Formation of the Statolith in the Ctenophore Mnemiopsis leidyi

Abstract

Abstract. 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.

Functional Consequences of the Asymmetric Architecture of the Ctenophore Statocyst

“…This study reports the functional consequences of a recently discovered design asymmetry in the geotactic organ of ctenophores (Tamm, 2014c). The architecture of the ctenophore statocyst previously had been considered to be radially symmetric about the longitudinal body axis (oralaboral axis) and in the two orthogonal planes of symmetry (sagittal and tentacular) along this axis.…”

“…The discovery of a bilateral morphological asymmetry of the statocyst with respect to the sagittal and tentacular planes (Tamm 2014c) prompted the present investigation into whether this newly revised structural plan affects the geotactic responses of ctenophores. I report here that it does: function follows structure, as Crick (1988) suggested, also in the statocyst of ctenophores.…”

“…This report investigates whether the newly revised structure affects its function. Specifically, does the recently discovered asymmetric architecture of the ctenophore statocyst, with respect to the sagittal and tentacular body planes (Tamm, 2014c), influence its functional responses during geotaxis?…”

“…1) (Tamm, 2014c). This non-radial symmetry of the statolith is due to two factors: a rectangular arrangement of the four balancers, which arise at the vertices of a rectangle with a 3:1 length-width ratio oriented along the tentacular plane ( Fig.…”

“…This non-radial symmetry of the statolith is due to two factors: a rectangular arrangement of the four balancers, which arise at the vertices of a rectangle with a 3:1 length-width ratio oriented along the tentacular plane ( Fig. 1) (Tamm, 2014c); and the addition of new lithocytes only at the two ends of the statolith by transport along the ciliary surfaces of the balancers (Noda and Tamm, 2014).…”