Anatomy and evolution of the pectoral filaments of threadfins (Polynemidae)

Abstract

The most remarkable anatomical specialization of threadfins (Percomorphacea: Polynemidae) is the division of their pectoral fin into an upper, unmodified fin and a lower portion with rays highly modified into specialized filaments. Such filaments are usually elongate, free from interradial membrane, and move independently from the unmodified fin to explore the environment. The evolution of the pectoral filaments involved several morphological modifications herein detailed for the first time. The posterior articular facet of the coracoid greatly expands anteroventrally during development. Similar expansions occur in pectoral radials 3 and 4, with the former usually acquiring indentations with the surrounding bones and losing association with both rays and filaments. Whereas most percomorphs typically have four or five muscles serving the pectoral fin, adult polynemids have up to 11 independent divisions in the intrinsic pectoral musculature. The main adductor and abductor muscles masses of the pectoral system are completely divided into two muscle segments, each independently serving the pectoral-fin rays (dorsally) and the pectoral filaments (ventrally). Based on the innervation pattern and the discovery of terminal buds in the external surface of the filaments, we demonstrate for the first time that the pectoral filaments of threadfins have both tactile and gustatory functions.

Molecular phylogeny of the threadfin fishes (Polynemidae) using ultraconserved elements

“…Threadfins are thought to use their filamentous fin rays to assist in the detection of food in areas with low visibility, as these fishes splay their fin rays away from the body while swimming near the bottom of their environment (Motomura et al, 2002). The recent study by Presti et al (2020) identified tastebuds on the surface of these filaments, further supporting the hypothesis that threadfin fishes use their fin rays to detect food. Polynemids are found in many of the world's tropical and subtropical environments, with the majority of species inhabiting coastal areas within the Indo-Pacific (Motomura, 2004).…”

“…The recent study by Presti et al . (2020) identified tastebuds on the surface of these filaments, further supporting the hypothesis that threadfin fishes use their fin rays to detect food. Polynemids are found in many of the world's tropical and subtropical environments, with the majority of species inhabiting coastal areas within the Indo‐Pacific (Motomura, 2004).…”

Morphological evolution and diversity of pectoral fin skeletons in teleosts

“…The other example is threadfins which have specialized free fin rays on the posterior side of the pectoral fin like the redwing searobin. Although two regions of normal fin rays and free fin rays are separated from each other, they possess only four proximal radials and accommodate the separation of fin rays by enlargement of the PR4 (Presti, Johnson and Datovo, 2020). Despite drastic morphological changes, the number of proximal radials is fixed at four in these species.…”

Direct development of the catfish pectoral fin: An alternative pectoral fin pattern of teleosts

“…The anterior margin of the scapulocoracoid contacts the developing intramembranous ossification-that is, the cleithrum bone. 2,4,5,11,18,46,47 The larval muscles are present as two main masses (adductor and abductor) separated by the endoskeletal disk. 12,28 The larval fin innervation is associated with the later muscle differentiation and consists of the first four anterior nerves entering the dorsal and ventral regions of the fin and forming the diffuse network in the muscles and fin membrane.…”

“…2,5,6 Given that the fin soft tissues evolve and develop with the skeleton under a precise coordination, 7 teleosts display relatively constant patterns of the fin musculature, 8 vascular system, 9 and innervation. 10,11 The teleosts' pectoral fin pattern is considered a rather conservative not only anatomically, but also developmentally. 5,7 It is widely accepted that pectoral fin has two distinct developmental states: larval and adult.…”