Abstract
Costly anti-predator traits tend to be expressed only in high-predation conditions. For the cyprinid fish genus Carassius, deeper body depth is more adaptive to avoid predation by gape-limited piscivorous fish, but it raises swimming costs. It is therefore predicted that the relative body depth will decrease when the prey fish has reached a size larger than the predator gape-size. This prediction was tested by allometric analysis of the relation between body depth and standard length of triploid asexual females of the Japanese crucian carp (Carassius auratus sspp.) sampled from 13 geographic populations. The overall allometric relation was not significantly different from isometry. The estimate of the common major-axis slope was close to 1 (near-isometry). The mean relative body depth differed significantly among populations. A significant positive correlation was found with the mean annual air temperature. The geographic variation suggests that local selection pressures vary. In conclusion, the hypothesis that larger fish will have lower body depth was not supported, perhaps indicating that deep body depth in large fish is adaptive for some reason other than defense against piscivorous fish.
Highlights
Longitude, °E be explained by unknown environmental differences among ponds or by the small sample size
The defense-allometry hypothesis predicts that the body depth of the large carp (>7 cm in standard depth) has a lower growth rate than the body length to reduce the expression of the costly anti-predator morphology
Contrary to that prediction, results show that the allometric relation estimated by the major-axis and instrumental-variable regression was almost isometric for triploid Japanese crucian carp in in 13 predator-sympatric populations
Summary
Longitude, °E be explained by unknown environmental differences among ponds (e.g., food availability, temperature) or by the small sample size. We test the defense-allometry hypothesis for body-depth morphology using geographic populations of Japanese crucian carp (Carassius auratus sspp.). If we apply linear regression analysis, the slope estimate is biased downward[25] When both variables are measured in the same units, as is true in the morphometry, the major-axis regression is a recommended method to estimate and to test the logarithmic linear functional relation in analyses of allometry[25,26]. Instrumental-variable regression ( known as “the covariance ratio method”27) is another recommended method that does not assume equal residuals, but which requires a third measurement of the “instrumental” variable (another morphological trait)[27,28] We use both major-axis and instrumental-variable regressions for logarithmic line-fitting in allometry analyses of the body depth of Japanese crucian carp
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