Oogonial proliferation in fishes is an essential reproductive strategy to generate new ovarian follicles and is the basis for unlimited oogenesis. The reproductive cycle in viviparous teleosts, besides oogenesis, involves development of embryos inside the ovary, that is, intraovarian gestation. Oogonia are located in the germinal epithelium of the ovary. The germinal epithelium is the surface of ovarian lamellae and, therefore, borders the ovarian lumen. However, activity and seasonality of the germinal epithelium have not been described in any viviparous teleost species regarding oogonial proliferation and folliculogenesis. The goal of this study is to identify the histological features of oogonial proliferation and folliculogenesis during the reproductive cycle of the viviparous goodeid Ilyodon whitei. Ovaries during nongestation and early and late gestation were analyzed. Oogonial proliferation and folliculogenesis in I. whitei, where intraovarian gestation follows the maturation and fertilization of oocytes, do not correspond to the late oogenesis, as was observed in oviparous species, but correspond to late gestation. This observation offers an example of ovarian physiology correlated with viviparous reproduction and provides elements for understanding the regulation of the initiation of processes that ultimately result in the origin of the next generation. These processes include oogonia proliferation and development of the next batch of germ cells into the complex process of intraovarian gestation.
The Atlantic mudskipper, Periophthalmus barbarus, is an amphibious fish that successfully overcomes the numerous physical challenges of capturing prey in a terrestrial environment. However, it is unclear what changes in the morphology and function of the feeding apparatus contribute to the mudskipper's successful transition from aquatic to terrestrial capture of prey. In particular, how does the mudskipper achieve effective prehension of land-based prey using its percomorph feeding apparatus? To address that question, we performed a morphological analysis of the feeding apparatus of P. barbarus based on microcomputed tomography scanning, histological sectioning, and dissections as well as a kinematic analysis based on high-speed video and X-ray video to quantify the movements of the oral jaw apparatus elements. Our results show that the neurocranium remains in a fixed position relative to the pectoral girdle as the fish pivots over its pectoral fins toward the prey. The premaxilla rotates dorsally and protrudes downward over the prey. The dentary is rotated ventrally over an angle of 120°, which is facilitated by an intramandibular joint. These motions of the neurocranium, premaxilla, and dentary reorient the mouth aperture so it is parallel to the substrate, thereby allowing the jaws to be placed over the prey. The prey is grabbed between the oral teeth or scooped into the mouth primarily via rapid closing motion of the lower jaw. This analysis of P. barbarus clarifies the morphological and kinematic characteristics required by fish to become successful terrestrial feeders at the environmental transition between water and land.
Small fishes living in fast-flowing rivers face a harsh environment as they can easily be swept away by the rapid currents. To survive such circumstances, teleosts evolved a wide variety of attachment mechanisms, based on friction, negative pressure or both. Balitorinae (Balitoridae, Cypriniformes) are exceptional in using their whole body as an adhesive apparatus. We investigated the morphological adaptations of Balitorinae by studying the osteology and myology of four species (Beaufortia leveretti, Sewellia lineolata,Pseudogastromyzon myersi, and Gastromyzon punctulatus) using clearing and staining, serial cross-sections and CT-scanning. A kinematic analysis was performed to study the respiration and feeding mechanisms and to identify key structures in these mechanisms. Our research showed that the whole body of Balitorinae acts as a suction disc, with friction-enhancing structures (unculi) on the thickened anterior rays of the paired fins. The abruptly rising head profile, supported by the extremely enlarged lacrimal bone and the flat ventral body surface facilitate effective substrate attachment. During attachment, the pelvic girdle is pulled anterodorsally, suggesting the formation of a negative pressure underneath the body. Detachment by water inflow underneath the body is prevented by three mechanisms. 1) Barbels control the water inflow by detachment and reattachment to the substrate. 2) Most water present underneath the body is removed during inspiration. 3) Excess water is regularly removed by movements of the posterior pectoral fin rays. The balitorine body is thus modified as such that it allows effective attachment, while not impairing respiration. Comparison with other teleosts living in similar environments shows that most species use more locally concentrated modifications of the paired fins and/or the mouth for attachment. The high diversity in teleostean adhesive apparatuses and associated myological modifications suggest a substantial functional convergent evolution, without necessarily highly convergent anatomical adaptations.
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