Functional consequences of the asymmetric architecture of the ctenophore statocyst

Abstract

Ctenophores, or comb jellies, are geotactic with a statocyst that controls the activity of the eight ciliary comb rows. If a ctenophore is tilted or displaced from a position of vertical balance, it rights itself by asymmetric frequencies of beating on the uppermost and lowermost comb rows, turning to swim up or down depending on its mood. I recently discovered that the statocyst of ctenophores has an asymmetric architecture related to the sagittal and tentacular planes along the oral-aboral axis. The four groups of pacemaker balancer cilia are arranged in a rectangle along the tentacular plane, and support a superellipsoidal statolith elongated in the tentacular plane. By controlled tilting of immobilized ctenophores in either body plane with video recording of activated comb rows, I found that higher beat frequencies occurred in the sagittal than in the tentacular plane at orthogonal orientations. Similar tilting experiments on isolated statocyst slices showed that statolith displacement due to gravity and the resulting deflection of the mechanoresponsive balancers are greater in the sagittal plane. Finally, tilting experiments on a mechanical model gave results similar to those of real statocysts, indicating that the geometric asymmetries of statolith design are sufficient to account for my findings. The asymmetric architecture of the ctenophore statocyst thus has functional consequences, but a possible adaptive value is not known.