Two types of uncini in Polycirrus (Polychaeta: Terebellidae: Polycirrinae) revealed using geometric morphometrics

Abstract

Geometric morphometric methods were used to investigate shape variation in the neuropodial chaetae (uncini) of the polychaete genus Polycirrus (Terebellidae: Polycirrinae). Illustrations of 82 uncini from 47 specimens/taxa (one to three uncini per specimen/taxon) were digitized and 10 landmarks, representing putatively homologous points, were identified on the outline of each uncinus. Two distinct types of uncini were identified and described graphically using a Principal Component technique (Relative Warps Analysis) and non‐metric ordination (multi‐dimensional scaling). Type 1 uncini with a short occipitium and flat base and Type 2 with a long occipitium and arched base represent states of a new character available for future phylogenetic studies of the group. The shape of the uncini alone subdivided the genus into two groups, one containing 14 taxa and another with 33 taxa. The type of uncini correlated well with the presence of long pinnate notochaetae and the last occurrence of notopodia, enabling further subdivision of the genus into four provisional groups; a key is provided to distinguish the groups. The landmarks identified on the surface of Polycirrus uncini are considered homologous with those on the uncini of other Terebellidae and therefore landmark‐based geometric morphometrics could be applied to investigate shape changes in other uncini‐bearing Terebellidae.