Echinoderm Tube Feet Don't Suck! They Stick!

Image taken by Barry Fackler

A few months ago, I wrote a post that summarized ongoing developments showing how the tube feet in echinoderms (best seen in sea stars and sea urchins) was primarily based on a 2-part glue/adhesive system.

Do tube feet actually use suction as has been historically thought/taught??

The whole "tube feet use suction" paradigm is a powerful one that has been observed since some of the earliest work on starfish in the 1840s. 

Its a powerful and seemingly straightforward idea. Tube feet have what appears to be a suction cup on the tip of their tube feet, and so, therefore, shouldn't it work like one??

Could this long-standing notion... BE WRONG???

Image by "ratexla"/Josefine Stenudo

The suction cup idea is pervasive and can be seen in many pop culture references.

How pervasive?  The authors of the paper I use below cite Peach the sea star from the recent movie Finding Nemo by Pixar- Peach uses the popping sounds that one associates with a rubber sucker!!

A new, recent paper from the bioadhesion labs of Patrick Flammang in Belgium, and Romana Santos and Elise Hennebert in Portugal have demonstrated several experiments that in fact, tube feet rely on adhesion (as outlined here before) and NOT on suction. This paper is OPEN ACCESS and can be found here at the Proceedings of the 7th European Echinoderm Conference! 

The paper is important to people who study echinoderms but is very straightforward and pretty easy to understand...  The authors work primarily on two species as test subjects:  

The common N. Atlantic sea star Asterias rubens

Image by Juncea

and the European urchin Paracentrotus lividus...

Photo by Marco Cortesi

Even before this recent work on adhesion in tube feet, there had been indicators, some years ago that suction was not the only force in tube feet at play. Why?

First-A study from 1985 (Thomas & Hermans) showed that echinoderms have been observed adhering to screens, meshes and grates-so how would a suction cup work if they were being applied on a porous surface with no way to create a vaccum?

Second.Tube feet leave footprints such as this one which leave behind residue suggesting a glue or adhesive was at play..

In order to test whether suction played an active role in adhesion (in other words they attempted to DISPROVE the role of suction), the authors approached the problem with some very insightful observations/ experiments.

1. Observing the tube feet directly!

The physics of your basic suction cup model is pretty straightforward. The suction cup creates a large suction cavity between the attached foot and the substrate (i.e., the ground).  

When you put a suction cup down, you press the top down and pull it up. This creates the suction cavity that attaches the suction cup to the ground..that's what you would expect.

Tube feet from Asterias (the starfish) and Paracentrotus (the urchin) were sampled immediately after it was clear they were attached, and photographed with a Scanning Electron Microscope. Histological (i.e. tissue) sections were also taken...

The top two pics (A+B) show an unattached tube foot.. But C through F? all show those attached to the bottom.

Figure  1 from Hennebert, Santos and Flammang, 2012

What they found? There was NO "suction cavity" between the tube foot edge and the substrate (i.e. the ground). The tube foot disc surface was actually flat and flush with the substrate surface. Thus, no physical evidence for suction could be observed.
The authors indicate that suction may still play a secondary role, serving in conjunction with the adhesion/glue but for the most part it doesn't look like suction is a primary influence here.

2. Measuring the Attachment Strength of the tube feet

Next, Hennebert and her coauthors measured the attachment strength of the tube feet relative to different variables. These included

A. Measuring the strength or tenacity (in terms of Force or Tenacity) of sea urchins as they hung from a glass plate at different angles.
They tested the adhesion of the tube feet on glass relative to detachment force (how hard they pulled) and pulling angle (the direction). That is they tried to pull it off and at different angles on a smooth glass surface.

fr. Fig. 3A in Hennebert et al. 2013

If this were truly suction then the tube feet would slide (i.e., no resistance) and the amount of suction would decrease. There was no (or at least no statistical) relationship between the detachment force and the pulling angle.

B. Measure strength and tenacity on a porous bottom

This one was more straightforward-if tube feet are anchored by suction, then an imperfect bottom (i.e. substrate) won't really work well as a good anchoring ground. 

The authors used a sheet of plastic with holes present in the surface.  They measured tube feet with a device that measure the force and tenacity and then recorded the footprints based on whether they completely, partially or did not cover the holes.  

Prediction:  If the tube feet use suction-the force measurements for strength and tenacity would be significantly affected. But if adhesion was at play, then the holes should make no difference.

Basically, this experiment mirrors the early observations of watching starfish or urchins moving around on a metal grate or mesh. How important can suction be if the animal can move on a non-porous surface?

 No statistical differences were found between the different groups (i.e., the tube feet that walked over a complete, partial or covered hole). 

CONCLUSION!  And so, not only has prior work (see earlier blog post) shown the huge role of adhesion/glue in the way tube feet work but now, the original historical model..i.e., tube feet use suction has been pretty effectively undermined if not disproven outright!
Its possible of course that there are further refinements to how all of this works in sea cucumbers and crinoids but starfish and sea urchins have always been the "model organism" for studying tube feet in echinoderms. 

One of the oldest and most widely known perceptions about echinoderms? Not the case. Evidence is slowly building up against it and an important lesson in science that even the most long-standing ideas can be overturned when you look at the facts with the right questions!

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!