Patterns of morphological and behavioral transition during the early ontogeny of windowpane, Scophthalmus aquosus, were quantified to examine the sizes (total length, TL) at which ontogenetic events took place. One of seven categorical transitions (i.e., flexion) and eight of 15 meristic and morphometric transitions (e.g., fin ray and pterygiophore development) occurred between 5 and 10 mm TL. Over 50% of the morphological characters examined were at intermediate stages of development when windowpane were between 10 and 30 mm TL (e.g., eye migration, pelvic fin asymmetry, squamation). The transitional morphology exhibited over this size range coincided with a period of behavioral transition (i.e., pelagic to demersal). By the time windowpane had reached 50 mm TL, morphological transition in all characters was complete and windowpane were capable of burying completely in substrate. Size-independent measures of variability (coefficients of variation, CVs) in character transitions were similar across character types, with the lowest CVs consistently recorded for morphometric transitions (1.73–1.81%). All transitions exhibited minor deviations from the fitted response curves (meristics, behaviors, morphometrics: r2 = 0.78–0.99 (range), P < 0.0001; morphological stages: χ2 = 4.8–25.5 (range), P < 0.05). By providing an objective framework for modeling ontogenetic progression, this study facilitates the prediction of the ontogenetic state of an individual knowing only its size and provides suggestions for establishing causal relationships between morphological, behavioral and ecological transitions in this and other species.

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