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

Abstract

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 19th century and early 20th 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.

Phylogenetic Classification of Living and Fossil Ray-Finned Fishes (Actinopterygii)

“…Diagnostic apomorphies. Morphological apomorphies for Holostei include (1) posterior extent of median rostral bone in adults reduced (L. Grande 2010), (2) anterior arm on antorbital with a tube-like canal (L. Grande 2010;Xu et al 2018), (3) adults with two vertebral centra fused into the occipital condyle (L. Grande 2010), (4) pterotic bone absent (L. Grande 2010), ( 5) adults with paired vomer (L. Grande 2010), ( 6) coronoid process of mandibula involves more than one bone (L. Grande 2010), ( 7) supraangular bone present (L. Grande 2010), (8) caudal region with both paired and median neural spines (L. Grande 2010), ( 9) normally all primary rays in caudal fin branched (L. Grande 2010), (10) fringing fulcra present on upper and lower margins of caudal fin (L. Grande 2010), (11) presence of anterior and posterior clavicle elements (L. Grande 2010), ( 12) four hypobranchials present (L. Grande 2010), ( 13) long nasal process that is tightly sutured to the frontals attaches immovable premaxilla to braincase (L. Grande 2010;Xu et al 2018), (14) anterior portion of premaxilla pierced by olfactory foramen and lies in nasal pit (L. Grande 2010), ( 15) sphenotic with dermal component (L. Grande 2010;Xu et al 2018), and ( 16) presence of a larval attachment organ that is a compound super-organ located at the front of the snout (Pinion et al 2023).…”