Our special guest post this week comes from Dr. Liz Clark of Yale University (you may have heard of it?) in New Haven, Connecticut, USA. She is bringing some biomechanics-fu to echinoderms– the weird marine critters like seastars and sea urchins. Did you see her 9-awesome-things-about-echinoderms blog post on Anatomy to You? You should. And you should check this out– and check out our new paper on this topic, which just came out! Remember: all images below can be clicked to zoom in. That’s so fun!
Eversible Stomach-Churning Rating: 2/10; no Uni sushi here.
I remember the first time I saw one. I was at the Duke Marine Lab staring at a chunk of dredged-up oyster shells in a glass dish, when all of a sudden a mass of big, black spines obscured my view. I looked up from the microscope to see a creature with a round body the size of a nickel and a flurry of long, skinny, spiny arms skulking hurriedly across the dish. It wasn’t quite a spider- the five-fold symmetry gave its echinoderm affinity away- but it wasn’t quite a starfish, either. Starfish appear graceful as their tiny tube-feet make hurried and unseen movements underneath them to transport them slowly across the sand- appearing nearly motionless to the naked eye. This animal, on the other hand, was making rapid, whip-like strikes with its arms so that it clambered forward, rapidly and fearlessly scaling the uneven terrain of the shells in a bold attempt to escape the dish. I was hooked. I had to know who this monster was, and learn as much about it as I could.
Brittle star arm set up to study its ossicle-joint mobility with CT scanning (below).
That was the day I was introduced to the brittle star. The name “brittle star” is a bit of a misnomer, since they are really anything but. Brittleness implies rigidity and stiffness, suggesting they have a delicate nature with the impossibility of repair or to adapt, which couldn’t be farther from the truth. Their long arms are incredibly flexible, each made of around 100 tiny segments that allow them to bend in any direction or loop them around in circles. I bet that their name comes from the ease at which they can cast off their arms, which they do intentionally to escape predators or pesky researchers trying to grab them, which deceitfully suggests fragility when in fact their arms are incredibly sturdy and packed with powerful muscles. They can flawlessly regenerate their arms, and, in the meantime, even after they lose several of them, they adjust their strategy for locomotion so that they keep prowling across the seafloor unphased. Their physical flexibility and ability to repair and adapt in the face of damage makes them anything but brittle. The Japanese name for brittle star roughly translates to “spider-human-hand,” which I think much more accurately captures the ethos of this group.
Brittle stars have internal skeletons, and each segment of their arms are made of a cluster of small skeletal elements (ossicles). Researchers in the past have made the assumption that differences in the shape of these ossicles between species change how they move, but I wasn’t so sure. So, John and I decided to work together to figure it out.
We didn’t dive into the freezer for this one- sorry to disappoint all of the diehard fans of John’s freezer out there (but in my defense can you imagine how tough it would have been to even find them in the sea of rhinos, giraffes, and crocs?!). [JOHN: awwwwwww!! It’s more of a wall keeping in the wildlings, than a sea right now though!] Instead we ordered some brittle stars off the internet! The first thing we did was make some measurements of how flexible the arms of brittle stars are when they’re alive. Then we digitized their skeletons by micro-CT scanning them so we could see the articulations between the ossicles and the segments in 3D. We scanned them in a few different positions so we could see the articulations between the ossicles as their arms bend. Then we incorporated all of that data into a 3D model that allowed us to visualize what’s going on in the inside of brittle star arms as they move them around.
We made several different models using this strategy to see if different ossicle shapes change how their arms move. We looked at the differences between arm ossicles in two different species– Ophioderma brevispina and Ophiothrix angulata, which represent two of the three different major morphologies of brittle star arms. We also looked at the difference in the movement mechanics at the tip and base of the arms in O. brevispina, since the ossicles at the tip are thin and elongated compared to wide and flat at the base.
We found that the tip of the arm of Ophioderma brevispina was more flexible than the base due, at least in part, to the shape of the ossicles. We also found several major differences between the two species, including the location of their joint center and the degree to which they could laterally flex. However, none of these differences were easily attributable to any specific morphological feature that set Ophiothrix angulata and O. brevispina apart, which cautions against making assumptions of brittle star functional capabilities by only looking at the shape of the ossicles. We also found that some of the smaller ossicles within each segment shift their position to accommodate arm flexion, when they were originally thought to limit the motion of the arm! We only looked at a few individuals of two species, but the methods for model-building we developed provide a framework to incorporate a broad sample of brittle star species in the future. We’re curious if the results we found stand when more brittle stars are brought into the mix!
It was incredible to take the journey from initially being surprised and captivated by the movement of these animals to eventually building 3D digital models to discover how they are able to do so. It made me realize that opportunities to be inspired by the natural world are around every corner, and that there are so many interesting questions out there that are still unanswered. Thanks to John and our other team members Derek Briggs, Simon Darroch, Nicolás Mongiardino Koch, Travis Brady, and Sloane Smith for making this project happen!
