A multivariate approach to morphological study of shell form in cowries (Gastropoda, Cypraeidae): a case study with Umbiliaarmeniaca (Verco, 1912)

Abstract

Multivariate approaches to morphological study of shell form have rarely been applied to cowries (Gastropoda: Cypraeidae) with preference, instead, for comparing formulaic notations of shell form that report averages (i.e., means) for key morphometrics such as shell dimensions, their ratios, and counts of apertural teeth. Although widely applied, the "shell formula" does not account for variation among individuals or support statistical comparison between taxa. This study applied a multivariate approach to analyse shell form within the four accepted subspecies of the cowrie, Umbiliaarmeniaca (Verco, 1912) and included a previously unstudied, and most northerly, population of U.armeniaca from Lancelin, Western Australia. Multivariate analyses readily separated the recognised subspecies of U.armeniaca (U.a.armeniaca, U.a.diprotodon, U.a.clarksoni and U.a.andreyi), but did not separate the Lancelin population from U.a.andreyi, indicating that the former represents a northward extension of U.a.andreyi that is not morphometrically distinguishable. These results provide improved understanding of infraspecific differences in shell form of U.armeniaca across its broad distribution, and demonstrate the utility of multivariate morphometric methods for statistical comparison of shell form between taxa. This approach is complimentary to existing research practices and has broad potential application in future morphometric studies of both extant and fossil taxa within the family Cypraeidae.

Keywords: Cowry; Umbilia; gastropod; marine; morphometrics; shell form; taxonomy.

Figure 1.

Approximate distributions of the four recognised subspecies of Umbiliaarmeniaca and the newly discovered Lancelin population of the species.

Figure 2.

Specimens of the four recognised subspecies of UmbiliaarmeniacaAU.armeniacaarmeniaca, trawled off Ceduna, Great Australian Bight, 90–120 m, 105 mm BU.armeniacadiprotodon, taken by diver, Thorny Passage, Port Lincoln, South Australia, 35 m, 102 mm CU.armeniacaclarksoni, taken by diver off Cape Le Grande, Esperance, Western Australia, 30–35 m, 94.1 mm DU.armeniacaandreyi, collected using ROV, off Augusta, Western Australia, 150 m, 84.2 mm and EU.armeniaca from the Lancelin population, collected using ROV, off Lancelin, Western Australia, 200 m, 68.9 mm.

Figure 3.

Diagrammatic representation of how a multivariate approach to morphological study of shell form for cowries was applied in this study, using Umbiliaarmeniaca as an example (see Suppl. material 1 for an outline of code used) A data were sourced by examining unstudied specimens (primary data) or sourced from prior studies (secondary data) B data for morphometrics deemed representative of shell form were C transformed to Z-scores and atypical specimens either validated (primary data) or censored (secondary data) before D computing a resemblance matrix based on Euclidean distance between specimens E non-metric multidimensional scaling (nMDS) was then used for dimensionality reduction to F permit visualisation in a new space of two dimensions and G statistical testing was employed to validate visual observations by estimating the probability that a priori assigned groups (taxa or populations) shared the same centroid and dispersion within multidimensional space.

Figure 4.

AnMDS ordination (stress = 0.16) of the resemblance matrix for Umbiliaarmeniaca, where shaded ellipses indicate the 95% confidence interval of group (subspecies or population) centroids and plot characters indicate data source B–H Associations between ordination structure and morphometrics influencing this structure, where the green lines illustrate B length C height:length ratio D width:length ratio E height:width ratio F normalised columellar tooth count G normalised labral tooth count, and H relative mass contour lines.

Figure 5.

Box plots showing univariate comparisons of A shell length B height:length ratio C width:length ratio D height:width ratio E normalised columellar tooth count F normalised labral tooth count, and G relative mass among the studied Umbiliaarmeniaca groups (subspecies or population). Black diamonds represent group means, boxes illustrate first and third quartile as box edges and median as central line. Shared alphabetic superscripts identify group means that are not statistically different (Holm-adjusted P ≥ 0.01) for a morphometric.