Small purple sea urchin spine we found in the rocky intertidal at Pt. Pinos beach in Pacific Grove, California. After seeing the grooves and socket on the spine I did a quick literature search to learn more about the formation and structure of sea urchin spines.
Sea urchins can move their spines independently of each other, if you have been to any touch pools at aquariums you may have seen or felt this. These spines can be used for a variety of purposes, such as burrowing into soft rock, protecting against predation, moving around, think walking on stilts, and for holding onto floating algal pieces. Each spine is a single crystal of magnesium calcite, a crystal structure of calcium carbonate (Donnay and Pawson 1969), with a base that forms a ball-and-socket joint with the “test”, or shell, of the urchin. The grooves seen above the base is where the spine transitions from the porous calcite crystal socket to non-porous crystal septa, structure that is elevated towards us, with material bridging the base of the septa, forming the grooves. This structure makes the spines very resistance to breaking under force along the spine, allowing it to break off at the ball-and-socket joint at the test (Burkhardt et al . 1983). These spines are fairly sharp and if you step on a urchin you can end up with multiple spines stuck in your foot.
References
Donnay, G., and D.L. Pawson. 1969. X-ray diffraction studies on echinoderm plates. Science. 28: 1147-1150.
Burkhardt, A., W. Hansmann, K. M ä rkel, H.J. Niemann. 1983. Mechanical design in spines in diadematoid echinoids (Echinodermata, Echinoidea). Zoomorphology. 102: 189-203.