Abstract
Flatfishes, such as flounder, are the world's most asymmetric vertebrates. It is unknown if the development of lateralized swimming behavior during metamorphosis is an adaptive response to bilaterally asymmetric eye positioning, or if this results from a vestibular response to thyroid hormone. This study describes larval development in left-sided, right-sided and bilaterally symmetric variants of southern flounder (Paralichthys lethostigma). Behavior and skull asymmetries precede metamorphosis, and the development of lateralized behaviors was independent of eye position in larvae treated with thyroid hormone and in symmetrical variants. Therefore, lateralized behavior is not an adaptive response to eye translocation, but rather must result from changing vestibular responses to thyroid hormone.
Highlights
The flatfishes (Order Pleuronectiformes) have the most asymmetric shape and lateralized behavior of any vertebrate
Flatfish are a natural paradigm for studying the development of behavioral and morphological asymmetry, as their larvae initially resemble typical fish with upright swimming and bilaterally symmetric bodies, but metamorphose abruptly into fish that swim on one side and lie on the ocean floor
The first part of this study dissociates eye migration from tilted swimming and settling to show that these adult behaviors result from a response to thyroid hormone that is independent of eye migration in the southern flounder (Paralichthys lethostigma)
Summary
The flatfishes (Order Pleuronectiformes) have the most asymmetric shape and lateralized behavior of any vertebrate. Flatfish metamorphosis is mediated entirely by thyroid hormone (TH; Inui and Miwa, 1985; Miwa et al, 1988; Schreiber and Specker, 1998). It is not clear if flatfish tilted swimming and settling behaviors are adaptive responses to changing eye position, or if these behaviors result from a vestibular response to thyroid hormone. The dorsal light reflex, which causes a typical upright swimming fish to tilt towards a lateral light source until equilibrium is established between visual and gravitational sensory input, may play a role in the development of flatfish lateralized behaviors as the migrating eye changes the perceived angle of light incidence (Graf and Baker, 1990; Neave, 1985). The first part of this study dissociates eye migration from tilted swimming and settling to show that these adult behaviors result from a response to thyroid hormone that is independent of eye migration in the southern flounder (Paralichthys lethostigma)
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