Sea stars, sea urchins and most other members of the phylum Echinodermata move along the ocean bottom using structures known as tube feet. These tube feet, called ambulacrae in science-speak, are unique to this group of animals. They are basically hollow cylinders tipped with a powerful sucker.
When I demonstrate how they work to children I like to use the analogy of a regular suction cup like the one we all use to keep our "baby on board" signs attached to our car's rear window. I'll stick a dry one to a smooth surface and it inevitably drops right onto the floor. But by wetting it first it will stick very tightly. Like the suction cup, tube feet need water to make them work.
Of course, this is a gross simplification. For the whole story you first need to know about a complex organ found only in echinoderms called a water vascular system. The wvs is basically a plumbing system within the animal's body that uses hydraulics to bring water to the feet and make them move and stick. Yes, like the internet, the water vascular system is a series of tubes.
drawing from Barnes.
Let's use this handy diagram above and see how this thing works. Notice that this is from a starfish (note the five radiating arms) but this model pretty much applies to all members of this phylum.
We'll start with the opposite end of the pipeline where the water first enters the animal's body. Water is brought in through a small button-like structure on the top called a madreporite, or "sieve plate". As the name suggests, it functions as a filter so that sand, plankton or other tiny objects don't come in and clog up the tubing. In many species the sieve plate is difficult to see because of the surrounding spines or its color is the same as the rest of the body. In my local common sea star (Asterias forbesi) it is light red. I've doctored the photo below so that you can see its placement more clearly. This sieve plate is often mistaken for an eye. The eyes are on the tips of arms.
Once the filtered water passes through the madreporite, it flows down a drain (called a stone canal because it is heavily calcified) into the ring canal, which is a circular duct that runs around the circumference of the central disk of the sea star's body. Note in the photo above that the five arms radiate from a small, circular central body. Actually, this central disk is easier to see in brittle stars:
The job of the ring canal is to evenly distribute the incoming water to the plumbing's five offshoots, called lateral canals. In sea stars each arm has its own lateral canal running down the middle of it and this in turn carries water to the short radial canals, each of which ends at a tube foot.
Ok, so that's the basic plumbing. Now for the tube foot itself. At the base of the foot, but still inside the animal's body, is a small sac called an ampulla which holds the fluid. When the ampulla contracts, water shoots down the hollow foot and extends it. The suction cup at the tip forms a seal around the substrate (rock, sand, aquarium glass, etc). Once the seal is formed the ampulla relaxes. This draws water back into the sac creating vacuum pressure which first makes the suction cup stick and then shortens the tube foot, pulling the animal forward a tiny bit. If I've failed to make the ampulla/foot/sucker interaction clear enough, then this picture should do the trick:
So the movement of a sea star involves hundreds of these ambulacrae going through this process over and over. If you watch one of these animals carefully you'll notice that its slow march across the bottom is kind of jerky looking (by that I mean not smoothly gliding along as a snail would). This is because the feet move independently of each other rather than in a coordinated, in-sinc way.
There are a few other features of the wvs, like the Tiedemann bodies (part of the immune system) and polian vesicles (which help control the water pressure in the entire organ) that I'll skip over. Also, the water vascular system is partly responsible for gas exchange, feeding, excretion and creating and transporting cells required for regeneration, but this diary is long enough as it is.
Although I will address regeneration in part. Echinoderms are masters at replacing lost body parts. If an arm of a sea star is torn off a new one will grow back with all of the internal parts, including the lost piping of the wvs. Tube feet are especially easy to grow back, and given their strength, it's lucky for the animal that they do. If you've ever pulled a sea star from a rock chances are you've torn off dozens of tube feet each time. The suction can be so strong that the foot will actually snap off at its base before the sucker releases its grip. Urchins are especially prone to losing feet in this way (its powerful grip on a rock is a primary defense against predators).
The length and number of tube feet vary from species to species. Those that live mainly on rocky bottoms will have more than those that live on mud or sand. While sea stars have only oral tube feet (found only on the bottom of the body where the mouth is), urchins have feet all over their bodies. The length of the tube feet depends on the length of the spines in the urchin's case, since for them to be able to grip anything the feet obviously have to be longer than the hard spines are. Urchins use these aboral feet on top of the body to help right themselves if overturned and also, in the case of many short-spined species, to cover themselves with rocks and shells as camouflage.
Not all tube feet are equal. Urchins have a ring of especially large feet surrounding their mouths known as buccal podia which helps them direct food into the Aristotle's lantern (the urchin's mouthparts, which I diaried about here). And on many types of sea stars each arm has a slightly longer, but suckerless, tube foot at the very tip. This is a sensing foot which detects chemicals produced by its prey and helps to determine the direction the sea star moves in as it hunts.
Another determiner of movement direction is light, which is sensed by the tiny light-sensitive eyespots on the tips of the arms, mentioned briefly above. Most species are photonegative, meaning they move away from light. This will bring them to more protected areas such as crevices between rocks, where predators are less likely to find them. Most of these species feed at night. A few species are photopositive and move toward light. In these rare cases they can actually be led around a darkened aquarium with a flashlight.
Fun Fact: This question comes up all the time so I'll preemptively answer it here. Echinoderms do not have a front or a back. On a sea star any of the five arms can lead the way. This is pretty handy in that to change direction the animal doesn't have to turn around. It simply leads with a different arm.
This diary was originally published to Daily Kos on October 20, 2006.
Other diaries in this series can be found here.