First description of a musculoskeletal linkage in an adipose fin: innovations for active control in a primitively passive appendage

Thomas A. Stewart,Melina E. Hale

doi:10.1098/rspb.2012.2159

Abstract

Adipose fins are enigmatic appendages found between the dorsal and caudal fins of some teleostean fishes. Long thought to be vestigial, degenerate second dorsal fins, remnants of the primitive gnathostome condition, adipose fins have since been recognized as novel morphologies. Unique among the fins of extant fishes, adipose fins have uniformly been described as passive structures, with no associated musculature. Here we provide the first description of a musculoskeletal linkage in an adipose fin, identified in the sun catfish Horabagrus brachysoma. Modified supracarinalis posterior muscles insert from the dorsal midline anterior to the adipose fin by tendons onto the fin base. An additional pair of posterior adipose-fin muscles also inserts upon the fin base and lay posterolateral to the fin, superficial to the axial muscle. This musculoskeletal linkage is an evolutionary innovation, a novel mechanism for controlling adipose-fin movement. These muscles appear to exemplify two approaches by which fins evolve to be actively controlled. We hypothesize that the anterior muscles arose through co-option of an existing fin linkage, while the posterior muscles originated as de novo fin muscles. These findings present adipose fins as a rich system within which to explore the evolution of novel vertebrate appendages.

Highlights

The evolution of vertebrate morphology has involved repeated innovations of new musculoskeletal linkage systems
The musculoskeletal linkage system we identify in H. brachysoma is an innovation that reflects a gained potential to actively control fin movement and is the first such mechanism to be described in an adipose-fin system
These data complement the palaeontological record and permit specific hypotheses to be proposed about the homology of the constituent parts, their phylogenetic order of acquisition, and the selective forces that might have promoted the origination of active control in this appendage

Summary

Introduction

The evolution of vertebrate morphology has involved repeated innovations of new musculoskeletal linkage systems. Adipose fins represent an important exception; situated between the dorsal and caudal fins of many teleostean fishes, they are novel structures They have originated at least twice in actinopterygian phylogeny, once in the Otophysi clade, excluding Cypriniformes, and again in the Euteleostei, excluding Lepidogalaxias salamandroides [7,8]. These muscles are a derived, specialized condition representing a new, gained functional potential in this appendage We propose that these muscles control adipose fin position, the first description of such a mechanism in an adipose fin. These results inform general patterns of morphological and functional elaboration in novel and primitively rudimentary vertebrate appendages

Material and methods

Results

Discussion