The bilaterally asymmetrical larval form of Stomopneustes variolaris (Lamarck).

This study describes the echinopluteus and juveniles of the Indo-Pacific echinoid Stomopneustes variolaris. Late 4-armed larvae had left postoral arms that were longer and more deeply red pigmented than the right arms. Two weeks into development, the sixth pair of arms, the posterolaterals, began to form as these larvae achieved an arbacioid form. The right posterolateral arm grew long, was heavily pigmented at the tip, and was oriented perpendicularly or obliquely to the main body axis. The left posterolateral arm was relatively short, with little pigment. Two of several hundred larvae examined showed different patterns. One, with a juvenile rudiment on the right side, had arms that were a mirror image of those of typical larvae. A second larva, without a rudiment, had equal postoral arms and long, deeply pigmented posterolateral arms. These patterns suggest a developmental link between the asymmetry of the larval arms and the formation of the juvenile rudiment. Adult Stomopneustes also often showed a fixed asymmetry, with the test higher and spines shorter on the side toward interambulacrum 3 and the test lower and spines longer on the opposite side (ambulacrum I). Cleared 1- and 2-day juveniles did not show any obvious asymmetry in the location of apical plates that form from the larval spicules, so there is no evidence for a morphological link between asymmetrical larvae and adults.