title: Modifications of the falciform process in the eye of beloniformes (Teleostei: Atherinomorpha): evolution of a curtain-like septum in the eye.
abstract: In order to comparatively analyze curtain-like septa in the eyes of visually orientated "close-to-surface-predators" among atherinomorph teleosts, we examined the eyes of 24 atherinomorph species under a binocular microscope with regard to the falciform process and related structures in the vitreous cavity. Additionally, falciform process samples were analyzed by transmission electron microscopy. All the studied representatives of the Cyprinodontiformes and Atheriniformes, and of one of the beloniform suborder, Adrianichthyioidei, possess a "typical" processus falciformis. In the eyes of the representatives of the other beloniform suborder, Belonoidei, however, pigmented structures that originate in the region of the optic disc and protrude into the vitreous cavity were noted. In the Hemiramphidae (halfbeaks) and Exocoetidae (flying fishes) these pigmented structures have a more cone-like shape, whereas in the Belonidae (needlefishes) and Scomberesocidae (sauries) horizontally oriented heavily pigmented curtain-like septa occur that divide the vitreous cavity dorsoventrally. It is suggested that the "typical" processus falciformis represents a plesiomorphic feature within the Atherinomorpha, whereas the pigmented modifications of the falciform process must be seen as a synapomorphic character state of the Belonoidei. The curtain-like septum of the Belonidae and Scomberesocidae might have evolved from the cone-like structures that are found in the Exocoetoidea. The functional significance of the pigmented structures in the eye is as yet not clear, except for the curtain-like septum found in Belonidae. It might play a role in visual orientation near the water surface at Snell's window.
authors.affiliation: Department Biologie II, D-80333 München, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Reckel
authors.forename: Roland R
authors.initials: RR
authors.lastname: Melzer
mesh_terms: D000818:Animals; D023761:Beloniformes / Q000033:anatomy & histology* / Q000502:physiology; D005075:Biological Evolution; D005123:Eye / Q000648:ultrastructure*; D005247:Feeding Behavior / Q000502:physiology; D014644:Genetic Variation / Q000502:physiology; D046529:Microscopy, Electron, Transmission; D009799:Ocular Physiological Phenomena; D010802:Phylogeny; D010857:Pigment Epithelium of Eye / Q000502:physiology / Q000648:ultrastructure; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology / Q000648:ultrastructure; D014785:Vision, Ocular / Q000502:physiology*; D014822:Vitreous Body / Q000502:physiology / Q000648:ultrastructure