Okay. So, nobody guessed the answer to the daily photo quiz…
It is one of two compound eyes on a horseshoe crab.
Not one of its seven OTHER eyes.
Just to be clear, I should tell you that a horseshoe crab is not really a “crab” at all, at least not in the sense of inclusion in the Crustacean Clubhouse. More closely related to scorpions and spiders, horseshoe crabs are ocean dwelling arthropods.
Walking is its primary means of locomotion, thanks to ten pairs of walking legs, but juveniles will swim, upside-down, by flapping their gills. Perhaps I should mention that horseshoe crabs have “book gills,” six sets of flat sheets of oxygen-exchanging tissue that do, with some imagination, resemble books.
For sustenance, horseshoe crabs will eat worms, algae, mussels, clams, dead fish and other carrion. They simply crawl over a meal and grind the food into mush betweent the spines on the upper parts of their legs, then shove the resulting glop into their mouths.
The “tail” (called a telson) may look scary, but it’s really just a tool, useful for steering and especially for flipping itself back onto its feet if it gets flipped onto its back by surf or some mean ol’ human.
The horseshoe crab is a genuine “blue blood:” its blood cells contain “hemocyanin” with copper, instead of “hemoglobin” with iron. As a result, the blood turns blue instead of red when exposed to oxygen. Immune cells in horseshoe crab blood are so good at detecting and eliminating bad bacteria that they are used to test vaccines and some medical equipment before those things are used on humans.
Okay, okay, back to those eyes. The compound eyes help the horsehoe crab recognize a potential mate. The other seven eyes, scattered around the body, detect changes in light levels, especially moonlight, and also pick up movement. In addition to all those eyes, horseshoe crabs also have light receptors in the tail area. It seems that when it comes to vision, what they lack in quality is made up for in quantity.