Eulimnadia texana (Packard, 1871)

Hethke, Manja & Weeks, Stephen C., 2020, Fig. 25 in Fig. 20. Sesarmops mora n in Paralbunea dayriti, Zoological Studies 59 (33), pp. 1-11 : 5-6

Eulimnadia texana

Size

PC1 and PC2 explain 82.4% and 9.9% of the variance in the log-transformed size dataset. All nine linear variables have positive loadings on PC1, which are illustrated by vectors in the biplot of figure 5A. So, all variables increase towards positive scores of PC1, which can thus be regarded as a general size axis, although log-transformation added an aspect of shape. Figure 5A also shows that specimens in cup 15 (= population density of 12.8 inds/400 ml, 14 days after hydration) mostly occupy negative scores on PC1 while single-individual cups (red) occupy positive scores on PC1. Hence, size in high-density cups is generally smaller than in low-density cups. Specimens of intermediate population densities occupy intermediate areas along PC1. Both, PC1 and PC2, are driven by Av, while PC2 is also strongly influenced by variables Arr and u.

NPMANOVA of log-transformed linear measurements indicates that differences in size are significant across different population densities ( Table 2A; p (same) = 0.0002). However, pairwise comparisons reveal that only density extremes (cups 1 and 15) yield significantly different carapace sizes compared to other population densities. Intermediate densities cannot be separated. In comparison to length ( Fig. 4 ), size (PC1 scores) by population density can also be well fitted by a linear relationship for Eulimnadia texana ( Fig. S1 ; that of Eocyzicus argillaquus is best fitted by a logarithmic relationship).

During the outlining process, we noticed that the anterior dorsal extremity (landmark D in Fig. 1 ), which highly influences values of Av, was sometimes difficult to identify. Hence, we carried out a second analysis with a reduced set of log-transformed linear measurements (H, L, Ch, u) that resulted in an overall similar plot, except that PC2 now explained a larger proportion of the variance in the dataset (17.1%). All four variables drive PC1 at almost equal proportions. H, L, and Ch increase towards positive scores of PC2 while u decreases. In general, there seems to be little difference between the analyses of nine and four linear variables. NPMANOVA yielded almost the same result except for one pairwise comparison (cups 15 and 10.1) that cannot be statistically distinguished in table 2B.

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. 4. A, Bivariate fit of max. y (estimated from von Bertalanffy equation) by standardized population density 16 days after hydration. Natural log regression provides the best fit. B, Bivariate fit of mean length by mean number of growth bands (per cup).

Fig. 1. Linear carapace variables used for A, Eocyzicus argillaquus and B, Eulimnadia texana, following Defretin-Lefranc (1965) and Tasch (1987):A, most anterior point of the valve; B, most posterior point of the valve; C, most ventral point of the valve; D, anterior extremity of the dorsal margin; E, posterior extremity of the dorsal margin; U, midpoint of the larval valve (located on the umbo, but not necessarily the midpoint of the umbo). a, vertical distance of A to A'; b, vertical distance of B to B'; c, horizontal distance of C to A''; Arr, horizontal distance of E to B'; Av, horizontal distance of D to A'; Ch, length of the dorsal margin; Cr, horizontal distance of U' to A'; u, vertical distance of Ch to highest point of the umbo; L, valve length; H, valve height.