Starfish Leave footprints! aka How Starfish Tube Feet Work! (21st Century version)

Image by veronica07

I betcha you guys didn't know that starfish left FOOTPRINTS did ya??  I don't just mean impressions on the sand..I mean, put a starfish on a smooth surface, like glass or rock and it leaves you one of these...

WHAT??  How does THAT work?

Hasn't the model always been that starfish tube feet work like suction cups (such as here) ????  Why would they leave footprints on a smooth surface if they work like suction cups??


Image by Symbiotics

THAT is a good question.

To answer it, I consulted the works of Patrick Flammang and his research lab at the University of Mons in Belgium. One of many papers addressing the biology of tube feet is utilized here (written by Elise Hennebert and others in Flammang's lab)

So, it turns out that the whole tube foot adhesion process is a LOT more complicated than the whole "its a suction cup" explanation that used to be offered in text books.

It turns out that the whole chemistry and physical processes of tube foot adhesion is actually quite complex and has very useful applications in adhesives, glues and so forth.

There is an important lesson to be learned here. This field of study started largely as an academic pursuit, to learn about the mechanisms used by these animals to survive in their habitat but may soon lead to something with many practical applications..

See this article awhile back about how Gecko adhesion has led to new types of adhesives! Adhesives that work under water would presumably have a multitude of uses..

How does this whole tube foot adhesion/stick to the ground thing work??   Here's a breakdown of the process as taken from Hennebert et al. (2008) in the Journal of Structural Biology...

1.Contact! The process begins with the contact of the tube foot with the ground/substratum.

There is a special layer called the "fuzzy coat" (shown below in red) which is present on the surface of the disk epidermis making contact with the bottom. 

Echinoblog Art Dept.- Note that "Smuck" is not known to be a true starfish tube foot noise!

2. "Film" Upon contact two different kinds of special "adhesive cells" release substances onto the  substrate (aka the ground) which form an even film (shown in green below). 

Echinoblog Art Dept. Did you know that ! can be used to make everything more exciting?!!

3. "Mesh" At this point one of the adhesive cells releases a substance that starts to form a thick "meshwork structure" (shown below in yellow) in the fuzzy coat. The mesh expands and "bulks up" creating more structure within the fuzzy coat.

Echinoblog Art Dept. Coloring is fun!

Here is an SEM of what this looks like...

fig. 1B from Hennebert et al. 2008

 4. Detachment.  When the tube foot releases, a THIRD substance-a "de-attaching" substance is released from the disk epidermis. This is something that permits the tube feet to detach easily and at the animal's will.

This leaves the "fluffy layer" and the "bulked" material behind...

Here is an SEM of what this looks like after the tube feet have been pulled away

Figure 2C from Hennebert et al. 

5. Footprint And the wet parts of the "fluffy layer" dry out leaving the "bulked" material aka the starfish (or whatever) footprint..

Here is what that part which is left behind looks like in 3D via Transmission and Atomic Force Microscopy

Fig. 3 from Hennebert et al.

And here is an image of the tube foot "foot print" as seen directly off a piece of glass..

And so...
What about that classic paradigm about suckers on sea stars and other echinoderm tube feet basically being suction cups???

Well, nothing definitive has been published as yet..but Flammang's lab has published on these adhesive processes in tube feet for over 10 years. A title from an oral presentation at one of the last European echinoderm conferences by Elise Hennebert, Romana Santos and Patrick Flammang here, entitled "Echinoderms don't suck" does seem to imply the days of that mechanism are numbered.

Bear in mind that this doesn't even necessarily end with this one tube foot type. There are at least two other forms of tube feet and exploring the various mechanisms at play could be fruitful indeed...

In addition to the lab run by Patrick Flammang at the University of Mons, this lab in the UK (shown in this video) has also undertaken research into this area to understand the nature of tube foot adhesives...

UPDATE:  To see the follow up to this story and to see how "echinoderms don't suck" go here!