Structure and function of the median finfold in larval teleosts

Abstract

SUMMARYThis paper offers a structural and mechanical analysis of the median finfold in larval teleosts. The median finfold is strengthened by bundles of collagen fibres, known as actinotrichia. We demonstrate that these structures contribute to increase the mass of backward accelerated water during swimming. The amount, dimensions, orientation and growth of actinotrichia were measured at various locations along the finfold in several developmental stages of common carp (Cyprinus carpio) and zebrafish (Danio rerio). Actinotrichia morphology, using light microscopy (e.g. diameter, orientation) and electron microscopy (which revealed their anchoring at proximal and distal ends), correlated with expected lateral forces exerted on the water during swimming. An analytical model is proposed that predicts the extent of camber from the oblique arrangement of the actinotrichia and curvature of the body. Camber of the finfold during swimming was measured from high-speed video recordings and used to evaluate the model predictions. Based on structural requirements for swimming and strain limits for collagen, the model also predicts optimal orientations of actinotrichia. Experimental data confirm the predictions of the model.

Fin modules: an evolutionary perspective on appendage disparity in basal vertebrates

“…Myxiniformes also often possess long median ventral finfolds, which although sometimes interrupted around the cloaca, are continuous with the caudal fin. These elongated median fins are reminiscent of the median larval finfold observed during the early ontogeny of more advanced fishes [150, 151]. They are also reminiscent of the extensive dorsal and ventral finfolds found in the more basal cephalochordates, which are continuous around the tail, but also around the anterior tip of the notochord [152–154].…”

“…Although patterning modules refer to the development of endo- and exoskeletal supports [39, 40, 219], the effect of the patterning module could extend to the resorption of the larval median finfold. More precisely, during fish larval development, there are generally dorsal and ventral median finfolds that are continuous with a caudal finfold [150, 151]. During development, these finfolds are resorbed except in places where dorsal, anal, and caudal fins will develop [151].…”

Early development of the longsnout seahorse Hippocampus reidi (Syngnathidae) within the male brood pouch

“…The functional significance of the larval median fin fold remains unresolved. However, several hypotheses have been proposed: the fin fold might be required for effective undulatory swimming, or could enhance respiration in pre‐hatched larvae (Van den Boogaart et al, ). Van den Boogaart et al .…”

“…Van den Boogaart et al . () explains that development of the primordial fin fold reflects the demands imposed by changes occurring in swimming style during ontogeny. Absence of the primordial fin fold in seahorse and pipefish (Syngnathidae) larvae is probably due to the lack of swimming activity within the brood pouch.…”

“…The functional significance of the larval median fin fold remains unresolved. However, several hypotheses have been proposed: the fin fold might be required for effective undulatory swimming, or could enhance respiration in pre-hatched larvae (Van den Boogaart et al, 2012). Van den Boogaart et al (2012) explains that development of the primordial fin fold reflects the demands imposed by changes occurring in swimming style during ontogeny.…”