“Those things don’t move, do they?” This is one of the most common questions I hear at the Seattle Aquarium from guests crowded around the intertidal touch tanks. The object to their subject is invariably the humble echinoderm. “Yes,” I inform them, “they certainly do move.” ” But HOW?” Ah, now we have hit upon the heart of the matter, or rather, the foot. How DO they move? Surely the seastar, sea urchin and cucumber are simply subject to the push and pull of the tides, going where the water flows without a care for how they get there. Well, anyone who has ever tried to pull a Pisaster off the rocks will tell you otherwise. They know that these creatures are not only capable of moving, but many are capable of NOT moving, clinging to surfaces with a kung-fu grip. So what is the secret to their pedal prowess? Well, the mechanism is one which man has employed in his endeavors since ancient times. The answer simply put, is hydraulics.
So far as we know, man invented the hydraulic pump sometime around the 3rd century BCE. And the echinoderm? Well, they have been employing hydraulic power since the Cambrian period, roughly 525 million years ago. Some might say they beat us to the technological punch. So how exactly does this unlikely water vascular system (as it’s called) actually work? The answer will vary by species but there are relative similarities between all members of Echinodermata which are ever present…and amazing.
The basic structure of this incredible system consists of a few ‘simple’ parts. Firstly, a madreporite (also called hydropore), which acts as a basic incurrent/excurrent and pressure regulatory portion for the water vascular system. It functions to equalize pressure of the system with the ambient water pressure, much in the same way SCUBA equipment adjusts air pressure to match depth. In some echinoderms (seastars & urchins) this madreporite is located on the exterior of the animal, but in the sea cucumber, this pore is internal and is able to regulate pressure via the compression of soft tissues. Next we come to the stone canal, a short tube which leads from the madreporite to the ring canal, the hub of water distribution for the rest of the body. Attached to this ring canal are a number of radial canal, usually numbering five, reflecting an echinoderms pentaradial symmetry. Also attached to the ring canal you will often find polian vesicles. These vesicles are blind sacs which hold excess water under pressure and may be utilized to add more fluid to the system if it is ever needed. Finally, we get to the crux of the matter, the all-important tube feet! The tube feet are fleshy stalks which may or may not have suction cup-like ends and are used to grasp, pull, hold, move and otherwise manipulate the environment and their position within that environment. Muscular and/or elastic canals act to operate these structures, and some may be associated with bulbs and ampullae that are capable of activating tube feet by local water pressure. An ingenious system for locomotion if ever there was one. Amazing!
Lucky for me, most guests at the Aquarium prefer demonstrations to wordy descriptions. So in answer to their question I will simply find the nearest seastar, and gently flip it on its ‘back’. “Watch.”