The larval attachment organ of the Bowfin Amia ocellicauda Richardson, 1836 (Amiiformes: Amiidae) and its phylogenetic significance.

Larval attachment organs (LAOs) are unicellular or multicellular organs that enable the larvae of many actinopterygian fishes to adhere to a substrate before yolk-sac absorption and the free-swimming stage. Bowfins (Amiiformes) exhibit a sizable LAO on the snout, which was first described in the late 19 century and early 20 century. In this study, we document the LAO of Amia ocellicauda (Richardson, 1836) using a combination of scanning electron and light microscopy, and histochemistry. We examined material representing three stages with SEM ranging in size from 5.8-11.2 mm in notochord length and one stage histochemically. We compare the LAO of Am. ocellicauda to that of the lepisosteid Atractosteus tropicus Gill, 1863 and show that while the LAO of Am. ocellicauda and At. tropicus are both superorgans, the two differ in the ultrastructure of the entire organ. Amia ocellicauda possesses two distinct lobes, with the organs arranged on the periphery with none in the middle, whereas At. tropicus also possesses two lobes, but with the organs scattered evenly across the superorgan. The individual organs of Am. ocellicauda possess adhesive cells set deep to support cells with adhesive substance released through a pore, whereas At. tropicus possesses both support cells and adhesive cells sitting at a similar level, with adhesive substance released directly onto the surface of the organ. We additionally provide a table summarizing vertebrate genera in which attachment organs have been documented as well as discuss the implications of our study to hypotheses of the homology of attachment organs in the Holostei. This article is protected by copyright. All rights reserved.