What is that weird offset circle (or patch?) that you see on the surface of a starfish?? (see the pic below in the red circle!)
This funny-looking, little offset patch/circle/whatever is called the madreporite or sieve plate. As you can see, it is composed of a bunch of grooves which are each, in turn part of a porous calcium carbonate plate covered with pores. The word "madreporite" is named for its resemblance to a genus of hard coral called Madrepora whose surface is covered by numerous small openings. As we'll see there are numerous small openings (i.e., pores) that cover the surface of the madreporite as well.
So, What does the Madreporite DO anyway??
So ALL sea stars have a madreporite. We can see how it varies among different taxa, including something as different as the asteriid Asterias above and the goniasterid Eknomiaster below...
Not only do all sea stars have a madreporite, but ALL echinoderms do as well. In some sea stars there are even multiple madreporites!Sea stars (and other echinoderms) have a unique circulatory system that basically processes and circulates sea water throughout the animal's body. Imagine if your body was basically just OPEN to sea water and funneled through a bunch of tubes that make your tube feet and stomach work.
That's basically how starfish (and their relatives) get around and hold themselves up. Other than the calcium carbonate endoskeleton in echinoderms-the only other thing that holds them up?? Water pressure.
But even an open system like this needs to have an intake valve to process water for the animal's basic bodily functions-like movement and internal circulation and etc... That is what the madreporite is for..
Based on a series of really neat experiments using small fluorescent microbeads, much of the fundamental physiology of how the madreporite and its relationship to the Water Vascular system was worked on in a series of papers by a physiologist named John C. Ferguson including this one. and this one.
He basically introduced the dye/microbeads into the water around each subject starfish and watched the green show up wherever the water took it!
Based on the water flow, the whole thin works like this:
One important thing to remember is that ALL of this is covered by a thin skin or epithelium, which is in turn covered by countless numbers of tiny, beating hair-like structures known as cilia. These are constantly beating and moving, thus creating a current (yes-INSIDE the animal's body cavity) !!!
Water enters into the madreporite via the beating of the cilia into the various pores and enters channels into the body. Water moves along the canal connected to the madreporite and enters into the circular or RADIAL canal present in the disk. Water is then further propelled throughout the body to the water vascular system, which supports the tube feet, permitting the animal some movement!
Now, water can get into the body through a variety of different openings (and all of these help support the body cavity), but SOME of the water that enters through the madreporite is actually PROCESSED by small little pockets on the main ring called Tiedemann's bodies.
It turns out that small amoeba like cells and other substances are secreted into the seawater as it processes through the Tiedemann's bodies and throughout several different pockets on the central water vascular system ring. The amoebae like cells proceed throughout the body to take care of various body functions.
All the time, the tiny cilia (hair-like structures) are just beating away..moving water currents throughout the body.
Water continues to move and some of it eventually makes it into the radial canals (which branch off the central ring) and into the arms and tube feet. So, water processing through the body does eventually reach the "bottom" where the tube feet emerge from the skeleton!! This ultimately gives the support to move and do stuff...
So, wait. ALL starfish have these? Even the weird ones???
Here's where I add some of that magic Echinoblog madreporite spice! For some reason, people always assume that madreporites are the same in every species. Not so. Here, for example, is a specimen of the large, multi-armed Crown-of-Thorns-Starfish, Acanthaster planci...
What's interesting about that?? Multiple arms. Multiple MADREPORITES!!! (see red circles)
When you think about it. This makes a lot of sense. You're bigger. You've got MORE tube feet going on. So, naturally, you would need MORE water and etc. to be driving those 10 additional arms!! You can also find multiple madreporites on big ass things like Pycnopodia helianthoides-the sunflower star..
Here, we have a deep-sea starfish (Dytaster grandis) which buries itself in mud or some kind of unconsolidated sediment. The madreporite is huge and covered by spiny platform shaped plates that may serve to keep mud from clogging up the pores..
But studies of madreporite function in these animals isn't really well known.... but sometimes, it does seem to have some functional relationship..
Now, don't forget that most other living Echinoderms have a madreporite!! (or some comparable parallel structure-crinoids don't technically have a madreporite)
Here for example is an ophiuroid (aka a brittle star). Ophiuroids form a separate class of echinoderms, but are closely related to sea stars. Their madreporites are found on the ORAL surface near the mouth... (as opposed to the aboral or top surface in proper sea stars/starfish).
Madreporites on Sea Urchins are located on the aboral side (i.e., the top)
whereas the madreporites in sea cucumbers are actually INTERNAL..