Quantifying sea urchins covering behavior by image analysis

Many sea urchin species collect debris on their aboral surface, a behavior collectively described as "covering behavior". In the Sultanate of Oman, the flower sea urchin, Toxopneustes pileolus, systematically shows this behavior, accumulating pieces of dead coral, pebbles, and fragments of various mollusks shells on its test. We compared the amount, size distribution, and relative volumetric mass of the covering material in three T pileolus populations using both underwater image analysis and physical analysis of collected debris. The underwater photographic method to estimate test cover was a good predictor of the actual amount of debris on the test (R2 = 0.85). Thxopneustes pileolus, preferred covering itself with the largest pieces of debris available in the surface sediment, but did not select pieces according to relative density. There were no significant differences in percentage cover neither among urchins of different diameters nor among urchins collected in different populations. We discuss these results in relation to various advanced hypothesis on the function of the covering behavior.