The ctenophore apical organ is structurally and functionally unique in the animal kingdom (Tamm, 2014a); and it contains other sensory elements besides the statocyst, including putative photoreceptor cells that express opsins (Schnitzler et al., 2012) and pressure receptors (Tamm, 1982), with possible influences on geotactic mood.
My aim was to discover whether these bilateral asymmetries in statocyst architecture affect the function of the statocyst during geotactic behavior of ctenophores.
Changes in geotactic sign are often accompanied by variations in beat frequencies and transient irregular activities of the imaged comb rows.
All of the animals that displayed a consistent sign of geotaxis during the tilting experiments and were used for analysis of beat frequencies were negatively geotactic. This finding is not surprising, since ctenophores were collected at the water surface during fairly calm weather, and were predominantly in an up mood when selected for this study.
At the initial orientation (0[degrees]) of a negatively geotactic Mnemiopsis in the cteno-tilter with the oral-aboral axis vertical and the aboral pole (statocyst) facing up, only occasional waves of beating were seen on all eight comb rows, as described previously for Pleurobrachia and Beroe in an up mood (Tamm, 1980, 1982).
The downward deflections of the lowermost balancers at 90[degrees] and 270[degrees] were analyzed because intact animals in the cteno-tilter were negatively geotactic with fast beating of the lowermost comb rows at these orientations.