Cnida Morphology as Taxonomic Tools within Tube-Dwelling Anemones (Ceriantharia, Cnidaria)

Abstract

Cnidome morphology appears to be a valuable tool for anthozoan (Octocarallia, Ceriantharia, and Hexacorallia) taxonomy. Ceriantharian cnidomes consist of b-mastigophores, isorhizas, spirocysts, and ptychocysts, including different subtypes. The b-mastigophores are the most valuable ceriantharian cnidae for species identification. The Ceriantharian b-mastigophore terminology is congruent to the b-mastigophores of Carlgren, b-rhabdoids, including the “faltstück” of Schmidt, spirulae, and also potentially the penicilli of den Hartog. The apical tightly folded, inverted b-mastigophore shafts, the “faltstück”, are important species-specific characteristics due to their various patterns. The mesenterial structures known as craspedonemes, cnidorages, and acontioids also have high taxonomic value. Drop-shaped b-mastigophores might be characteristic nematocysts for mesenteries. The occurrence of isorhizas and striations on the inner ptychocyst capsule walls are other species’ characteristics. The morphological similarities of spirulae and penicilli to b-mastigophores within Hexacorallia are highlighted by naming spirulae and penicilli as b-mastigophores/spirulae and b-mastigophores/penicilli subtypes, respectively. The slight morphological distinction between spirulae and penicilli will doubtfully justify the suborders Spirularia and Penicillaria. The spirocysts presence in Ceriantharia and Hexacorallia indicates a closer relationship between Ceriantharia and Hexacorallia than between Ceriantharia and Octocorallia. Octocorallia are the only anthozoans without spirocysts. This work underlines the importance of cnidome morphological analysis for taxonomic identification and classification.