Yolk Loading in Oocytes of Darters and Its Consequences for Life-History Study

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Yolk loading in oocytes of oviparous fishes may affect size and variation of developing eggs at different stages in the production of a clutch. Three sequential stages of clutch or ovum development are recognized: mature, ripening, and ripe. We tested four hypotheses that describe differences in size and variation of the masses of developing eggs among the three stages of development: (1) yolk loading does not occur between mature-oocyte and ripe-egg stages, resulting in constant means and standard deviations of egg weights during clutch development; (2) yolk loading occurs at a similar rate for all oocytes in a clutch, maintaining a constant standard deviation as mean weight increases; (3) yolk loading favors smaller oocytes in a clutch, allowing them to catch up in weight with larger oocytes and decreasing the standard deviation as mean weight increases; (4) yolk loading occurs until oocytes reach a certain size, at which point it ceases completely in those oocytes reaching the maximum size (ceiling effect), decreasing variation as mean weight increases and resulting in a negatively skewed distribution of sizes with a blunted upper tail (heavier weights). Mean oocyte or egg mass was compared with mean oocyte or egg diameter to determine whether or not the latter measurement adequately demonstrates change in size among stages due to yolk deposition. Mean weight of developing eggs increased significantly between mature-oocyte and ripening-oocyte stages. Within-female and within-stage variances decreased significantly between stages, and skew was nonsignificant for all stages, which showed there was a catch-up and supported Hypothesis 3. Diameters of oocytes and ova increased significantly between successive stages of development. Correlations between mean diameter and mean weight of oocytes and eggs decreased successively between stages, and the difference in correlation coefficients between mature-oocyte and ripe-egg stages was significant. Based on these data, we propose standard methods for the study of propagule size in darters. Our results may be broadly applicable to other taxa of fishes.