mexicanaandP. the macula sacculi possesses dorsal and ventral bulges. Both researched ecotypes ofP. mexicanashowed variant mainly in the form and curvature from the macula lagenae, within the curvature from the macula sacculi, and in the width of the otolithic membrane. == Conclusions == Our study for the first time provides detailed insights Teijin compound 1 into the auditory periphery of a cyprinodontiform inner ear and thus serves a basisespecially with regard to the application of 3D techniquesfor further research on structure-function relationships of inner ears within the species-rich order Cyprinodontiformes. We suggest that other poeciliid taxa, or even other non-poeciliid cyprinodontiforms, may display similar inner ear morphologies as described here. == Introduction == Fishes show an amazing diversity in hearing abilities[1],[2], inner ear structures[3][6], and otolith morphology[7],[8]. Until now, however, only few studies investigated aspects of inner ear morphology within the diverse order Cyprinodontiformes (e.g., green swordtails,Xiphophorus hellerii[9]; guppies,P. reticulata[10], both Poeciliidae; redtail splitfin,Xenotoca eiseni[3], Goodeidae). The Central American Atlantic molly,Poecilia mexicana(Cyprinodontiformes, Poeciliidae) has been FLJ39827 established as a model to study various aspects of local adaptation and ecological speciation[11],[12]. We therefore choseP. mexicanafor our study of cyprinodontiform inner ear morphology.For the Atlantic molly several populations are described that have adapted to extreme environmental conditions (e.g., cave life[13][15]). Considering the amazing degree of local adaptation to divergent habitat types observed in this widely distributed species[11],[12], we investigated the intra-specific variability in inner ear morphology by including three populations belonging to two different ecotypes (cavevs.surface habitats). So far, only Teijin compound 1 very limited information exists on intra-specific variation Teijin compound 1 of inner ear morphology in teleost fishes (for otolith shape variation amongP. mexicanapopulations see refs[16],[17]). The inner ear of modern bony fishes (Teleostei) is composed of the upper labyrinth (pars superior) containing the three semicircular canals and the utricle and the lower labyrinth (pars inferior) which is build up by the two end organs, saccule and lagena. In each of the end organs a calcareous biomineralisate (otolith) is connected via an otolithic membrane (the OM is actually not a biomembrane, but a fibrillar extracellular pellicle;[18]) to the ciliary bundles of the hair cells extending from the sensory epithelium (macula). According to their position in the inner ear, three different otolith types can be distinguished, namely the otolith of the utricle (lapillus), saccule (sagitta), and lagena (asteriscus). Otoliths can either cover the entire macula or only a part of it[3],[5],[6],[19]; in the latter case, the remaining macula area is covered by the otolithic membrane only. Still, the interrelationships between otolith morphology, and features like otolithic membrane structure, or the structure of the respective sensory epithelium are not yet fully understood[20],[21]. Moreover, there exist only few studies dealing with all three otolithic end organs and their functional interactions (e.g.,[22],[23]). Our study provides the first detailed investigation of a cyprinodontiform inner ear while combining an array of different methodological approaches (-CT analyses, scanning electron microscopy, and confocal laser scanning microscopy after immunocytochemical staining). Specifically, we focused on the position of otolithsin situ, the three-dimensional structure of sensory epithelia, and orientation patterns of ciliary bundles of the Teijin compound 1 sensory hair cells. The primary aim of our study was to provide a basis for further investigations with regard to structure-function relationships of inner ears of cyprinodontiform fishes. In addition, by comparing cave and surface forms ofP. mexicana(on a qualitative basis) we made a first attempt to describe intra-specific variability and to detect potential differences between ecotypes that might be interpreted as adaptations to extreme environmental conditions (toxic hydrogen sulfide coupled with perpetual darkness). == Results == == Gross inner ear morphology == The lagena and utricle inP. mexicana(N= 6) are both smaller than the saccule (Figure 1AB). The utricle is positioned rostro-laterally to the saccule and a wide opening connects both end organs. The lagena is positioned postero-laterally to the saccule (Figure 1A2). The lagena of the surface ecotype shows an almost parallel orientation to the main axis of the fish (Figure 2A2), while in the cave ecotype it was found to be more laterally-bent (Figure 2B, C2). Moreover, all three semicircular canals.