The Influence of Tetrodotoxin (TTX) on the Gut Microbiome Diversity, Composition, and Metabolism of Tiger Pufferfish (Takifugu rubripes)
Tiger puffer Takifugu rubripes, which is a commercially important fish species in Japan, is well known to possess a potent neurotoxin tetrodotoxin (TTX). Hatchery-reared tiger puffer are known to become non-toxic when fed non-toxic diets in an environment where TTX-bearing organisms are absent. Bearing TTX in pufferfishes has an ecological significance because TTX in pufferfishes functions for predator avoidance in their early life history. Further, TTX is physiologically functional as a stress-relieving substance by decreasing crowding stress and agonistic interactions among T. rubripes juveniles. It is known that tiger puffer spawn offshore in a bay, larvae migrate to inshore regions, and juveniles inhabit tidal flat and estuary basin as a nursery ground. Since the salinity fluctuates in the estuarine habitat, tiger puffer juveniles in the nursery ground often encounter different salinity levels. The Â fluctuation of environmental salinity may be a potential stressor to tiger puffer because they are known Â to be seawater-dwelling stenohaline species.Lee et al. (2005) explored the survival of tiger puffer juveniles from freshwater to full strength seawater, and found that juveniles can maintain blood osmolality within their physiological range between 25-100 % seawater (8.5-34.0 ppt). Therefore, we set the salinity at 8.5 ppt as the isosmotic condition, and changed salinity to 1.7 ppt (5% sea water) as low salinity stress where the salinity is at the lower limit of salinity tolerance, or to 34.0 ppt as full-strength seawater. Considering that wild juvenile tiger puffer is toxic and TTX is physiologically functional as a stress reliever, we hypothesized that there is some correlation between TTX and the preference of hypotonic conditions of juvenile tiger puffer gut microbiome. Therefore, we will examine the effects of oral TTX administration and chronic salinity changes on  diversity, composition, and functional potential of the gut microbiota of the hatchery-reared non-toxic juvenile tiger puffer.