Ontogeny and Homology of the Claustra in Otophysan Ostariophysi (Teleostei) Ralf Britz 1 * and Matthias Hoffmann 2 1 Department of Zoology, The Natural History Museum, London SW7 5BD, UK 2 Lehrstuhl fu ¨ r Spezielle Zoologie, D-72076 Tu ¨ bingen, Germany ABSTRACT We studied the ontogeny of the claustrum comparatively in representatives of all otophysan sub- groups. The claustrum of cypriniforms has a cartilaginous precursor, the claustral cartilage, which subsequently os- sifies perichondrally at its anterior face and develops an extensive lamina of membrane bone. The membrane bone component of the claustrum and its close association with the atrium sinus imparis, a perilymphatic space of the Weberian apparatus, are both synapomorphies of cyprini- forms. The characiform claustrum is not preformed in cartilage and originates as a membrane bone ossification, a putative synapomorphy of that taxon. Among siluri- forms, the claustrum is present only in more basal groups and originates as an elongate cartilage that ossifies in a characteristic ventrodorsal direction, possibly a synapo- morphy of catfishes. Gymnotiforms lack the claustral car- tilage and claustrum. We review all previous hypothesis of claustrum homology in light of the above findings and conclude that the most plausible hypothesis is the one originally proposed by Bloch ([1900] Jen Z Naturw 34:1– 64) that claustra are homologs of the supradorsals of the first vertebra. J. Morphol. 267:909 –923, 2006. © 2006 Wiley-Liss, Inc. KEY WORDS: Weberian apparatus; claustrum; supraneu- rals; supradorsals; Otophysi; Cypriniformes; Characi- formes; Siluriformes; Gymnotiformes “…embryology is the only line of investigation which offers any prospects of a satisfactory and final solution to the problem.” (Bridge and Haddon, 1893:260) The Weberian apparatus is a highly modified mor- phological complex that characterizes one of the largest subgroups of teleosts, the Otophysi (Sage- mehl, 1885; Rosen and Greenwood, 1970; Fink and Fink, 1981, 1996). As originally described by Weber (1820) in the carp, loach, and wels, this apparatus comprises modified peri- and endolymphatic spaces of the inner ear, parts of the anterior vertebrae modified into the Weberian ossicles, and a modified swimbladder (Chranilov, 1927). The function of the Weberian apparatus is the reception, transmission, and perception of sound, as shown in elegant exper- iments by von Frisch (1923), Stetter (1929), and von Frisch and Stetter (1932). Because of their highly derived anatomical struc- ture, homology of the skeletal parts of the Weberian apparatus, i.e., the neural complex, the Weberian ossicles, and the os suspensorium, has been a matter of debate. A consensus has not been reached and a number of hypotheses exist. We recently started to revisit the homology of the modified skeleton of the Weberian apparatus by applying a comparative on- togenetic approach (Hoffmann and Britz, in press). The present article focuses on the first of the Webe- rian ossicles, the claustrum, originally not included in the series of “ossicula auditoria” by Weber (1820). The issue of homology of this small ossicle has re- ceived considerable attention in recent years (de Pinna and Grande 2003; Bird and Mabee, 2003; Coburn and Chai, 2003; Grande and Young, 2004; Grande and de Pinna 2004). We describe the devel- opment of the claustrum in representatives of the different subgroups of Otophysi, review all previous hypotheses of its homology, and discuss them in the light of our findings. MATERIALS AND METHODS Our study is based exclusively on a large number of cleared and double-stained (Taylor and van Dyke, 1985) specimens of otophysan taxa. We chose this approach for the reasons detailed in Hoffmann and Britz (in press). We use high-quality color photo- graphs of our specimens, as we did in our previous article (Hoffmann and Britz, in press), so that the reader sees what we saw. The photos were taken at various magnifications with a Zeiss MC 30 camera attached to a Zeiss Axioplan or with a Jenoptik ProgRes C12plus digital camera attached to a Zeiss Tessovar. Cypriniformes. Cyprinidae: Cyprinus carpio: British Museum of Natural History (BMNH) 2005.7.5.138- 151, 14 specimens, 6.2 to 17.2 mm standard length (SL). Puntius sp. Odessa: BMNH 2005.7.5.152-251, 100 specimens, 3.6 to 12.2 mm SL. P. fasciatus: BMNH 2005.7.5.252-290, 39 specimens, 4.8 to 13.5 mm SL. P. filamentosus: BMNH 2005.7.5.291-351, 61 specimens, *Correspondence to: Ralf Britz, Department of Zoology, The Natu- ral History Museum, Cromwell Road, London SW7 5BD, UK. E-mail: r.britz@nhm.ac.uk Published online 21 April 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jmor.10447 JOURNAL OF MORPHOLOGY 267:909 –923 (2006) © 2006 WILEY-LISS, INC.