A Modified View on Octocorals: Heteroxenia fuscescens Nematocysts Are Diverse, Featuring Both an Ancestral and a Novel Type

Abstract

Cnidarians are characterized by the presence of stinging cells containing nematocysts, a sophisticated injection system targeted mainly at prey-capture and defense. In the anthozoan subclass Octocorallia nematocytes have been considered to exist only in low numbers, to be small, and all of the ancestral atrichous-isorhiza type. This study, in contrast, revealed numerous nematocytes in the octocoral Heteroxenia fuscescens. The study demonstrates the applicability of cresyl-violet dye for differential staining and stimulating discharge of the nematocysts. In addition to the atrichous isorhiza-type of nematocysts, a novel type of macrobasic-mastigophore nematocysts was found, featuring a shaft, uniquely comprised of three loops and densely packed arrow-like spines. In contrast to the view that octocorals possess a single type of nematocyst, Heteroxenia fuscescens features two distinct types, indicating for the first time the diversification and complexity of nematocysts for Octocorallia.

Phylogenomic Analyses Support Traditional Relationships within Cnidaria

“…[ 17 ], their interpretation of anthozoan phylogeny requires that spirocysts are lost in Octocorallia. The cnidome of Octocorallia includes only a limited suite of nematocysts (none of which are unique to the group: see [ 32 , 44 ]) and no single-walled cnidae, so it is improbable that these have been transformed into another type of cnida. The alternative explanation for this feature under the preferred phylogeny of Stampar et al .…”

Phylogenomic analyses support traditional relationships within Cnidaria

“…The cnidome of Octocorallia includes only a limited suite of nematocysts (none of which are unique to the group: see [32,44] ) and no singlewalled cnidae, so it is improbable that these have been transformed into another type of cnida. The alternative explanation for this feature under the preferred phylogeny of Stampar et al [17] is that the spirocysts of Ceriantharia and of other Hexacorallia are convergent.…”

The Transcriptome of the ZoanthidProtopalythoa variabilis(Cnidaria, Anthozoa) Predicts a Basal Repertoire of Toxin-like and Venom-Auxiliary Polypeptides

“…At the tissue level, cnidarians are structurally simple, being composed only of two epithelia: ectoderm and endoderm. Organelle-like toxin-containing capsules with eversible tubules, called nematocysts, are integrated in one or both of these epithelia, depending on the taxon ( Fautin 2009 ), and impart to the organism the abilities to self-defend against predators and to capture prey ( Yoffe et al 2012 ). Despite the exceedingly simple basic body organization of cnidarians, Mackie ( Mackie 2002 ) attributed the evolutionary success of the group to nematocysts, “the cnidarians’ secret weapon,” while the mixture of toxic substances that these nematocysts contain was reported by Mariscal ( Mariscal 1974 ) as “among the largest and most complex intracellular secretion products known.” Although there are more than 11,000 cnidarian species ( Daly et al 2007 ), the vast majority of the currently reported toxins (approximately 300) were collected from a small number of well-studied species, including sea anemones, jellyfishes, sea wasps, and hydra ( Brinkman and Burnell 2009 ; Sher and Zlotkin 2009 ; Frazao et al 2012 ), while the remaining toxic peptides and proteins of the anthozoans (e.g., Palythoa and Protopalythoa ) remain understudied.…”