Identification of Three Species in Acanthocystis turfacea Carter 1863 Species Complex, and Notes on Its Symbiosis With Algae

Abstract

Relatively large Acanthocystis turfacea Carter 1863 are the type species of the centrohelid genus Acanthocystis Carter 1863, characterized by bifurcated spine scales. In this study, several isolates and strains identified as A. turfacea are re-evaluated based on morphological, morphometric, and molecular evidence. The Len strain corresponds to A. turfacea sensu stricto, while the Luga and Kos isolates, previously considered A. turfacea paucilituatus Nicholls 2023, are elevated to species level as Acanthocystis paucilituatus n. stat. A distinct Sin strain, with a smaller cell diameter and lacking symbiotic algae, is described here as a new species, Acanthocystis ladogensis n. sp. Molecular phylogenetic analysis based on 18S rRNA gene sequences confirms that these taxa form a well-supported clade within the genus Acanthocystis, and supports the hypothesis that bifurcations at the distal ends of spine scales represent an ancestral feature. Observations under long-term culture demonstrate that A. turfacea maintains a stable symbiotic association with symbiotic algae. The taxonomic status of the A. turfacea species complex and correlations between molecular and structural data are discussed.