The Oman shark, Iago omanensis, is a small, placental viviparous species encountered in great numbers in the deeper waters of the Gulf ofAqaba, Red Sea. It reproduces year-round, providing an opportunity to study ontogenesis of organ systems at various stages of development. This study examines the morphological and cytological development of the mechanoreceptive lateral line (LL) system and the electrosensoryAmpullae of Lorenzini. Female I. omanensis were collected bimonthly from the Gulf of Aqaba at depths of 300–800 m and sacrificed with an overdose of MS222. Their uteri were dissected and the embryos separated and fixed for light and electron microscopy.Atotal of 260 embryos of varying dimensions were studied. The first primordia of neuroectodermal LL neuromasts are seen in embryos of 18mmTL. These then sink into the dermis, ripen, and develop tubuli that join to form the LL canal systems, especially developed on the head. In contrast, the primordia ofAmpullae of Lorenzini start out as groups of embryonic cells situated subdermally. In embryos of 24–26mmTLinitially they develop into tubuli. With growth, the ampullar alveoli gradually widen at their ends to form the sensory epithelium. The ampullar tubuli elongate, bringing the alveoli to sites over the rostrum and head, where the ampullar capsules are formed. The presynaptic electrosensory cells are attached to afferent neural extensions forming sensory rami which extend, as in adult sharks, to the dorsal nucleus in the medulla. In preterm juveniles of 150–160 mm TL, the LL system and the Ampullae of Lorenzini are fully developed cytologically. The results of this study support the hypothesis that the LL system and electrosensory Ampullae of Lorenzini develop as separate modalities and that their structural similarity is due to the origin from the embryonic neuroectoderm. The dichotomy of their evolution occurred in very early ancestry as an ecomorphological adaptation to different sensory functions.
The Oman shark, Iago omanensis, is a small, placental viviparous species encountered in great numbers in the deeper waters of the Gulf ofAqaba, Red Sea. It reproduces year-round, providing an opportunity to study ontogenesis of organ systems at various stages of development. This study examines t...
مادة فرعية
The Anatomical Record