Getting to the root of fireworms' stinging chaetae—chaetal arrangement and ultrastructure of Eurythoe complanata (Pallas, 1766) (Amphinomida)

Abstract

Amphinomid species are since long known to cause urtication upon contact with the human skin. Since it has been reported that amphinomid chaetae are hollow, it has repeatedly been suggested that poison is injected upon epidermal contact. To test predictions for the structural correlate of such a stinging device we studied the structure and formation of chaetae in the fireworm Eurythoe complanata (Amphinomida). Neither the structure of the chaetae nor their formation and their position within the parapodium provide evidence for their function as hollow needles to inject poison. The chaetae even turned out to be not hollow, but containing calcareous depositions. The latter most likely cause artificial ruptures of delicate chitin lamellae in the inner of the chaeta when treated with acidic fixatives. Inorganic calcium compounds harden the chaetae and make them brittle so that they break easily. Additional information on the structure of the chaetal sac, the site of formation and the acicula do not contradict the position of the Amphinomida within Annelida as revealed by phylogenomic studies.

Are Fireworms Venomous? Evidence for the Convergent Evolution of Toxin Homologs in Three Species of Fireworms (Annelida, Amphinomidae)

“…However, conflicting evidence regarding the function of chaetae as a venom delivery apparatus and the inability to identify glandular tissue near the base of the chaetae has generated some doubts ( Eckert 1985 ; Schulze et al. 2017 ; Tilic et al. 2017 ).…”

“…2017 ). Tilic et al. (2017) concluded that the intrachaetal cavity is artificial and found no evidence of associated venom glands, which led them to suggest that the physiological reactions triggered by fireworm stings result from direct injuries rather than venom injection.…”

“…2017 ). More recently, in their ultrastructural study of Eurythoe complanata , Tilic et al. (2017) found no evidence of chaeta functioning as hypodermic needles and suggested that the chaetae are not hollow, but filled with a calcium carbonate matrix that might dissolve leaving a central cavity when exposed to acidic fixatives.…”

“…The conflicting evidence regarding the possible function of the calcareous chaetae as a toxin delivery system and the fact that no secretory tissue has been identified near the base of the chaetae has generated doubt about the venomous nature of fireworms ( Tilic et al. 2017 ).…”

A new genus and species of bristle worm from Beibu Gulf, South China Sea (Annelida, Polychaeta, Amphinomidae)

“…It has been reported that amphinomid chaetae are hollow and filled with complanine, a trimethylamine compound which is transmitted to predators and causes intense irritation through highly brittle, calcareous harpoon notochaetae (Arias 2013; Day 1967; Kudenov 1993; Penner 1970). By contrast, Tilic et al (2017) showed that chaetae of Eurythoe complanata (Pallas, 1766) are not hollow; the skin reactions are upon direct contact injury rather than from venom injections. Amphinomid species have either elongate or fusiform bodies, with caruncles generally extending posteriorly over several anterior chaetigers, branchiae ranging from bipinnate to tufts comprised of digitiform rami, with one dorsal and ventral cirrus per parapodium (Fauchald 1977; Gathof 1984; Kudenov 1995).…”