Bilateral symmetry and locomotion: do elliptical regular sea urchins proceed along their longer body axis?

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Vagile animals usually have bilaterally symmetrical bodies and proceed with their mouth-end first. Regular sea urchins have, however, radially symmetrical bodies with their mouth facing the substratum and show no preference in which side of the body should be anterior in their locomotion. The elliptical sea urchins in the subfamily Echinometrinae are exceptional among regular sea urchins in having elongated bilateral bodies. We studied whether they showed the preference in the direction of locomotion using Echinometra sp. type A. Directional preference was not observed in the proceedings in an open space. However, they proceeded preferentially with their long axis coinciding with the direction of locomotion when they moved along the water surface or along the wall of the aquarium. The speed of locomotion was the same irrespective of the direction of proceedings and of whether sea urchins moved freely or moved along the water surface or along the walls. We suggest that the bilateral body form and the habit of long-axis lead of this sea urchin have adaptive significance to increase the protected body surfaces, not to facilitate the efficiency in locomotion.

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