Puffer fish, a delicacy in Japanese cuisine, contain a lethal toxin that can lead to food poisoning and a swift death if improperly prepared. The fish’s liver and ovaries contain a powerful neurotoxin called tetrodotoxin (TTX). Though its purpose is unclear, puffer fish also have analogs of TTX that are non-toxic. Researchers at Nagoya University have proposed that puffer fish use these toxin analogs to communicate using smell.
For the research, the team studied grass puffer fish. Grass puffers are considered a problem for recreational anglers as they eat bait and break fishing lines. Others enjoy their appearance and some people keep them as pets.
Since childhood, Professor Hideki Abe was delighted by the grass puffer’s “charming appearance.” As a biologist, he was more intrigued by their spawning behavior. “I was fascinated by the hypothesis that their toxin may be involved in their spawning behavior as a pheromone,” he explained. Pheromones are chemicals that are perceived by fish using their olfactory senses and affect the behavior of others.
The grass puffer accumulates the neurotoxin TTX to discourage would-be predators. They also accumulate a non-toxic analog of TTX called TDT. To better grasp this phenomenon, the research team investigated the compound. The results showed that TDT serves as a smell that attracts other grass puffers and the olfactory sensory cells that they use to sense it.
Surprisingly, the team discovered that rather than responding to the toxic TTX, the puffer fish responded to its non-toxic analog, TDT, implying that it behaves as an odorant that attracts grass puffers.
The attractive properties of TDT were studied by putting grass puffers into a fish tank, and then introducing it to only one side. Responding to the chemical, the grass puffers gathered in the part of the tank where it was added. The results were not expected given previous TTX studies.
“The TTX used in earlier studies was derived from living organisms and had low purification; therefore, we assumed that it might not only contain TTX but other analogs, including TDT, which might have actually induced the odor response,” said Professor Abe.
By using pure TDT, long-held beliefs about the role of TTX in pheromone signaling may be overturned.
“Prey animals contain both the toxin and its analog, so grass puffers may use the smell of TDT as a signal to find prey animals bearing TTX, such as flatworms, starfish, gastropods, and skeleton shrimp,” said Professor Abe.
Because TTX and TDT are located in the ovaries, it is released from the eggs of a female during spawning. This was found to protect eggs from predators while also attracting males.
These results are a new perspective and clarify the biological functions of the non-toxic analog. The researchers expect their work to impact future research on the reproductive behavior of puffer fish.
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