Echinoderm: the name means spiny skin but we aren’t talking about porcupines, unless we rename sea urchins the porcupines of the sea. Sea urchins are the spiniest members of the marine phylum Echinodermata. Other echinoderms are starfish, brittle stars, the lesser known crinoids, and the sea cucumbers, the least spiny of the lot. They are a strange lot, with no eyes or brains, radially symmetrical, with rows of tube feet around a central mouth from which the stomach sometimes protrudes, and yet they are distant relatives of the chordates, our own phylum.
Echinoderms are a venerable group, with fossils dating back to the Cambrian Period. They were more numerous in past ages, with the Crinoids reaching their peak in the Permian, but most were wiped out in the great extinction event that occured at the end of that Period. Thirteen thousand species of Echinoderms are known from the fossil record but only about 7500 species survive today.
The best known echinoderms are the starfish, order Asteroidea. They are slow motion predators, crawling about the ‘benthos’, the bottom of the ocean, from the intertidal zone to great depths. Usually 5 legged, but sometimes more, the starfish body radiates out from a central, ventral mouth. The undersides of the legs have those strange, hydraulically operated tube feet that carry it across the seascape looking for its prey. Since they have no eyes, they aren’t really looking. Instead starfish use smell and feel to find their prey. They don’t have to move fast since mostly what they are after are even slower than they are: molluscs. A starfish finds a hapless clam, oyster or snail, covers it with its body and then uses those tube feet to pry open the mollusc. They dont have to open it very far, once there is any crack in the molluscan defenses, the starfish extrudes its stomach out through its mouth and into the body of its prey. Digestive juices squirt out and dissolve the muscle and guts and then the stomach surrounds it and digest it, leaving an empty shell, as the starfish tucks in its stomach and moves slowly on.
Brittle stars (Order Ophiuroidea) are faster and more flexible versions of the starfish, with serpent like legs. They are usually scavengers, coming out at night to find food and hiding from bigger predators in the daytime.
Feather stars (Order Crinoidea) are plankton feeders and look more flower-like than animal. Crinoids can crawl but mostly they sit on sponges or rocks and wave their feathery legs about, picking up detritus and plankton in the water, then moving the tasty bits down their legs to the central mouth.
Sea urchins (Order Echinoidea) are the herbivores of the echinoderm world. Their guts and gonads are edible but encased in a hard spiny shell that protects them from all but the most determined predators. They crawl about on their spines and graze on seaweeds and sea grasses. They dominate some benthic ecosystems, occurring in their thousands. Their sharp, brittle spines are also toxic and a good protection against predators, just like their terrestrial counterparts, the porcupines.
Last but certainly not least are the unlovely sea cucumbers (Order Holothuroidea). Visitors to the Barrier Reef see lots of cucumbers grazing around the inner reef flats. They crawl all over the sandy parts, eating the sand as they go and pooping out long sand-worms at the other end after digesting out all the micro-organisms in their sandy diet. Sea cucumbers are softer than starfish but they have those tube feet underneath for crawling around on. They are important members of the reef ecosystem, acting like marine earthworms. They look wormy too, or like big slugs, but their larval stages and organs reveal them for what they truly are: echinoderms without the spines. Like starfish, they can also evert their stomachs out of their mouths, but rather than using this as a hunting technique, they do it to discourage predators. They also spit when you pick them up.
So why do scientists think that Echinoderms are relatives of Chordates? We have to look at larval development to see. All these animals have free-living, planktonic larvae that begin as eggs, which develop into balls of cells called the blastula and then into a worm-like stage that is similar to the worm-like stage our own embryos go through after the blastula stage. In most invertebrates, the first opening or pore to develop in the blastula becomes the mouth of the adult animal. But in Echinoderms and Chordates, the first hole, the blastopore, becomes the anus. From there the majority of invertebrates, known as Protostomes, develop a dorsal gut and a ventral nerve chord. Echinoderms and Chordates, the Deuterostomes, develop a ventral gut and a dorsal nerve chord. Later the nervous chord disappears in adult Echinoderms, but in their larval stages, their similar development indicates a common ancestor with the ancestors of the the chordates. So next time you look at a starfish, say hello to a distant, very distant, cousin.