Phylogeny and morphology of Himerometroidea (Echinodermata: Crinoidea) feather stars in Singapore

Evaluating the diversity of crinoids (Echinodermata: Crinoidea) in Singapore accurately has been challenging due in part to inadequacies in crinoid taxonomy. Most species descriptions have neither incorporated morphological variation of juveniles into diagnostic characters nor analysed species’ molecular phylogenetic affinities. In this study, we performed detailed morphological examination and phylogenetic analyses of the mitochondrial cytochrome c oxidase subunit I marker on Himerometroidea crinoids collected from the Singapore Strait, and showed that specimens could not be reliably identified to species based on existing taxonomic descriptions. Specifically, juveniles identified as Heterometra spp. and Zygometra sp., and adults as H. cf. producta, solely based on morphological features were revealed to comprise three distinct species: Zygometra cf. comata, Heterometra schlegelii, and Dichrometra sp. The juveniles were found to have sizes smaller than the ranges given in existing descriptions, with considerable variabilities in traits that limited the utility of morphology for species taxonomy. Features contributing to misidentification of specimens and their application in past species diagnoses are discussed. Revised descriptions of these species, as well as the recently revised Homalometra crenulata, are also presented. More broadly, this study emphasises the need to revisit diagnoses of crinoid species according to various ontogenetic stages and to uncover more congruent morphological characters based on a robust molecular phylogeny.