Asymmetric craniofacial remodeling and lateralized behavior in larval flatfish

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.