Hitching a ride

posted by Christopher Laumer / on March 27th, 2010 / in Arthropods, Parasites, Symbiosis

When collecting bugs out in the field, it can be easy to get more than one bargained for. Many flies, beetles, and other mobile beasties such as the harvestman shown above (Megalopsalis sp. from New Zealand) find themselves regular host to hitchhikers of arachnid origin: the five orangish globules nestled among the bases of this unfortunate individual’s limbs are parasitic mites hunkered down for the long haul.

Indeed, many tens of thousands of mite species spend their early lives attached to a host, slowly drawing nutrition from its internal fluids until they become large enough to drop off wherever their hosts have carried them, where they begin life as free-living adults. However, these parasitic freeloaders aren’t the only kind of tenants one will find on harvestmen; there are also diverse sorts of more benign bedfellows who climb aboard with no appreciable harm to their unwitting ride.

One group of these tag-alongs is the pseudoscorpions – distant relatives of true scorpions, living secretive lives in forest soil and tree bark – which many would doubtless find abjectly terrifying… if they ever got larger than a few millimeters. Below, you can see a pseudoscorpion hanging for dear life onto the leg of another Megalopsalis. There are reports of pseudoscorpions waiting eagerly around a flower for a bee pollinator to grab onto, or clustering around the pupal bores of flies just before the airborne adult emerges. The traditional interpretation of this behavior suggests that climbing aboard larger, more mobile animals is an adaptation meant to transport these tinier critters to a wider range of habitats. Others, however, have suggested that perhaps pseudoscorpions simply grab onto whatever passes by them in the hopes that they might be able to eat it.

Strangely enough, even pseudoscorpions can bear hitchhikers, as the parasitic mites on this neobisiid I collected during field work in Alabama attest (gray bugs next to the greenish dots). It’s hard not to be reminded of the poet Jonathan Swift’s famous verses:

“The vermin only teaze and pinch
Their foes superior by an inch.
So, naturalists observe, a flea
Has smaller fleas that on him prey,
And these have smaller still to bite ’em,
And so proceed ad infinitum.”

First three photographs by Gonzalo Giribet. Last photograph by Christopher Laumer.

Tube vision

posted by S. Zachary Swartz / on March 17th, 2010 / in Echinoderms, locomotion, optics

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.

CreatureCast – Diving for Jellies

posted by Sophia Tintori / on March 3rd, 2010 / in History, Jellies, Podcast, Siphonophores

Here in the Dunn lab, siphonophores are our favorite animal and the focus of much of our research.

Dr. Phil Pugh is a good friend of the lab, and he also happens to have described more new species of siphonophores than anyone who has ever lived. In the video below, he describes what it’s like to come across a siphonophore in the deep sea with a submarine. What looks like one long body in this video is actually a free-swimming colony of clones — many genetically identical bodies that are all attached. But each body in the group isn’t just like its neighbor. They each do a specific job for the colony. Some individuals will swim, some will catch food, and some will reproduce.

More on siphonophore biology can be found in papers here and here. But we’ll talk about all that later — for now, just take a look.

Footage courtesy of the Bioluminescence lab at the Monterey Bay Aquarium Research Institute. Music graciously provided by Raf Spielman of The Golden Hours. Edited by Sophia Tintori. This podcast is published under a Creative Commons Attribution Non-Commercial No Derivatives 3.0 license.