Ontogenesis and cytomorphology of the nasal olfactory organs in the Oman shark, Iago omanensis (Triakidae), in the Gulf of Aqaba, Red Sea.

BACKGROUND

Sharks (Selachi) are among the largest predators in deep and shallow seas, feeding on live and dead prey. Olfaction is one of the central senses by which they forage, especially at night and in deep water. The organs responsible for this function are the olfactory rosettes, which are situated in their nares. This study follows the ontogenesis and cytological development of the olfactory rosettes of the Oman shark, Iago omanensis, found in the Gulf of Aqaba, Red Sea, at depths of 150-1500 m.

METHODS

The sharks were collected bimonthly by means of a specially designed vertical standing net and sacrificed by an overdose of MS222. The olfactory rosettes were extracted from the adults and embryos, then fixed and prepared for EM and LM studies.

RESULTS

Iago is a placental, matrotrophic species with a maximal dimension of 800 mm TL (total length). It reproduces all year round, giving birth to a maximum of four (occasionally five) young of 170-180 mm TL. In newborn and adult fish the nasal olfactory organs are as described for other sharks, composed of olfactory lamellae with secondary folds. The number of lamellae increases during embryogenesis up to a maximum of 28-32 in adults. The primary nasal placodes first appear in larvae of 10-14 mm on the dorso-lateral part of the head and then become gradually displaced to the ventral position, typical for adults. Ontogenesis of the nasal rosettes is characterized by a gradual development of the lamellae and their secondary folds, with a concomitant ripening of the sensory elements (ciliated, microvillar, and rod-like bearing cells), as well as glandular and supporting cells and cells containing kinocilia that agitate the nasal water flow.

CONCLUSIONS

The released young possess functional olfactory organs and developed neural transmission across the olfactory bulb and tract, to the olfactory lobes in the brain, enabling them to forage from birth. Presented data show the occurrence in I. omanensis of two types of ciliated and microvillar cells. Ciliated and rod-bearing sensory neurons are described for the first time in sharks.