Formation of the statolith in the ctenophore Mnemiopsis leidyi.

The aboral sensory organ (apical organ) of ctenophores contains a statocyst with a single large statolith. The statolith comprises living cells (lithocytes), each containing a large membrane-bound concretion. The statolith is supported on the distal ends of four compound motile mechanoresponsive cilia (balancers) which control the beat frequencies of the eight locomotory comb rows, and thereby the orientation of animals to gravity. In Mnemiopsis leidyi and Pleurobrachia pileus, lithocytes arise in the thickened epithelial floor of the apical organ on opposite sides along the tentacular plane. Lithocytes progressively differentiate and migrate toward the apical surface where they bud off next to the bases of the balancers. New lithocytes are transported up the balancers by ciliary surface motility to form the statolith (Noda, 2013). The statolith has a superellipsoidal shape due to the rectangular arrangement of the four balancers and the addition of new lithocytes to its ends via the balancers. The size of the statolith increases with animal size, starting at the highest rate of growth in younger stages and gradually decreasing in larger animals. The total number of developing lithocytes in the epithelial floor increases rapidly in smaller animals and reaches a plateau range in larger animals. Lithocytes are therefore produced continually throughout life for enlargement of the statolith and possibly for turnover and replacement of existing lithocytes. The dome cilia enclosing the statocyst were observed to propagate slow, low-ampitude waves distally. The dome cilia may act as an undulating screen to prevent foreign objects in the seawater from being transported non-specifically up the balancers to make a defective statolith.