Ecology of tetrodotoxin toxicity and ectoparasite burden of Hong Kong newt populations

Many amphibians have defensive chemicals in their skin. Newts from the family Salamandridae possess a potent neurotoxin - tetrodotoxin (TTX). TTX toxicity has been studied intensively, with the aim of revealing the function and origin of TTX in this family. Research has found within- and between-population variations in North American newts like Taricha, possibly related to co-evolution with its predator, symbiotic bacteria, or several life history traits (e.g., sex and life stage). However, our knowledge about TTX ecology in the Asian newt clade is relatively limited. Additionally, a group of water mites, Lurchibates, specifically parasitizes Asian newts, but the interaction between these parasites and their hosts is poorly understood. It is hypothesized that TTX plays a role in regulating the host-parasite interaction between the newts and the mites. To fill the gap, I used the Hong Kong newt (Paramesotriton hongkongensis) and its parasitic water mite (Hygrobates aloisii) as a study system. I quantified TTX toxicity in six Hong Kong newt populations by field collection of skin samples. The ecological context of TTX variation was explored regarding different reproductive phases, sex, population, and life stage of the newts. I also surveyed the parasitism pattern of H. aloisii, evaluated its impact on the body condition of its host, and assessed if TTX toxicity helps the host defend against mite infestation. The results showed that TTX levels and parasitism on P. hongkongensis varied both spatially and temporally. TTX levels ranged from undetectable to 15.6 μg/g, with a mean toxicity level of 0.271 μg/g. Male newts possessed a higher level of TTX in the breeding season compared to the non-breeding season, while females did not. Also, the inter-sex difference was only prominent in the non-breeding season, while the TTX level was roughly equal during the breeding season. TTX levels also differ significantly within and between breeding populations. Parasitism of adult H. aloisii was limited to breeding newts and appeared to be completely absent in some populations. While the infestation intensity increased gradually, the accumulation patterns differed between the two life stages of H. aloisii at the end of the breeding season of the newts. Despite skin lesions of infested newts being observed, the results did not confirm that mite infestation severely affects the body condition of newts. TTX toxicity in P. hongkongensis does not contribute to the chemical defense against infestation by H. aloisii. This host-parasite system seems to have evolved to maintain a stable relationship, where the newts can tolerate mite infestation without severe impacts. Overall, this study illustrates the variability of TTX in P. hongkongensis at the population level and contributes to a better understanding of the ecology of TTX and host-parasite interactions for Asian salamanders.