KAJIURA, stephen m; TRICAS, timothy c; FORNI, jesica b; SUMMERS, adam p; florida atlantic university; university of hawaii at manoa; university of california, irvine; university of california, irvine: Olfaction in sphyrnid sharks
Hammerhead sharks (Family Sphyrnidae) are characterized by a unique head morphology in which the olfactory capsules of the cartilaginous neurocranium are laterally expanded. The enlarged olfactory capsules contain correspondingly large olfactory organs. It has been hypothesized that the widely spaced incurrent nares and large olfactory organs of sphyrnid sharks might confer directional localization of odors and enhanced olfactory sensitivity compared to their carcharhinid relatives. We tested these hypotheses by comparing the morphology of the peripheral olfactory system in all eight extant sphyrnid species as well as two carcharhinid species. The distinct prenarial grooves, characteristic of most sphyrnid species, channel water from along the anterior margin of the cephalofoil laterally into the incurrent nares. This effectively reduces the separation distance between left and right nares to a distance comparable to that of carcharhinid sharks. In addition, although sphyrnid sharks typically possess a greater number of olfactory lamellae than carcharhinid sharks, the dorso-ventrally compressed cephalofoil contrains the proportions of the lamellae such that the total lamellar surface area does not differ between comparably sized scalloped hammerhead and sandbar sharks. An electro-olfactogram technique was employed to quantify the response of juvenile scalloped hammerhead sharks to a variety of proteogenic amino acids. Cysteine yielded the greatest response magnitude and dose-response curves indicated that this species is able to reliably detect concentrations less than 1 x 10 -7 molar. These values are comparable with values for other elasmobranch fishes and, taken in conjunction with the morphological data, seem to indicate that sphyrnid sharks might not possess any extraordinary olfactory capabilities.
