Bilaterality in the regular sea urchin Anthocidaris crassispina is related to efficient defense not to efficient locomotion

Abstract

Regular sea urchins show pentaradial symmetry: they have round bodies made of five almost identical sectors. There is no preference in which sector should be anterior in their locomotion. Anthocidaris crassispina is exceptional among regular sea urchins. Morphometry showed that their tests had radial symmetry but the overall body shape showed bilateral symmetry because of the uneven distribution of spine length. This bilateral axis did not correspond to the ambulacrum III—interambulacrum 5 axis, which is the bilateral axis of irregular sea urchins. Their behavior in relation to this bilaterality was observed in the aquarium. At rest, most of them stayed in the upper corner of the aquarium keeping the sector with the shortest spine length (the shortest sector) toward the water surface. At walking, most of them moved on the vertical wall just under the water surface keeping the shortest sector directing to the water surface. When they walked in the open space, they moved in a random direction with respect to the bilaterality of the spines. The bilaterality and the behavior of this sea urchin were interpreted as the adaptation to defense not to efficient locomotion.