Growth and formation of statoliths in Aurelia coerulea examined by using 34S- and Sr-labels

Statoliths of Aurelia coerulea were successfully labeled with S-34 and Sr and were visualized for the first time by nanoscale secondary ion mass spectrometry (NanoSIMS) and electron probe microanalysis (EPMA). Medusae were incubated in artificial seawater with an enrichment of S-34 and Sr for 25 hours. Statocysts for analyses were fixed at two time-points: immediately after the exposure (zero-day) and after 3 weeks of incubation in non-enriched seawater (post-incubated). In the analyses by NanoSIMS, deposition of S-34 was observed as an inner ring enclosed within the crystal in post-incubated specimens. In zero-day specimens, S-34/S-32 ratio in the peripheral region of statolith was much higher than those in the core region. In the analyses by EPMA, small statoliths with dense deposition of Sr were located in the basal part of statocysts in zero-day specimens. The larger statoliths tended to be located more distant from the basal part. These findings were consistent with the hypothesis that the basal part of statocysts is the area of origin of statoliths. Statoliths without deposition of Sr tend to be larger in size than those with deposition, suggesting the saturation of growth after crystals attain a certain size.