Life can sometimes be pretty dangerous for larval California newts(Taricha torosa). If the adults' favourite diet of worms is scarce,they cannibalise their larvae for an alternative snack. Adult newts in turn avoid being munched by secreting the paralysing poison tetrodotoxin (TTX) from their skin as a defence against predators. Richard Zimmer from the University of California, Los Angeles, was surprised to see that when larvae caught a whiff of TTX they would flee to the nearest shelter, suggesting that TTX was also acting as a warning signal. By delving into the newts' complex chemical world, Zimmer and his colleague Ryan Ferrer found that interactions between different chemicals affect the behaviour of both larval and adult newts, in very different ways.
The team were alerted to the fact that the story might be more complicated when they saw research by Jacob Kerby and Lee Kats showing that when adults are dining on their favourite worms, larvae stay put. Worm body fluid contains a lot of the amino acid arginine, which is also a feeding cue in aquatic systems. Could arginine be suppressing the larval newts' response to cannibalistic adults? Similarities in the chemical structure between arginine and TTX led Zimmer and Ferrer to suspect that these two chemicals were interacting and somehow affecting larval behaviour.
Following their hunch, the team collected eggs from the field and hatched them back in the lab to find out how the larvae would react to TTX and arginine (p. 1768). They placed the larvae in specially designed flow tanks, and targeted a stream of TTX solution towards them, finding that TTX on its own caused the larvae to swim for shelter, as they expected. When they blocked their noses with inert silicon gel, the larvae didn't flee, showing that they were `smelling' TTX in the water. Next they mixed arginine with TTX, to simulate adults gorging themselves on worms, and found that the larvae didn't try to escape to a refuge, showing that arginine cancelled out the escape response caused by TTX. Because of the similarity in structure between TTX and arginine, the team suspect that these two chemicals are probably competing for olfactory binding sites in the nose: arginine binds instead of TTX, meaning that the larvae effectively can't smell TTX.
But how robust was the larvae's response? The team chemically manipulated the structure of the different groups on the arginine molecule, and found that only changes to the guanidinium group – the structure arginine shares with TTX – prevented arginine from blocking the larvae's response to TTX, suggesting that this group is key to how the newts' noses detect TTX and arginine.
Next, Ferrer and Zimmer trekked to the field site in Malibu, California,carrying all their equipment in backpacks, to test how the adults responded to arginine (p. 1776). To introduce the chemicals into the stream water from a distance without disturbing the newts, the team devised a system involving 3 m long transparent hollow rods connected to pumps. `The animals can be fairly skittish, so we had to remove all visual cues,' says Ferrer. Testing the response of adults to amino acids released from damaged worms, including others like alanine and glycine, they found that the newts responded most strongly to arginine by swimming towards the source of the smell, and by raising their snouts into the odour plume or burying themselves into the stream bed. Newts with blocked up noses didn't respond, and they also couldn't detect arginine if the molecule had been modified in any way, showing that the adult newts had a much more specific response to arginine and were probably `smelling' it in a different way.
`The big differences in behaviour caused by arginine between larvae and adults suggest that there is a change [in the nose] happening at metamorphosis' says Ferrer. So depending on whether you are little or big,arginine could be a signal that dinner is served, or that you don't have to swim for cover...yet.