Morphological and biomechanical changes of the feeding apparatus in developing southern flounder, Paralichthys lethostigma

The feeding biomechanics of premetamorphic, metamorphic, and postmetamorphic southern flounder, Paralichthys lethostigma, were investigated to better understand the origin and design of adult pleuronectiform feeding mechanisms. Larval P lethostigma were sampled from culture tanks every day from first feeding through metamorphosis. Fish were then fixed, cleared, and double stained for cartilage and bone. Postmetamorphic juvenile and adult fish were obtained from aquaculture facilities, fixed, and the muscles and bones of the head dissected. All fish were digitally photographed from both sides of the head. Measurements from digital images included head depth, head length, and quadratal angle (a measure of articular-quadrate position). Measurements were also made of closing in-lever, opening in-lever, and out-lever moment arm lengths for the determination of lower jaw opening and closing mechanical advantage. In premetamorphic larvae, quadratal angle increased from 40 degrees to 80 degrees, opening lever ratio increased from 0.10 to 0.37, and closing lever ratio increased from 0.06 to 0.40. From these measurements and observations of cleared and double-stained specimens, it was determined that lower jaw depression and elevation changed from a hyoid-based to an opercular-based mechanism prior to the onset of metamorphosis. With migration of the right eye to the left side of the head, quadratal angle remained relatively unchanged at 72 degrees to 84 degrees, opening lever ratio decreased from a high of 0.32 to a low of 0.14, and closing lever ratio decreased to as low as 0.17. Postmetamorphic fish exhibited little change with a quadratal angle of 83 degrees to 84 degrees, an opening lever ratio of 0.19, and a closing lever ratio of 0.17 to 0.19. Paired measurements made on the left (ocular) and right (blind) sides of the head indicated that quadratal angle was asymmetrical during metamorphosis (P = 0.003, alpha = 0.017). Mechanical advantage for lower jaw elevation was also bilaterally asymmetrical following metamorphosis (P = 0.002, alpha = 0.013). Because mechanical advantage for lower jaw depression was not directionally asymmetrical in metamorphic or postmetamorphic P. lethostigma, functional asymmetry (lateral jaw flexion) is not predicted for jaw opening. These results suggest differences in the design and function of feeding mechanisms for premetamorphic, metamorphic, and postmetamorphic P. lethostigma.