New Methods of Morphometric Analyses on Scyphozoan Jellyfish Statoliths Including the first Direct Evidence for Statolith Growth Using Calcein as a Fluorescent Marker.

Statoliths are the only hard structures in the gelatinous bell of most scyphozoan medusae and investigations on these structures could promote investigations of the understudied population dynamics and phylogeny of jellyfish. We examined the statoliths of Aurelia aurita jellyfish of different ages by light microscopic and microtomographic measurements supplemented by scanning electron microscopy. The morphometric analyses confirmed that statolith numbers and sizes increase during jellyfish development and revealed that newly-formed statoliths had similar shapes that may change during statolith growth. Nevertheless, most statoliths had a typical compact rod shape with an aspect ratio of 1-2.5 at all ages and we suggest that the composition of statolith shapes may be taxa specific. We developed a new approach allowing exact measurements of statolith growth for the first time. The application of calcein as a fluorescent marker resulted in clear fluorescent lines within the statoliths, allowing calculations of the statolith side face growth increments (0.1 µm/day; n=252). A single-crystal analysis revealed that the calcein incubation did not affect the statolith crystal structure. In conclusion, calcein labeling is an excellent method to follow the growth of bassanite statoliths.