Migratory Restlessness and the Role of Androgen for Increasing Behavioral Drive in the Spawning Migration of the Japanese eel.

Ryusuke Sudo,Katsumi Tsukamoto

doi:10.1038/srep17430

Abstract

Migratory restlessness refers to a type of locomotor activity observed just before the onset of a migration. This behavior is primarily known in birds, where it is considered to be an indicator of the urge for migration. In contrast, little is known about migratory restlessness in fishes. To confirm migratory restlessness in a fish, we measured the locomotor activity of the Japanese eel, Anguilla japonica during its migration season. Migratory-phase silver eels showed higher locomotor activity in aquaria than yellow eels at the non-migratiory growth-phase. Silver eels stayed outside of their shelters for longer durations in dark periods than yellow eels and were active even in light periods when yellow eels were inactive in the shelters. Silver eels had higher levels of the androgen hormone 11-ketotestosterone at the end of experiment than yellow eels. Administration of 11-ketotesosterone to yellow eels induced higher levels of locomotor activity than that observed in non-treated controls. These findings suggest that anguillid eels exhibit migratory restlessness just before their spawning migration and that 11-ketotestosterone may be involved in the onset of this behavior.

Highlights

Migratory restlessness is the seasonally occurring behavior of caged migratory birds that is expressed by high locomotor activity during migration seasons and is considered to be an indicator of the urge for migration[1]
Triggering factors for their spawning migration[7]. These findings suggest that silver eels exhibit migratory restlessness before starting their spawning migration
For confirming whether migratory restlessness in eels is reflecting internal drive for spawning migration or not, we have to demonstrate that higher activity only occurs in migratory silver eels that are held in tanks isolated from meteorological factors

Summary

Introduction

Migratory restlessness ( known as Zugunruhe) is the seasonally occurring behavior of caged migratory birds that is expressed by high locomotor activity during migration seasons and is considered to be an indicator of the urge for migration[1]. After a long period of growth in inland waters or estuaries, yellow eels undergo marked morphological and physiological changes as they transform into the silver eel phase, which commence to migrate back to the open ocean to spawn[3,4]. For confirming whether migratory restlessness in eels is reflecting internal drive for spawning migration or not, we have to demonstrate that higher activity only occurs in migratory silver eels that are held in tanks isolated from meteorological factors. The objectives of this study were to confirm the presence of migratory restlessness in eels and to examine the role of androgens in the drive to start the spawning migration in eels by measuring the locomotor activity of 11-KT treated eels

Objectives

Methods

Results

Conclusion