A calm ocean can seem uninteresting from above. At first glance it's this big expanse of basically nothing, but just beneath the surface lie wonders. The same can be said of sea stars. When we find them at low tide sea stars barely move. Many people aren't even sure that they're animals, and we regularly had people at the touch-pools of the Seattle Aquarium ask if they were fake.
Well he does exaggerate how much he can bench, but I wouldn't call him fake
Courtesy Jerry Kirkhart via Flickr
Right under the surface of sea stars, and the other echinoderms like sea cucumbers and urchins, lies what I think is one of the coolest adaptations of any animal on earth: catch connective tissue. This material is the source of echinoderms' amazing ability to become completely rigid, or jello soft.
Before we get into the meat of how this stuff works let's think about how it might be useful. At any one time different sections of a star's body can be rock hard, near liquid, and everything in between. This is an amazingly good strategy for an animal that moves over uneven terrain. Imagine you're a sea star and you're trying to find a tasty mussel to eat. As you crawl along your leading arm comes into contact with a big boulder. Well no big deal, you can make that arm go soft and bend to any angle you might need to climb onto it. As you reach the top of the stone you notice a strong current trying to blow you away. Again no problem, you can make a couple of arms go rigid to add strength to your grip which keeps you from waving around. Once you're held on nice and tight one of the arms that you're not using to cling for dear life can go soft and tap around the rock's surface in search of prey. Below you can see sped up video of a blue linckia star (Linckia laevigata) moving, and you can get a good idea of what I'm talking about. Check out how flexible the sections bending around the edges of the coral are, and how stiff the parts on the flat.
Clearly this catch connective tissue is some useful stuff, but how does it work? In humans, our skin and connective tissues are made up of fibrils (small bundles of strands that make up a fiber) of collagen held together by connecting proteins.
Collagen fibrils (the big strands going up) and the proteins holding them together
Courtesy Zeiss Microscopy via Flickr
This is true for catch connective tissue as well. The difference is that we have a more or less set amount of proteins holding the fibrils together whereas echinoderms can change the number of links. The more proteins holding the fibrils together, the less they can slide around on one another, and the more rigid the whole tissue. The fewer the proteins, the more the fibrils can slide around, and the softer the tissue. It's almost like bundling sticks with rubber bands. In what might be the greatest naming ever, the molecules that trigger the hardening and softening of the tissue are called tensilin and softenin. Many echinoderms can even reduce the number of proteins so low that they can literally walk away from sections of their body. This is called autotomy (dibs on the band name) and it's a great way to escape predators that might do much more damage by ripping off one of their appendages. Thankfully echinoderms have incredible regenerative abilities, so they can regrow parts of their bodies. There's a wonderfully quirky, yet easy to understand, video explanation of this incredible tissue at creaturecast.org.
Ana R. Ribeiro, Alice Barbaglio, Cristiano D. Benedetto, Cristina C. Ribeiro, Iain C. Wilkie, Maria D. C. Carnevali, Mário A. Barbosa, (September 14th, 2011) "New Insights into Mutable Collagenous Tissue: Correlations between the Microstructure and Mechanical State of a Sea-Urchin Ligament" PLOS ONE, DOI: 10.1371/journal.pone.0024822 Accessed via
Yasuhiro Takehana, Akira Yamada, Masaki Tamori, and Tatsuo Motokawa, (Jan 15, 2014) "Softenin, a Novel Protein That Softens the Connective Tissue of Sea Cucumbers through Inhibiting Interaction between Collagen Fibrils" PLOS ONE. 2014; 9(1): e85644.
Published online, DOI: 10.1371/journal.pone.0085644,
Accessed via http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3893245/
Dunn lab and Creature Cast