Monday, October 31, 2016
Sea urchins are visually arresting, hazardous to swimmers, and to some cultures delicious. The marine invertebrates are important links in the marine food chain. Fish pick at the urchins, which feed on bits of algae.
Sea urchins are perhaps best known for their armor of spines. But their mouths may be even more daunting—urchin teeth can literally chew through stone without getting dull.
Urchins feed on algae and invertebrates, a recent study found that the sound of their teeth scraping on reefs can cause a rise in ocean noise.
The teeth are mosaics of two kinds of calcite crystals: fibers and curved plates. The crystal shapes are arranged crosswise to each other and are bound together with a superhard cement of calcite nanoparticles.
Between the crystals are layers of weaker organic material. By striking the teeth with microscopic, diamond-tipped probes, the scientists found that the teeth break along these organic layers.
The scientists think the organics are predetermined weak spots in the teeth that allow parts of the material to "tear" away, similar to perforations in a sheet of paper. This means the teeth, which grow continuously, can regularly shed damaged areas to keep a well-honed edge.
On October 15, standing near the summit of Hawaii's Mauna Kea and looking away from a gorgeous sunset produced this magnificent snapshot of a Full Moon rising within the volcanic mountain's shadow. An alignment across the Solar System is captured in the stunning scene and seeming contradiction of bright Moon in dark shadow.
The triangular appearance of a shadow cast by a mountain's irregular profile is normal. It's created by the perspective of the distant mountaintop view through the dense atmosphere. Rising as the Sun sets, the antisolar point or the point opposite the Sun is close to the perspective's vanishing point near the mountain shadow's peak.
But extending in the antisolar direction, Earth's conical shadow is only a few lunar diameter's wide at the distance of the Moon. So October's Full Hunters Moon is still reflecting sunlight, seen through the mountain's atmospheric shadow but found too far from the antisolar point and the Earth's extended shadow to be eclipsed.
Image & info via APODhttp://apod.nasa.gov/apod/astropix.html
Image Credit & Copyright: Greg Chavdarian