Evidence of accumulation of tetrodotoxin (TTX) in tissues and body parts of ectoparasitic copepods via their feeding on mucus of TTX-bearing pufferfish

De novo transcriptomic analysis to identify candidate genes potentially related to host recognition during infective stage of Caligus fugu (Crustacea: Copepoda)

Attachment site recognition for infecting a definite host is crucial for obligate ectoparasites. Caligus fugu, a marine copepod parasite primarily found on pufferfishes, has a unique host-/organ-specificity. The infective stage of the species exclusively infects host fins as the attachment site. However, so far, little is known about the mechanism underlying the specificity of C. fugu. In this study, transcriptomic profiles of the infective first copepodid stage (CI) along with the pre- and a post-infective stage, i.e., second nauplii (NII) and second copepodids (CII), respectively, were investigated. The de novo assembled transcriptome of C. fugu showed that a high number of transcripts showed high homology to those of relative species, Caligus rogercresseyi (94.7 %) and Lepeophtheirus salmonis (91.0 %), suggesting that only a small portion of species-specific genes contribute to interspecific differences, such as host-seeking. Importantly, no gene was noted from the odorant receptors and gustatory receptors families in C. fugu transcripts, similar to L. salmonis genome. Genes related to chemosensing such as the ionotropic glutamate receptors (iGluR) or ionotropic receptors (IRs), viz., GRIA2, GRIA3, GRID2, GRIK2, GRIK3, IR21a, IR25a, IR40a, and IR93a, likely involved in host-seeking, were highly expressed during CI among the three stages. In addition, inositol 1,4,5-triphosphate receptor-associated 2, another potential candidate gene involved in host-seeking, was significantly upregulated in CI compared with that in NII and maintained at the same level in CII. Our present transcriptomic data should offer a foundation for further investigations on various biological aspects, such as the host-/organ-specificity of sea lice.

Tetrodotoxins in Larval Development of Ribbon Worm Cephalothrix cf. simula (Palaeonemertea, Nemertea)

The toxic ribbon worm, Cephalothrix cf. simula (Palaeonemertea, Nemertea), possesses extremely high concentrations of tetrodotoxin (TTX). Although TTX has been found in the eggs of this species, the fate of the toxin in the ontogeny of the animal has not been explored. Here, using high performance liquid chromatography with tandem mass spectrometry and immunohistochemistry with anti-TTX antibodies, we examined levels, profile, and localization of TTX and its analogues (TTXs) in larvae of C. cf. simula throughout 41 days post-fertilization. A detailed investigation of cells in sites of TTX-accumulation was performed with light and electron microscopy. Newly hatched larvae possessed weak TTX-like immunoreactivity in all cells. With subsequent development, intensity of TTX-labeling in the ectodermal structures, mesodermal cells and apical cylinder of the apical gland increased. In the ectodermal structures, an intense TTX-labeling was observed in the multiciliated, type II granular, type I mucoid, and basal cells of the epidermis, and in the type III granular cells of the mouth gland. In the mesoderm, TTX was localized in the muscle and unigranular parenchyma-like cells. Eggs and larvae of C. cf. simula contained five TTXs, with two major toxins - TTX and 5,6,11-trideoxyTTX. Level and relative proportion of TTXs did not differ significantly among developmental stages, confirming that larvae obtained toxins from maternal eggs and were able to retain it. The results of this study provide insights into the formation of TTX-bearing apparatus of C. cf. simula through the larval development.

The quite low cross-reactivity of Kawatsu's anti-tetrodotoxin monoclonal antibody to 5,6,11-trideoxytetrodotoxin, 11-nortetrodotoxin-6(S)-ol, and 11-oxotetrodotoxin, the major tetrodotoxin analogues in pufferfish

The monoclonal antibody against tetrodotoxin (TTX), prepared by Kawatsu et al. (1997), has been used in several TTX-related studies. Herein, we confirmed the quite low cross-reactivity of this antibody to three major TTX analogues in pufferfish using competitive ELISA: 5,6,11-trideoxyTTX (<2.2%), 11-norTTX-6(S)-ol (<0.3%), and 11-oxoTTX (<1.5%), with reactivity against TTX being 100%. We further confirmed that the presence of these analogues did not cause a marked overestimation of TTX in pufferfish extracts using competitive ELISA.

An Overview of the Anatomical Distribution of Tetrodotoxin in Animals

Tetrodotoxin (TTX), a potent paralytic sodium channel blocker, is an intriguing marine toxin. Widely distributed in nature, TTX has attracted attention in various scientific fields, from biomedical studies to environmental safety concerns. Despite a long history of studies, many issues concerning the biosynthesis, origin, and spread of TTX in animals and ecosystems remain. This review aims to summarize the current knowledge on TTX circulation inside TTX-bearing animal bodies. We focus on the advances in TTX detection at the cellular and subcellular levels, providing an expanded picture of intra-organismal TTX migration mechanisms. We believe that this review will help address the gaps in the understanding of the biological function of TTX and facilitate the development of further studies involving TTX-bearing animals.