ is present in all but three genera in the