Females of tetrodotoxin-containing ribbon worms of Cephalothrix simula species complex (Palaeonemertea, Nemertea) keep high toxicity and produce toxic eggs during long-term captivity.

Tetrodotoxin (TTX), also known as puffer fish venom, has gained wide popularity in recent years as an effective tool in the field of physiology, as well as a promising anesthetic and analgesic agent. However, to date, no economically viable method of TTX production has been found. In this work, using high-performance liquid chromatography with tandem mass spectrometry, we conducted a study of the dynamics of the content and ratio of TTX and its analogues (TTXs) in the eggs of captive nemerteans of the Cephalothrix simula species complex, as well as a study of TTX localization in nemertean eggs using confocal laser scanning microscopy. It has been shown that nemertean eggs and nemerteans themselves, when kept in captivity for a long time, can contain concentrations of TTXs comparable to individuals of the wild population. It was found that TTX in eggs is associated with yolk granules, for which morphological characterization was carried out for the first time. The data obtained in the work can be used as a basis for the development of a technique for keeping animals in captivity for as long as possible while maintaining/increasing their toxic potential for further toxin extraction from egg yolk.