Mystery of the Mud Star: Ctenodiscus REVEALED!!

So, everyone knows that various animals (and other living things) have adaptations to where they live. And certainly, many if not ALL of the various things I describe on the Echinoblog are an ongoing story of how these unique animals have evolved to deal with the environments they live in.

What's always curious is that echinoderms evolve in strange ways, which is what makes them so cool! Sometimes, there are such clear cases of HOW critters have such specific adaptations to living in certain habitats that its pretty amazing.

One case in point? The "Mud Star" Ctenodiscus crispatus ! (along with sister species C. australis and C. procurator)
Ctenodiscus lives primarily in MUD , mostly on continental shelves around the world. It is WIDELY distributed in the Arctic and subArctic as well as the subAntarctic and cold-water regions adjacent to these areas.

As you can see, where they occur, they are present in huge abundance. Single collections have resulted in hundreds of individuals!

(image from NOAA Ocean Explorer 2005)

That means that it lives in cold water regions almost ALL over the world. If that's not an evolutionary sucess story, then I don't know what is.

So, let's break it down...
Two of the important things that starfish need: They need FOOD. And they need to BREATHE.

The material for this blog is taken from work by Malcolm Shick here and here.

1. Ctenodiscus is an non-selective deposit feeder.
Simply put, they open their mouth and put ALL the mud in it. They get all sorts of sediment with good organic goo out of it, as its slowly digested and processed.
The animal is often found GORGED with mud filling the entire disk.
2. The Real Story: HOW Ctenodiscus BREATHES.
So, here's where the COOL stuff starts. HOW does a starfish BURIED in mud get WATER so that it can exchange gases over the body surface and BREATHE??

It does THIS:

(Fig. 10 from Shick et al. 1981)

Here's what happens: Ctenodiscus is buried near the surface of the muddy bottom, but water flows in and out of the various channels around the surface of the animal!!

How does the water current flow???

The body surface of starfish (and other echinoderms) are COVERED by a very thin ciliated skin. Cilia are tiny, microscopic, hair-like structures that cover the surface and when they beat, they can create a current flow over the body.

So, that is what happens normally in ALL echinoderms. How does THIS species, which lives buried in mud, create such a STRONG current that it actually can circulate water over its body surface??

It has TURBINES!!!
What? Where?? HERE.
See these horizontal and lateral channels between the spines? Those are special structures known as CRIBIFORM ORGANS. These channels lead into deeper channels that are further divided into pockets-ALL covered internally with cilia.

(Fig. 10 from Shick et al. 1981)

These cilia beat, creating a current that flows through these channels around the animal and over the papulae (i.e., the gills) so that Ctenodiscus is properly aerated.

Its like a bunch of canals ALL over the surface of the animal and water flows through every one of them. Acting like sort of a fluid insulation that brings oxygen to the surface of the starfish so it can respire.

BREATHING in MUD? But what happens when there's NO OXYGEN??
Where you've got mud, you sometimes get a condition called hypoxia. That's when the oxygen just doesn't reach a certain level of the sediment and you see a build up of a toxic chemical called hydrogen sulfide. That' that level of mud that turns black.
Ctenodiscus has a high tolerance to hypoxia, possibly one of the best known for any echinoderm (but there isn't much known about other taxa).
What happens? The mud star has what's called an anal cone aka an epiproctal cone (see above).

(Fig. 10 from Shick et al. 1981)

The cone is basically an outpocket of the body. Its extended UPWARDS through the sediment.

(Fig. 2 from Shick 1976)

Observations of Ctenodiscus under hypoxic conditions led to the illustration above. Basically, its thought that the cone gets more enlarged as hypoxia and hydrogen sulfide increases.

The extension of the cone extends through the surface, with the tip at the surface. For your typical 6.0 cm diameter animal, these animals can have a cone that can attain 3 to 4 cm and extend 2 to 3 cm above the mud. It can leave this extended for over an hour. As the picture suggests, it can move around and push through sediment as the mud shifts, and etc. So, it can move around.

This also serves to make the top surface of the animal thinner, allowing easier gas exchange and opening up a channel to the surface water above the sediment surface!!

Yowza.

MYSTERIES of the mud stars revealed!! Success comes in many forms!