Incipient regressive evolution of the circadian rhythms of a cave amphipod

Luis Espinasa,Emily Collins,Jenna Robinson,Joseph Kopp,Jennifer Rutkowski,Anthony Finocchiaro

doi:10.3897/subtbiol.20.10010

Abstract

The habitat of cave-adapted organisms is characterized by complete darkness and in some instances, an apparent lack of environmental distinction between day and night. It is unclear if cave-adapted organisms retain circadian rhythms that can be light-entrained. Stygobromus allegheniensis (Allegheny Cave Amphipod) is an eyeless troglobitic crustacean found in caves located in the Northeastern region of the United States. Two cave populations were examined for evidence of light-entrained circadian rhythms. The first population inhabits a small tectonic cave (Ice Caves, Sam’s Point Preserve, NY) and the second (Clarksville Cave, Clarksville, NY) inhabits a long cave system in limestone rock. Experiments conducted in both the field and the laboratory suggest that the capacity to exhibit motor rhythms has been conserved in at least some individuals of both populations. Nonetheless, their motor activity rhythms have high variability of period length between individuals and do not appear to be light-entrainable. It is thus proposed that in this species, light-entrainable circadian rhythms controlling motor activity have undergone incipient regressive evolution.

Highlights

In the evolutionary history of organisms, the loss or reduction of ancestral characters is a common event that occurs when a character is no longer needed for survival
To contribute to a better understanding of this important aspect of regressive evolution, we have studied motor rhythms in the eyeless cave amphipod, Stygobromus allegheniensis Holsinger, 1967
Specimens subjected to continuous darkness in the laboratory showed periods of rest in which they were observed positioned on their side with little to no movement, followed by periods of active movement where specimens were crawling throughout the petri dish (Figures 2 and 3)

Summary

Introduction

In the evolutionary history of organisms, the loss or reduction of ancestral characters is a common event that occurs when a character is no longer needed for survival. Examples of this phenomenon in nature include the regression of pelvic and posterior appendages in whales (Bejder and Hall 2002), teeth in birds (Chen et al 2000), and eyes and dermal pigmentation in cavefish (Jeffery 2001; McCauley et al 2004). Despite the scientific community’s interest in chronobiological research in species that do not experience day and night cycles, and the fact that cave-dwelling animals represent a powerful model for understanding the evolution of biological rhythms, few studies have been conducted in this field. To contribute to a better understanding of this important aspect of regressive evolution, we have studied motor rhythms in the eyeless cave amphipod, Stygobromus allegheniensis Holsinger, 1967

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