Starfish- Sticky feet

by Matthew Norton

The seafloor can create some rough terrain with jagged rocks, soft sands and sheer cliff edges being some of the obstacles that bottom dwelling animals must overcome to find food and escape predators. Crabs rise to the challenge with their jointed legs, as do octopuses with their sucking tentacles and lobe-finned fish with their thick, fleshy fins. But few animals are as capable at off roading as starfish (also called ‘sea stars’ for not being actual fish).

Starfish are famous for the five way symmetry of their bodies, which they shared to varying extents with their cousins, including sea cucumbers, sea urchins and brittle stars.
All members of the invertebrate animal group Echinodermata (commonly referred to as echinoderms).

If you have ever visited an aquarium you may be aware of how starfish can climb up vertical surfaces, particularly glass windows, with ease. It’s a feat they achieve with the rows upon rows of tube feet they keep on the underside, both along each arm and around the middle. We often think of these tube feet as mere suckers, but starfish also use a chemical adhesive (i.e. glue) to strengthen their hold. That is, until powerful muscles, connected to the tubes of pressurised water that props up the whole starfish body, unsticks each tube foot and allows it to move forward.

The tube feet of a northern sea star Leptasterias polaris stretched out and presumably looking for the next surface to adhere to (left). A sunflower sea star Pycnopodia helianthoides (right), who is capable of moving at the rate of 1 metre per minute with 15,000 tube feet. A real speed demon among the starfish.
Starfish can also use their tube feet to catch their prey and, where necessary, pull its shell apart to get at the tasty animal inside, just as the common starfish Asterias rubens is doing as it tackles a mussel (left). 
Circeaster pullus (right) demonstrating how starfish can push their stomach out through their mouth to digest their prey. In this case, the starfish is attacking a coral colony.

It might seem a bit pedantic to focus on how exactly starfish hold on to a surface, but small details, like the inclusion of glue, can make a huge difference. For instance, there is evidence to suggest that a single tube foot can resist being dislodged no matter what angle it is pushed from. If starfish relied on suction alone, their predators, or competitors, may have greater success if they attacked from the side.

Sticky feet might also be more forgiving on those occasions where suckers would fail to get a good grip, whether it be due to extremely rough terrain or bad aim. And considering the number of tube feet that some species of starfish possess, mistakes are inevitable. For animals with effectively no brain, it’s amazing they can coordinate their movement at all.

Using glue to stick to surfaces is also going to have some drawbacks, otherwise everyone would do it. One possible issue could arise from the sticky ‘footprints’ they leave behind, which could betray their presence. In a previous article, I mentioned that many submerged surfaces are, over time, covered in a layer of organic material called a ‘biofilm’. The concoctions of chemicals it contains is a valuable source of information for starfish, who can read the biofilm with receptors built into their tube feet. But this can also be true of their enemies, including other starfish. 

Scanning electron image of a biofilm incubated on a steel sample under laboratory conditions. Similar biofilms in the wild would also have this kind of variety of microorganisms and associated substances. A real treasure of information for animals who can sift through it all.

Tube feet are useful pieces of kit for starfish, as well as their diverse cousins. But with such a diversity of owners, there is also a diversity in shape and structure. One source even mentioned a type of tube foot with a ‘knob’ end, as described from the suggestive shape of its tip under a microscope. It would seem that evolution’s tendency for tinkering with a core design can produce some odd results.

In crinoids, the ancient cousins of starfish, their tube feet is used exclusively for feeding as they capture pieces of food floating in the water. Given that they either flap about in the water, like the red feather star, or are rooted to the seafloor, like the white sea lily, movement by tube feet is completely unnecessary.

From a human perspective

Few sea creatures are as immediately recognisable as starfish. I have seen the joy they bring firsthand through my current job at the National Marine Aquarium in Plymouth (not affiliated with this blog). They bring joy to so many visitors, particularly those young children who run straight up to the tanks that have starfish stuck to the glass.

Their popularity can also be seen in the stories and works of literature that starfish have inspired. These include fables, poems and even non-fiction books about business management. 

A drawing of a starfish that accompanied a poem by Lydia Sigourney called ‘Hope in God’.

The influence of starfish also extends into popular culture, with the most famous example being Patrick Star, the dimwittted, but lovable best friend of Spongebob Squarepants. He fits the general appearance of a starfish (minus the many tube feet), but Patrick’s character and quirks were also inspired by the real thing. Specifically, how starfish appear to be slow and dumb, only to surprise you with their active and aggressive lifestyle.

Starfish are also important in scientific research, particularly in a laboratory where they are relatively easy to breed and care for. There is particular hope for their role in stem cell research where, one day, we might be able to replicate a starfish’s ability to regenerate almost any part of their body. I very much doubt we can push this area of science into creating real life versions of Wolverine, Deadpool or any other comic book character gifted with super fast healing. But it could help to advance the treatment of medical conditions that can only be tackled sufficiently with stem cells.

The blue bat star, Asterina pectinifera, is particularly popular as a model organism in developmental biology for various reasons. They are resilient in laboratory conditions, being happy in a wide range of temperatures and not being a fussy eater. They also live in shallow water and so can easily be collected.
Starfish missing an arm. It could have been ripped off, or the starfish could have detached willingly as a defence reaction. Either way, it will be able to grow it back again if given time to do so.

Going back to how starfish move, there have been multiple projects to create ‘starfish robots’. Like other animal inspired robots, their designs replicate the shape and omnidirectional movement of the real thing. In many cases, the bulk of the starfish robot is made out of soft and flexible materials such as silicon. These would be closer to the constitution of real starfish compared to metal, or hard plastics.

If these artificial starfish were to be let loose on the seafloor (within reason) the ‘soft robot’ approach is probably the way to go. Considering the rough and unpredictable environments that real starfish have to contend with. Although, the ‘tube feet’ that one such robot starfish is equipped with are suckers only. An oversight that is probably common in the current roster of robot starfish designs as the inclusion of an onboard reservoir of synthetic glue, and a delivery system in each replicate foot, would surely be a logistical nightmare. 

As an addendum to the whole topic of robots and starfish, a few years ago there was a project to protect coral reefs from the destructive crown of thorns starfish by dispatching them with robots armed with lethal injections. I draw your attention to this in case you want to read more about starfish robotics. You may have to trawl through a few pages of headlines about robots killing starfish, like I did when I was researching for this article. 

But what would happen if these two branches of underwater robotics were brought together? We could end up with five legged terminators long before anything that resembles Arnold Schwarzeneggar. 

Thanks for reading

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All other images are public domain and do not require attribution.

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