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!

Is it Love? Mutualism in Leather Stars (Dermasterias) and Scale Worms (Arctonoe)!!

(this image from Pt. Lobos.com!)

So, with all this talk of Invertebrate Blog War, I thought I would veer away from all of that and instead talk about the more positive relationships that are observed between two different phyla of invertebrates.

This is based on a paper by Wagner et al. 1979 that details the relationship between two species that are found all along the west (Pacific) Coast of North America.

One of them is this species, a sea star, Dermasterias imbricata, aka the Leather Star or the Garlic Star.

(this image from Pt. Lobos.com!)

AND along with it is a worm. A member of the Polynoidae called Arctonoe, species- A. vittata.

(these images from the WallaWalla invertebrates site)

This worm lives in the tube foot grooves of Dermasterias. They basically take residence in the spaces between the tube feet and make themselves at home in all of the open space.

(this image from Pt. Lobos.com!)

This relationship was observed by many of the early Pacific coast naturalists and was labelled commensalism, that is two species that lived together but neither species had either loss or gain. Kind of a "roommates that pass in the night" kind of arrangement.

Here's a cool video that shows the worm ON the underside of the leather star and in/around the animal's tube foot groove.


These worms are commensal in MANY other marine invertebrates. That includes this limpet (in the red circle)

as well as other species of sea stars, and the giant gumboot chiton that lives on the west coast of North America...

These worms have apparently been shown to be chemically attracted to their hosts.

This doesn't really come as a surprise. If they live in the "house" that one of these species provides, then they should know how to find it.

But how far gone is the relationship between this worm and its host???

Wagner et al. wanted to know the extent to which Dermasterias (the starfish) were attracted to the worms in a "Y-shaped" aquatic maze. It was basically given a choice of the worm in one arm (A) versus control (B)..

Which arm of the maze would Dermasterias choose?

The worms have been shown to move toward their sea star host, but in the total number of trials... Amazingly in 16/20 trials, the sea stars also moved towards the WORM!

They also changed variables. They switched out the commensal worm with a free-living worm, other food items, the sea anemone Anthopleura. And even more amazing??
The sea stars PREFER the commensal OVER its favorite food item!

Another curious dynamic?? The stars will take the worms REGARDLESS of which host it lived in! They removed one from a limpet, placed it into the maze, and voila! The sea star STILL likes the worm !
So, its not so much a commensal relationship but a MUTUAL one!! Both of them get something out of it!

So...Why? What makes the starfish WANT a worm living in its tube foot groove and in/among its parts?

Wagner et al. speculate that this is tied to the worm feeding on either mucus, detritus or other prey.

Curiously, they notice that the worms might be hanging out on starfish that LACK pedicellariae, which are pincers or clamp-like stuctures that starfish use to remove surface detritus/defence etc.
As an end note, they caution that the experimental results aren't necessarily an indication of what is "real" in the wild. So, one hopes that someday, someone will pick up this study and follow up on these curious relationships, not to mention the other species that harbor these worms!

A short video about how Sea Urchins use their whole bodies as compound eyes. I wrote about it here.. But this Cal Academy Video stars my colleague Rich Mooi expounding on sea urchin goodness!! Go check it out!!