Morphology and molecular phylogeny of two new species in genus Freyastera (Asteroidea: Brisingida: Freyellidae), with a revised key to close species and ecological remarks

Freyastera (Downey, 1986) is a derived group of deep-sea starfish in the order Brisingida with 6 arms and slender ambulacral plates, whose taxonomy and phylogeny have rarely been investigated. In this research, two new species of Freyastera are reported from the northwest Pacific Ocean, both characterized by fairly long and numerous lateral spines and the presence of furrow spines. Freyastera delicata n. sp. is further distinguished from other species by having 6-8 spinelets arranged in a transverse line on each abactinal arm plate. Freyastera basketa n. sp. is similar to the Atlantic species Freyastera sexradiata (Perrier, 1885), but differs from the latter by the presence of pedicellariae on the abactinal surface and number of mouth spines and lateral spines. A revised key to Freyastera and potential Freyastera spp. from the genus Freyella (Perrier, 1885) is provided. Phylogenetic analysis was conducted using available molecular data for the order Brisingida. The intraspecific and interspecific divergence within Freyastera are recognized from a COI genetic distance matrix, in which some abnormal intraspecific distances are found between the known species, which might indicate the existence of cryptic species and an underestimated diversity in the genus. A phylogenetic tree of Brisingida is constructed based on a concatenated sequence dataset of COI, 16S and H3 gene fragments. The tree supports the affinity of Freyastera and Freyella, which together form a crown Glade within the order. The global distribution and in situ observations of Freyastera are discussed to provide information on their ecological characteristics and diversity.