This LMAC collage was inspired by a YouTube video of a sea urchin army:
I had studied 
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These teeth will never need the attention of a dentist. Despite the fact that urchins chomp on rocks and shells, their teeth never become dull. As a matter of fact, the more they chomp, the sharper the teeth get. As the teeth make contact with hard surfaces, material is selectively worn off. All the while the urchin's teeth continue to grow, so the lost material is perpetually replaced. Horatio Espinosa, of Northwestern University, compares it to sharpening a knife blade.
The urchin's five teeth work in unison and the jaw structure is referred to as Aristotle's Lantern. Aristotle described the mouth structure of the urchin as looking like a'"horn lantern'.
Do They Live Forever?
Almost. Urchins have one of the longest life spans on earth, some observers have suggest they may have the longest life span. They don't seem to suffer senescence. Even very old sea urchins can reproduce and show no signs of aging. Their lives are likely to end when they are eaten by a predator.
Unlike some animals (moths, for example), the sea urchin larval stage is very short.
Here's a YouTube video showing time-lapse larval development.
The larvae feed for about a month before becoming very small sea urchins. Sea urchins reproduce by releasing sperm and eggs into the water. While there are male and female sea urchins, no external sex organs distinguish the two. They look the same.
What do they eat?
Famously, they eat kelp. As a matter of fact, they eat so much kelp that, according to some California marine scientists,their appetite for this algae is endangering the survival of purple kelp in the Pacific.
However, sea urchins are omnivores that eat plant and animal matter alike. But as they eat, so are they eaten.
Sea Otter
Image credit: "Mike" Michael l. Baird. Used under CC 2.0 license
Sea Otters eat urchins, as do starfish, humans and the spiny lobster.
Sea Urchin Defense
Sea urchins use their spines to defend themselves. Wounds from these can be painful, or fatal. Some species of sea urchin are venomous and at least one, the flower sea urchin (Toxopneustidae) can be highly toxic.
Image credit: Samuel Chow. CC 2.0 license
Types of Sea Urchins
While I have written about spiny sea urchins, there are other types of urchins, including sand dollars and Spatangoida.
Eccenteric Sand Dollar
Image credit:D. Gordon E. Robertson CC 3.0 license
Spatangus Purpureus
Image credit: Rpillon CC 3.0
I'll end this excursion into the world of the sea urchin by linking to an amazing National Geographic video of a carrier crab, carrying a red sea urchin. Apparently this arrangement works for both species. The crab gets cover from the spiny sea urchin, and the sea urchin gets to move to new areas where it can feed and also reproduce.
Carrier Crab with Red Radiant Sea Urchin
Image credit: Sylke Rohrlach. CC BY-SA 2.0
My Collage
This collage went through so many interations (don't I always say that?). The artists in the #LMAC community have a strategic method. My method is completely intuitive. I start with an idea and then muddle along. Eventually the collage finds itself.
Here are a few steps from this week:
It began with a sea urchin, taken from Paint 3d that I warped with GIMP.
Then I went for hermit crabs, an octopus and sea urchin, also taken from Paint 3D :
Sea horses were part of the scene for a while, and there was a lot of warping (thank you, 
I used the warp function on Gimp to create the explosive effect. I drew the teeth on the fish, once again by using GIMP. All elements in the collage, kelp,fish, sea urchins, came from Paint3D.
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