Hydractiniid hydroids display a range of morphological variation from sheet-like forms (i.e. closely spaced polyps with high rates of stolon branching) to runner-like forms (i.e. widely spaced polyps with low rates of stolon branching), thus exemplifying the patterns of heterochrony found in many colonial animals. A sheet-like and a runner-like inbred line of Podocoryne carnea were produced to investigate this heterochronic variation further. Selection on colony morphology at the time of the initiation of medusa production resulted in dramatic differences by the F5 and F6 generations. Compared with colonies of the sheet-like inbred line, runner-like colonies exhibited smaller sizes at the initiation of medusa production, more irregular colony shapes and diminished stolon development relative to polyp development. In addition to these differences in colony morphology, runner-like colonies also exhibited larger medusae and a greater amount of gastrovascular flow to the peripheral stolons. To assess differences in the metabolic capacity underlying this variaton in flow, the redox state of the polyp epitheliomuscular cells was measured using the fluorescence of NAD(P)H. In response to feeding-induced changes in gastrovascular flow, runner-like colonies show greater redox variation than sheet-like ones, plausibly corresponding to the greater amounts of flow generated by the former colonies relative to the latter. Perturbing the system with dilute solutions of 2,4-dinitrophenol similarly indicates that runner-like colonies contain more functionally oxidizable NAD(P)H. The correlation between gastrovascular flow and morphological differences supports the hypothesis that the former mediates the timing of colony development, perhaps in concert with the observed variation in the redox state of polyp epitheliomuscular cells.

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