Quantification of Ontogenetic Transitions during the Early Life of a Flatfish, Windowpane (Scophthalmus aquosus) (Pleuronectiformes Scophthalmidae)
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.Abstract
Diagram of a juvenile windowpane showing the nine body regions (A1, A2, A3, B1, B2, B3, C1, C2, C3) that were examined for the presence of scales. Morphometric measurements, dorsal fin origin to the anterior-most tip of the upper jaw (DO to UJ length) and pelvic fin lengths, are also indicated
Progression of number of anal pterygiophores with respect to natural log transformed body size (total length, mm). A cumulative normal probability curve was fit to the actual data (▪) using nonlinear regression techniques (r2 = 0.98, lower panel). The corresponding normal probability curve (mean ± 1 SE) is also shown (solid line, upper panel). The dotted line (- - - -) is drawn through the mean size at which individuals had reached the midpoint of character development (half of the total predicted number of anal pterygiophores)
Progression of number of gill rakers on the first arch with respect to natural log-transformed body size (total length, mm). A cumulative normal probability curve was fit to the actual data (▪) using nonlinear regression techniques (r2 = 0.96, lower panel). The corresponding normal probability curve (mean ± 1 SE) is also shown (solid line, upper panel). The dotted line (- - - -) is drawn through the mean size at which individuals had reached the midpoint of character development (half of the total predicted number of gill rakers)
Transitions in flexion stages with respect to natural log transformed body size (total length, mm). Normal probability curves were fit to the binary data using probit analysis and mean sizes (○) at each transition ± 1 SE were generated. Preflex = preflexion, Flex = flexion, Lflex = late flexion, and Postflex = postflexion
Transitions in eye migration stages with respect to natural log-transformed body size (total length, mm). Normal probability curves were fit to the binary data using probit analysis and mean sizes (○) at each transition ± 1 SE were generated
Transitions in scale presence in nine body regions (A1, A2, A3, B1, B2, B3, C1, C2, C3, see Fig. 1) with respect to natural log-transformed body size (total length, mm). Normal probability curves were fit to the binary data using probit analysis and mean sizes (○) at each transition ± 1 SE were generated
Changes in body proportions during development: (A) Eyed side pelvic fin (▴) and blind side pelvic fin (○) natural log transformed lengths (mm) versus natural log transformed total length (mm) during morphological progression of windowpane analyzed with simple piecewise regression resulting in the regression equation, ln(y) = m × ln(x) + b + c[ln(x)—z] × N, where y is the morphometric measurement, m is the slope prior to the infection point, x is the total length of the individual, b is the y-intercept, c is the change in slope after the inflection point, z is the inflection point, and N = 1 for (x > z) and N = 0 for (x ≤ z). The solid line is the regression line for the eyed side (r2 = 0.98) and the dotted line (- - - -) is the regression line for the blind side (r2 = 0.96). (B) Dorsal fin origin to upper jaw natural log transformed length (•, mm) versus natural log-transformed total length (mm) during morphological progression of windowpane analyzed with multiple piecewise regression resulting in the regression equation, ln(y) = m × ln(x) + b + c[ln(x)—z1] × N + d[ln(x)—z2] × P, where, in addition to the parameters defined above, c is the change in slope after the first inflection point, z1 is the first inflection point, d is the change in slope after the first inflection point, z2 is the second inflection point, N = 1 for (x > z) and N = 0 for (x ± z), and P = 1 for (x > z2) and P = 0 for (x ≤ z2). The solid line is the regression line for the eyed side (r2 = 0.78). Refer to Table 1 for the mean sizes ± 1 SE at which these characters exhibited shifts in their relationship with total length
Sequence of ontogenetic transitions in windowpane using natural log-transformed body size (total length, mm) to gauge the occurrence of events. Entire transitions are depicted for meristic, morphometric, and behavioral characters while only initial and final transitions are shown for characters described using discrete stages (mean size at transition ± 1 SE). For meristic characters (▪), the size ranges over which the entire transition from zero to a full complement of elements are shown. For morphometric characters (♦), the size ranges over which allometric shifts in the relationship between body parts are shown. For characters described using morphological stages (•), the size ranges over which the initial and final transitions between character states occur are shown. For behavioral characters (▴), the size ranges over which entire transitions, from pelagic to demersal behavior and 0–100% burial are shown. For more detailed descriptions of characters see Materials and Methods. DFO to UJ = measurement from the dorsal fin origin to the anterior edge of the upper jaw