The green urchin and the pencil urchin are alike in many ways, but their differences matter in a big way when it comes to their ecological impacts. We originally posted this episode at the New York Times, where you can read more.
We have a front and a back and two legs. We walk around on our two legs. When we need to change the direction we are moving in, we first turn our body to face the new direction and then use our same two legs to keep going. It works for us.
But what about a round animal that also has an odd number of limbs? This is the question that Henry Astley, a graduate student in Tom Robert’s lab here at the Brown University Department of Ecology and Evolutionary Biology, set out to answer. Like their relatives the starfish, brittlestars have five arms. Unlike starfish, which crawl around with the thousands of sticky tube feet that line the bottoms of their arms, brittlestars get around by moving their whole arms. They can move much more quickly than starfish, scurrying under a rock or sprinting across the ocean bottom.
Surprisingly, nobody had previously described the details of how brittlestars get around with their arms. Do all five arms play an equal part all the time, or do only some of the arms move at once? Do they have a favorite front and back, or can any arm serve as the front or back?
In his paper published this week (“Getting around when you’re round: quantitative analysis of the locomotion of the blunt-spined brittle star, Ophiocoma echinata“), Henry answers all these questions. It turns out that most of the work of getting around is only done by two arms at a time. These arms move in a rowing motion, much like a sea turtle crawling along the beach, while the other arms stay out of the way. My favorite part of the story though, is how brittlestars turn. Rather than rotate their body to face a new direction, as we do, they just chose a new front and back and row with a different pair of arms. Not only do they not have a favorite front and back, they constantly change their front and back to change direction.
Karen Connolly, from Casey Dunn’s Invertebrate Zoology (Biol 0410) course at Brown University, tells the story of how echinoderms (starfish, sea urchins, and their relatives) can change the stiffness of their skin at will.
Music by Scott Joplin.
Here is another student contribution to the CreatureCast series, by Nathaniel Chu. Nathaniel is a sophomore at Brown University, and was in Casey Dunn’s Invertebrate Zoology course last fall. In this audio piece, Nathaniel talks (and sings) about sea stars, from their run-in with the oyster industry in the early 1900’s, to their profound influence on that stretch of land between high tide and low tide, known as the intertidal zone.
This podcast is published under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license. Photograph by Dr. D. Gordon E. Robertson. If you’d like, you can download this podcast here.
As a student of science, I love how even something close to home can take me completely by surprise. I study sea urchin development, and yet until recently I had no idea that urchins can see. I find this fascinating because they do not have eyes, at least not as I typically think of them. I first learned of this at a talk about feet, of all things. Feet may be the organs by which urchins largely experience their world. Sea urchins have hundreds of feet: thin, muscular tubes with suction cups at the ends. In the video below, you can see how their combined action allows the animal to move (slowly). The role of the tube foot goes beyond locomotion, however.
Urchins have many of the same genes that are associated with vision in other animals. But they don’t have anything that resemble eyes. Instead, these genes are expressed most in the tube feet and short appendages called pedicellariae. It’s been long recognized that sea urchins are light sensitive. Specifically, they tend to move away from it. Trickier, though, is determining how sensitive they are. Are sea urchins reacting to the presence or absence of light, or do they actually have spatial perception? Recent work by Blevins and Johnsen (2004) and Yerramilli and Johnsen (2009) suggests the latter. In these experiments, urchins would react to the presence of dark targets that looked like nice holes to crawl into in their tank. But they only recognized them if they were above a certain size, implying that their visual perception has a resolution of that certain size, and that they’re not just recognizing simple light or dark cues.
So urchins can move in relation to where the dark shapes are, and they have these photoreceptor genes in their feet. But a photoreceptor alone won’t provide spatial information. A creature needs a way to screen out light coming from the sides of the photoreceptors, and only recognize light coming directly at it, so that each photoreceptor is getting a unique reading. Then it can use those readings to get a sense of the differences between the different spaces in front of each receptor. Where does sea urchins’ resolution come from? It’s possible that their spines block out all light except that which is directly in front of any given photoreceptor. When combined, all of the photoreceptors—on the tube feet, pedicellariae, and probably the shell itself—may function as a giant compound eye like that of an insect, with each receptor only seeing what’s in front of it. And if fact, it turns out that the resolution of sea urchin vision actually correlates well with the spacing between their spines. The resolution is modest, but enough to allow for some complex behavior, helping them seek shelter, locate food, or flee from predators. Sea urchins lack a central nervous system or anything resembling a brain, so I find it amazing that they are able to process spatial information.
Videos and photographs taken by Adrian Reich of the Wessel lab. The top photo shows the wandering tube feet of the sea urchin Strongylocentrotus purpuratus. Next down is a video of S. purpuratus moving, then an image of the tube feet of S. purpuratus and its sea star relative Patria miniata. The bottom photo is a close up on P. miniata.