Structural analysis of the statocyst and nervous system of Praesagittifera naikaiensis, an acoel flatworm, during development after hatching

Acoel statocysts are fluid-filled chambers formed by two parietal cells underlying basal lamina as a capsule and contain a movable statolith cell called a lithocyte. The statocyst is needed for geotaxis; however, the mechanism of the gravity receptor system has not been elucidated. We focused on the geotactic ability of the acoel Praesagittifera naikaiensis, acquired during the development of the statocyst and its nervous system after hatching, and examined the three-dimensional relationship between the statocyst and its nervous system. Acoel geotactic ability was acquired between 0 and 7 days after hatching. No major changes in neural structures, namely a commissural brain, nerve cords, and commissures, were observed between juveniles and adults. The statocyst-associated commissure (stc), a commissural brain component, was circular and was located ventral to the statocyst but was not observed in neural connections to the capsule’s lumen. Fine structures of the statocyst revealed that the statolith developed after hatching. We hypothesized that geotactic ability needs the following conditions: (1) a sufficient concentration of calcium salt in the statolith; (2) stc must work as afferent neurons; and (3) a ventral polar cell, which is present outside the capsule, must be a sensory cell stimulated by the lithocyte.