Shell shape convergence masks biological diversity in gliding scallops: description of Ylistrum n. gen. (Pectinidae) from the Indo-Pacific Ocean

Abstract

The scallop genus Amusium Röding, 1798 is one of few genera of Pectinidae that includes taxa capable of long-distance swimming or gliding. Membership of the genus has been defined primarily by shell shape, and it currently includes only three species: the type species A. pleuronectes (Linnaeus, 1758), A. balloti (Bernardi, 1861) and A. japonicum (Gmelin, 1791). In this study, we use molecular data and aspects of shell morphology to resolve the systematics of the genus. Phylogenetic reconstruction of Pectinidae using nuclear and mitochondrial DNA sequence from four genes supports a polyphyletic Amusium. Differences in internal ribbing pattern provide morphological evidence for the recognition of the two clades identified in our phylogenetic analyses. In contrast, quantification of shell shape through geometric morphometric methods indicates that shape is a convergent phenotype and is not informative in terms of distinguishing between the two gliding lineages. Based on these results, we describe Ylistrum, n. gen, which includes two species previously assigned to Amusium. We provide characters that separate the now monotypic Amusium from the two species, Ylistrum balloti, n. comb. and Y. japonicum, n. comb.