Eulimnadia texana

Hethke, Manja & Weeks, Stephen C., 2020, Fig. 25 in Fig. 20. Sesarmops mora n in Paralbunea dayriti, Zoological Studies 59 (33), pp. 1-11 : 6-8

Eulimnadia texana

Shape

According to Fourier shape analysis (PC of Fourier coefficients in figure 5B, NPMANOVA in Table 3), there is an overall significant population density effect on carapace shape (p (same) = 0.0001). Nine of fifteen pairwise comparisons exhibit significantly different shapes. Most non-significant comparisons can be explained by similar population densities 14 days after hydration, except for two comparisons that include cup 10.2 (density 7.1 inds/400 ml), for which the p -value is, however, close to the significance level (set to α = 0.05).

Eigenvalues plotted against a random model (scree plot) resulted in two significant components. PC1 and PC2 explain 39.8% and 18.3% of the variance in the Fourier dataset ( Fig. 5B ). Extreme shapes along PC1 indicate a much larger relative umbo height in low-density cups ( Fig. 6 ).

The shape plot can be further interpreted using ordinary least squares regression of PC1 and PC2 scores versus ratios u /(u + H), H / L, Ch / L ( Fig. 7 ). According to figure 7A–C, PC1 is driven by relative umbo height, while the regression is not significant for the H / L ratio. Hence, the main difference in shape is not described by the H/L ratio, which is often used as a simple shape indicator for clam shrimp. But it forms one of several ratios driving PC2 ( Fig. 7E; p <0.001). PC2 is also influenced by relative length of the dorsal margin ( Fig. 7F ), with generally shorter dorsal margins compared to length in low-density cups.

Burnaby transformation of linear measurements

In contrast, NPMANOVA of the multivariate size dataset that has been transformed using Burnaby’s method did not yield overall statistically distinct groups ( Table 4; p (same) = 0.218). Pairwise comparisons reveal just one significant pair (cups 1 and 10.1). Hence, shape variation between hermaphrodites of different population densities is too subtle to use this method for shape discrimination.

Figures

Fig. 5. Population density effect on size and shape in Eulimnadia texana. Colours represent standardized population densities 14 days after hydration that range between 1 and 12.8 individuals/400 ml. A, Biplot of principal component scores and variable vectors (loadings) on PC1 and PC2 of a set of log-transformed linear measurements. B, Fourier shape analysis. Although the analysis is shape-only, outlines have been scaled to size for better comparison with the size plot. PC1 represents relative umbo size, while PC2 is driven by several ratios, including the H/L ratio and relative size of the dorsal margin (compare with Fig. 7).

Fig. 6. Size and shape variation in Eulimnadia texana. Pictured are the two extreme shapes that represent the most negative and most positive scores on PC1 in figure 5B. Individuals have been scaled to size (population densities of 12.8 individuals/400 ml, left, and 1 individual/400 ml, right; individuals 15.1_6 and 1.1_1).

Fig. 7. Ordinary least squares regression of PC1 and PC2 scores of the Fourier shape analysis in figure 5B versus ratios u/(u+H), H/L, Ch/L. Shape change in PC1 is correlated with higher u/(u+H) ratios. The H/L ratio does not drive PC1, but it contributes to PC2.