Mar Biol (2011) 158:1149–1161 DOI 10.1007/s00227-011-1637-3 123 ORIGINAL PAPER Comparison of the statolith structures of Chironex Xeckeri (Cnidaria, Cubozoa) and Periphylla periphylla (Cnidaria, Scyphozoa): a phylogenetic approach I. Sötje · F. Neues · M. Epple · W. Ludwig · A. Rack · M. Gordon · R. Boese · H. Tiemann Received: 29 September 2010 / Accepted: 1 February 2011 / Published online: 19 February 2011  Springer-Verlag 2011 Abstract The rhopalia and statocysts of Periphylla periphylla (Péron and Lesueur in Ann Mus Hist Nat Mar- seille 14:316–366,1809) and Chironex Xeckeri Southcott (Aust J Mar Freshw Res 7(2):254–280 1956) were exam- ined histologically and showed several homologous charac- teristics. DiVerences in sensory area distribution could be connected to a slightly diVerent functionality of equilibrium sensing. In P. periphylla, the statoliths (crystals) grow inde- pendently of each other; whereas in C. Xeckeri, one large crystal covers the smaller ones. The structures of both statoliths were examined in detail with single-crystal diVraction, microtomography and diVraction contrast tomography. The single compact statolith of C. Xeckeri consisted of bassanite as was previously known only for other rhopaliophoran medusae. An origin area with several small oligocrystals was located in the centre of the cubo- zoan statolith. The origin areas and the accretion of stato- liths are similar in both species. Our results lead to the assumption that the single bassanite statolith of C. Xeckeri (Cnidaria, Cubozoa) is a progression of the scyphozoan multiplex statolith. It is therefore suggested that the Cubo- zoa are derived from a scyphozoan ancestor and are a highly developed taxa within the Rhopaliophora. Introduction Evident in the fossil records from the Middle Cambrian about 600 millions years ago, the Cnidaria are one of the oldest animal phyla (Coates 2003). Earliest fossils already displayed characteristics that are still present in the medusae of the recent taxa (Adler et al. 2007; Cartwright et al. 2007). Despite more than 100 years of investigation, the phyloge- netic tree and systematics of the Cnidaria are still subject to considerable discussion. At present, the most common divi- sion is into the four classes; Anthozoa, Scyphozoa, Cubozoa and Hydrozoa (Schuchert 1993; Werner 1993). Some authors have, however, introduced an additional class, the Staurozoa (Marques and Collins 2004), while others distin- guish only three classes, grouping Cubozoa with Scyphozoa (Matsumoto 1995; Ruppert et al. 2004). Only the three taxa, the Hydrozoa, Scyphozoa and Cubozoa, develop pelagic medusa stages with marginal sense organs bearing statocysts (Russell 1970; Werner 1993). While the medusa stage could have developed once, twice or three times within the diVer- ent taxa (Thiel 1966; Werner 1973, 1993; Salvini-Plawen 1978; Schuchert 1993; Collins 2002) the statocyst, which is crucial in medusae orientation, must have developed together with the pelagic stage. Communicated by J. Purcell. I. Sötje (&) · H. Tiemann Biocenter Grindel and Zoological Museum, University of Hamburg, Martin-Luther-King-Platz 3, 20146 Hamburg, Germany e-mail: ilka.soetje@zoologie.uni-hamburg.de F. Neues · M. Epple · R. Boese Inorganic Chemistry, University of Duisburg-Essen, Campus Essen, Universitaetsstraße 5-7, 45117 Essen, Germany W. Ludwig · A. Rack European Synchrotron Radiation Facility, BP220, 38043 Grenoble Cedex, France W. Ludwig Institut National des Sciences Appliquées de Lyon, 20, Avenue Albert Einstein, 69621 Villeurbanne Cedex, France M. Gordon School of Marine and Tropical Biology, James Cook University, McGregor Road, SmithWeld, QLD 4878, Australia