Search and Destroy: Sawfish are handier with their blades than previously thought

ResearchBlogging.org

With their long, serrated snouts, the sawfish might strike you as a little like aquatic versions of Leatherface. Scientists used to think the two were behaviorally comparable: sluggish and maybe a little dimwitted, just waving their saw around blindly and waiting for something to run into it. New research by Barbara Wueringer and colleagues from Australia and the US, though, shows that the fish actually wield their saws with considerable skill. They’ve also learned that the sawfish’s nose knows, too, and is a complex sensor as well as a weapon.

The sawfish family has not been heavily studied (a shame considering that all seven species are endangered), and with only a few firsthand accounts of how the fish hunt, researchers could only speculate on how the saws were used. One of the more common suggestions was that they were bottom feeders and raked their snout through the sand to snag buried prey on the saw’s teeth. Other ideas were that the fish cut into the sides of whales or slashed their way through schools of fish.

To figure out what was really going on, Wueringer and her team captured young freshwater sawfish in northern Australia and watched them feed on mullet and pieces of tuna. The fish went after the food in two different ways, depending on whether it was floating in the water or lying at the bottom of the tank. In the water, they quickly slashed at the prey to impale it on their saws’ teeth or knock it to the bottom or into position to eat. Some of these strikes were strong enough to cleave the fish in half. At the bottom of the tank, the sawfish used the underside of their saws to pin prey down and then move them into position to ingest, and usually preferred to eat the mullet headfirst. This video shows all of these maneuvers.

The sawfish weren’t just spearing wildly, either, and the study found that their snouts are part lance and part smart missile, with buit-in tracking systems. Sawfish are closely related to rays, skates and sharks, and share their sensitivity to electrical fields and their ability to use them to navigate and detect other animals.  Wueringer discovered a few years ago that sawfishes’ saws are covered in electroreceptors, and when she presented the sawfish in this new study with electrodes that mimicked the electric signals prey would give off, the fish reacted the same way they did to the food.

The sawfish’s saw has turned out to be more impressive than anyone thought, but cause the fish a lot of trouble, too. Every sawfish species is listed as Critically Endangered by the IUCN Red List, in part because their saws are prized by shamans in Asia as tools for expelling demons and disease, and are easily caught on fishing hooks and lines.

Reference: Barbara E. Wueringer, Lyle Squire, Stephen M. Kajiura, Nathan S. Hart, & Shaun P. Collin (2012). The function of the sawfish’s saw
Current Biology, 22 (5), 150-151 : 10.1016/j.cub.2012.01.055

Image: “What a saw” by Lorenzo Blangiardi. Used under a Creative Commons license

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Man of Steel: Armor, not weapons, protects harvestmen from certain doom


ResearchBlogging.org

A lot of people mistake harvestmen for spiders, but there are two big differences between the two orders of arachnids. One, harvestmen do not scare the living shit out of me and I do not need to my girlfriend to kill any that wander into our house. Two, the eight-legged freaks commonly called daddy longlegs are awesome beyond your wildest imagination, whereas spiders are demons from Hell and are not awesome.

Among the 6,400 known species of havestmen, there are females who can give birth without the need for a male to fertilize thier eggs. There are males who mate with multiple females and then guard all the eggs, sometimes from egg-eating females they’ve recently mated with. There are harvestmen who enjoy each other’s company so much that they live together in groups of 70,000+ individuals. Then, there’s the granddaddy of wieners, willies, dongs and johnsons, the 400-million-year-old fossilized harvestman that possesses the world’s first known penis.* [Read more]

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How is a mantis shrimp like a punching bag? The way it takes a hit.

ResearchBlogging.org

Mantis shrimp are, ounce for ounce, some of the most fearsome predators that you can pull out of the ocean. The marine crustaceans of the order Stomatopoda (neither shrimp nor mantids, they got the name because of their physical resemblance to both) are tiny and unassuming, but can use their front claws to attack with incredible speed and tremendous force. Stomatopods armed with “smasher” claws (there are also those armed with spearing claws) regularly crack open crabs and snails with cudgels that work on the same principal as crossbows: a spring-and-catch mechanism allows potential energy to be built up and stored and then released all at once. When all that power is unleashed, stomatopods can bludgeon prey with 45 mph strikes (the fastest known limb movement in the animal kingdom) and 340 pounds of force.

These war hammers aren’t just for hunting meals, though. Stomatopods use them on each other in territorial disputes, too. Given what these strikes can do, how have mantis shrimp not power-punched each other into extinction?

There’s two parts to the answer. One is the way they hit each other. When sparring over turf, two mantis shrimp will usually exchange a few strikes to each others’ tails as a way of sizing each other up before committing to a full-on and rumble and mutually assured destruction. The second part is where they hit each other. These ritual test blows are made to each other’s telsons, armored tail segments that are strong enough to take the punishment.

To find out just how strong telsons are and how they withstand such force, Sheila Patek from the University of Massachusetts and Jennifer Taylor, from the University of Indiana took a few shots of their own at them. They got some mantis shrimp, let them live out a few final days eating grass shrimp in luxurious plastic cup accommodations and then put them in the freezer until they were dead, but not frozen solid. Then, they superglued the stomatopods to a strip of Plexiglass and dropped stainless steel balls on them.

The pair recorded the impacts with high-speed video cameras and used the data to calculate the tails’ coefficient of restitution, a value representing the elasticity of an object. The basic principle of the measurement is that the amount of elastic energy absorbed by an object can be measured by the loss of momentum of a colliding object suffers, so the figure is expressed as a ratio of and the post- and pre-impact velocities of the striking object. Coefficients of restitution are often used to characterize and regulate products that take their fair share of blows, like automobiles, body armor, sports equipment and even fruits and vegetables.

Patek and Taylor calculated the telson’s coefficient of restitution as 0.56. This is similar to a major league baseball, which has a coefficient of restitution between 0.45 and 0.50 when hit with a bat. The telson dissipated a significant amount of energy, 69%, when it compressed during impact with the steel balls. The incredible loss of energy implies that the telson absorbs impact inelastically, like a heavy punching bag does.

Patek and Taylor also used micro-ComputedTomography (a 3-D imaging method that uses penetrating waves) scans to examine mantis shrimp exoskeletons to see if they could find anything that might explain the telson’s resilience.

They found that the stomatopods’ tails are two times thicker than normal at three ridges, called carinae, that run along the telson. While the center area of the telson crumples inward upon impact, the carinae don’t deform. This provides a balance of stiffness and compliance that helps impact resistance by both absorbing energy and resisting penetration, a strategy human engineers have co-opted for designing armor.

Reference: Taylor JR, & Patek SN (2010). Ritualized fighting and biological armor: the impact mechanics of the mantis shrimp’s telson. The Journal of experimental biology, 213 (Pt 20), 3496-504 PMID: 20889830

Patek, S., Korff, W., & Caldwell, R. (2004). Biomechanics: Deadly strike mechanism of a mantis shrimp Nature, 428 (6985), 819-820 DOI: 10.1038/428819a

Images: Female Odontodactylus Scyllarus by Roy L. Caldwell, UC Berkeley, for the National Science Foundation. Odontodactylus Scyllarus by Flickr user prilfish, used under a Creative Commons license.

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