Seismic interpretation of a sea urchin


The algorithms developed to pick and track continuities and discontinuities in seismic data can be used to count the spines of a sea-urchin. Yes, very useful.

Sea urchins have sharp spines, and if you ever stepped on one you know that they can inflict you a painful experience. And I guess it is not pleasant for the urchin either. Urchins can repair, rebuild their spines: each of them is made of single-crystal calcite, as proven by X-ray imaging.

Anyway, in this exercise the spines are long gone: we only have the shell of a sea urchin, that biologists call test. The spines had been attached to the tubercles, the raised spots regularly arranged on the shell. The tubercles are clearly visible on the shell: there are many of them, of different size, and with different alignments. On a single close-up image their patterns are not obvious, but it’s a matter of perspective.


We don’t search the additional complexity of a strong perspective distortion: even with a nice full image of a shell, if you want to pick automatically the tubercles and their patterns, categorizing the picks, well it’s not an easy problem.

There are lots of small circular features, not all evident, mostly radially aligned, with staggered patterns and a fivefold symmetry.


To cut a long story short, the image below shows the results. Categories are shown with three colours. The cyan and yellow are aligned on a half-meridian, while the pink are more chaotically distributed. The intensity of dots shows the pick reliability.

The image preserve the fivefold symmetry of this organism. This is pentamerism.


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