The multiple-tentacle box jellyfish, Chironex indrasaksajiae (Sucharitakul, 2017) and Chiropsoides buitendijki (Horst, 1907), are venomous species found in Thai waters. They are responsible for numerous envenomations through their stinging organelles, nematocysts. These specialized microscopic structures discharge venom, yet detailed knowledge of their types and morphology in these species remains limited. This study updates the characterization of nematocyst types and features in C. indrasaksajiae and C. buitendijki using light and scanning electron microscopy for detailed examination. Four distinct nematocyst types were identified: banana-shaped microbasic p-mastigophores, oval-shaped microbasic p-rhopaloids, sub-spherical microbasic p-rhopaloids, and rod-shaped isorhizas. In C. indrasaksajiae, banana-shaped microbasic p-mastigophores exhibited significant intraspecific variability, ranging from 30.26 µm to 102.56 µm in length and 6.42 µm to 17.01 µm in width. Conversely, C. buitendijki showed a narrower size range, 72.17 µm to 98.37 µm in length and 10.73 µm to 16.48 µm in width, based on multiple individuals. The size ranges for the other nematocyst types were consistent across both species. This study enhances the understanding of nematocyst morphology in these box jellyfish, providing a foundation for further research on venom delivery mechanisms and improved management of jellyfish envenomations in Thai waters.

Nematocysts, specialized stinging cells in cnidarians, play a crucial role in both defense and prey capture, containing venomous, coiled tubes within a capsule. While box jellyfish are recognized as a medical threat, information on the nematocysts of species like Gershwinia thailandensis and Morbakka sp. from Thai waters remains sparse. This study explores the types and morphology of nematocysts found in the tentacles of these species using light and scanning electron microscopy. We identified three nematocyst types: club-shaped microbasic p-mastigophores, oval isorhizas, and oval microbasic p-rhopaloids. Notably, significant differences in capsule sizes were observed, especially in the microbasic p-mastigophores and isorhizas. The discharge tubules tapered from the proximal to the distal ends, featuring arrow-shaped spines in a helical pattern. A distinct lancet structure was present in both microbasic p-mastigophores and p-rhopaloids. These findings suggest that variations in nematocyst size and morphology may be linked to evolutionary adaptations, functional roles, and venom toxicity. Further research into venom discharge mechanisms could offer valuable insights into the ecological and medical importance of these cnidarians.