Some more fun facts about sea urchins, you can see my first post here. (image: staroftheeast on instagram)
Regular sea urchins have a five-fold symmetry which you can see most clearly in the pink ones, they are born as bilateral symmetric but develop five-fold symmetry as they mature.
However sand dollars are also sea urchins and they are called irregular sea urchins as they are often oval and radial symmetric.
The bodies of adult sand dollars, display radial symmetry. The petal-like pattern in sand dollars consists of five paired rows of pores. The pores are perforations in the endoskeleton through which podia for gas exchange project from the body. The mouth of the sand dollar is located on the bottom of its body at the center of the petal-like pattern.
(image: staroftheeast on instagram)
Unlike other urchins, the bodies of sand dollars also display secondary front-to-back bilateral symmetry. The anus of sand dollars is located at the back rather than at the top as in most urchins, with many more bilateral features appearing in some species. These result from the adaptation of sand dollars, in the course of their evolution, from creatures that originally lived their lives on top of the seabed to creatures that burrow beneath it. (source: wikipedia)
Sea urchins are omnivorous animals and therefore eat both plant and animal matter. The sea urchin mainly feeds on algae on the coral and rocks, along with decomposing matter such as dead fish, mussels, sponges ans barnacles.
Sea urchins' tube feet arise from the five ambulacral grooves. Tube feet are moved by a water vascular system, which works through hydraulic pressure, allowing the sea urchin to pump water into and out of the tube feet, enabling it to move. (image source: wikipedia)
The Phyllacanthus imperialis sea urchin, more commonly known as the Sputnik urchin have thick spines which leave a very different texture once they die and have no spines left. Around the big spines are tiny spines for more protection. (image source: galleryofinvertebrates)
We find them very seldom on our beaches here, but when we do we use them for some special jewelry:
Sterling silver Sea urchin earrings available here.
Brass and Enamel Sputnik sea urchin cuff available here.
Here you can see a sea urchin in action, it is pretty amazing!