MORPHOLOGY OF THE OLFACTORY APPARATUS IN LEPTOCEPHALUS LARVAE
Wightman, Molly Ann
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Leptocephalus larvae are a unique larval form that unites the Elopomorph fishes. This larval form has morphological and cellular characteristics that differ from most other teleost fishes. The visual system of leptocephali rod-dominated retinas, leading to poor photopic vision and low visual acuity that would hinder feeding on planktonic prey. Recent studies indicate they feed marine snow, which is composed of mucilaginous materials, bacteria and plankton. Their low visual acuity raises the question of how these organisms find this gelatinous food source in the wild. I hypothesize that these larval fishes use chemoreception in order to find marine snow, and that their olfactory apparatus thus differs morphologically from other teleost fishes. A variety of settlement-stage pelagic larvae were collected from Sebastian Inlet State Park in Florida and Andros Island, Bahamas. Leptocephalus larvae were represented by tarpon (Megalops atlanticus), ladyfish (Elops saurus), bonefish (Albula vulpes), and speckled worm eels (Myrophis punctatus). Three species with non-leptocephalus larvae were examined: Atlantic menhaden (Brevoortia tyrannus), Atlantic croaker (Micropogonias undulatus), and pinfish (Lagodon rhomboides). Their external olfactory and visual systems were analyzed using Scanning Electron Microscopy. Morphometric data were collected and compared to make comparisons between larval types regarding the dimensions and structures of these sensory organs. Developmental differences were also observed by comparing SEM images of larval and juvenile fishes. Three out of the four species of leptocephalus larvae had large, exposed olfactory pits, while speckled worm eels and all non-leptocephalus larvae had their olfactory apparatus embedded under the skin of the head, with distinct anterior and posterior nostrils. Although there was no consistent difference among the groups in dimensions of the olfactory apparatus, eye size, or ratio between the sensory organs, the open olfactory pits of tarpon, ladyfish and bonefish larvae would directly expose sensory cells to chemical signals from marine snow. The enclosed olfactory apparatus of the non-leptocephalus species develops during embryogenesis, suggesting that exposed olfactory surfaces are not as important as vision in detecting motile plankton prey. Speckled worm eel larvae entering the estuary had already developed closed olfactory pits and nostrils, presumably in preparation for their rapid assumption of a life style where they bury in sediments during the day. Bonefish and ladyfish showed a similar development of closed olfactory pits and nostrils after metamorphosis within the estuarine habitat. Studies of receptor densities, olfactory organ ontogeny, and testing the olfactory sensitivity of live larvae could help to further understand the life history of the Elopomorph fishes, and help with continued conservation and aquaculture efforts.