Ocular development in the Oman shark, Iago omanensis (Triakidae), Gulf of Aqaba, Red Sea

Ocular ontogenesis was studied in embryos of the placental viviparous shark , Iago omanensis, abundant in the Gulf of Aqaba, Red Sea, at depths of 150- 1500 meters. Samples of gravid females were collected bi-monthly, and their embryos extracted. The eyes of 220 of those embryos of various dimensions were dissected and routinely prepared for histological and electron microsc opic studies. The initial signs of eyes appear in embryos of 8 mm total len gth (TL). The primordial zone of germinal neural cells appears in 12 mmTL e mbryos and at 26 mm separation of the visual layer of the retina and the pl exiform layers is initiated. From this stage on until 60 mmTL the nuclear a nd plexiform parts of the retina continue to develop and outer segments of the visual cells begin to form. Concomitant with ripening of the inner and outer plexiform layers, the tapetal layers of melanocytes and tapetal plate lets of reflecting guanine also begin to ripen. The tapetum in Iago is of t he cellular type. In embryos of 140-145 mmTL (6-7 months old), as they appr oach term, the visual cells, their synaptic connections and the intermediat e cell types of the retina are all full developed. The melanocytes, rich in pigmentation, and sacs of tapetal platelets, penetrate deeply between the lamellated outer segments of the visual cells. Data are provided on growth parameters of the retinal cell layers and growth of the eyes during embryon ic development. According to the position of the nuclei of the visual cells , the retina of Iago appears to be duplex, with rods and cones. Anat Rec 25 6:389-402, 1999. (C) 1999 Wiley-Liss, Inc.