Morphological and cytological ontogenesis of the ampullae of Lorenzini andlateral line canals in the Oman Shark, Iago omanensis Norman 1939 (Triakidae), from the Gulf of Aqaba, Red Sea

The Oman shark, Iago omanensis, is a small, placental viviparous species en countered in great numbers in the deeper waters of the Gulf of Aqaba, Red S ea. 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 electrosensory Ampullae of Lorenzini. Female I. omanensis were collected bimonthly from the Gulf of Aqaba at dept hs; 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 micro scopy. A total of 260 embryos of varying dimensions were studied. The first primordia of neuroectodermal LL neuromasts are seen in embryos of 18 mm TL. These then sink into the dermis, ripen, and develop tubuli that join to form the LL canal systems, especially developed on the head. Ln con trast, the primordia of Ampullae of Lorenzini start out as groups of embryo nic cells situated subdermally. In embryos of 24-26 mm TL initially they de velop into tubuli. With growth, the ampullar alveoli gradually widen at the ir ends to form the sensory epithelium. The ampullar tubuli elongate, bring ing the alveoli to sites over the rostsum and head, where the ampullar caps ules are formed. The presynaptic electrosensory cells are attached to affer ent 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 cytolog ically. The results of this study support the hypothesis that the LL system and ele ctrosensory Ampullae of Lorenzini develop as separate modalities and that t heir structural similarity is due to the origin from the embryonic neuroect oderm. The dichotomy of their evolution occurred in very early ancestry as an ecomorphological adaptation to different sensory functions. Anat. Rec. 2 52:532-545, 1998; (C) 1998 Wiley-Liss, Inc.