The Scent of Danger: Tetrodotoxin (Ttx) as an Olfactory Cue of Predation Risk

Abstract

Larvae of the California newt (Taricha torosa) exhibit striking predator‐avoidance behavior, escaping to refuges in response to a chemical cue from cannibalistic adults. In laboratory flow‐tank experiments, stream water collected near free‐ranging adults induced hiding responses in 100% of the larvae tested. Solutions prepared by bathing adults (in field and laboratory) also evoked strong hiding behaviors. Insensitive to adult feeding status (fed or starved), and clearly not an excretory product, the chemical cue was released from adult skin (i.e., in swabs of adult backs, sides, and bellies). Tetrodotoxin (TTX) was found in skin swabs of adults and in bathwater at 1 × 10−7 mol/L using reversed‐phase high‐pressure liquid chromatography (HPLC). Concentrations of 1 × 10−7 to 1 × 10−9 mol/L TTX standard, and equivalent dilutions of bathwater, triggered hiding behaviors in larvae, with no subsequent sublethal toxicity. The presence of TTX‐sensitive cells within larval olfactory epithelium was confirmed by behavioral experiments and electrophysiological recordings. In contrast, larvae did not hide in response to two other, structurally mimetic compounds (saxitoxin and μ‐conotoxin GIIIB). Ontogenetically, larval behavioral responses to TTX and bathwater were strongest during weeks 3–5, diminishing to nil during week 7. No longer susceptible to adult cannibalism, larval indifference to the cue coincided with their ability to climb out of water and onto land. Thus, newt larvae escape cannibalism by detecting a poison (TTX) well known as a chemical defense for conspecific adults. Eliciting a behavioral response in one case and inhibiting neural activity in the other, this compound results in opposing physiological effects, with avoiding predation as the common goal. Accordingly, TTX joins a select group of keystone molecules, each having critical, but different, ecological consequences at multiple trophic levels. The unique combination of bioactive properties makes a compelling case for asymmetrical selection as a force driving the evolution of adult–larval trophic interactions.

The scent of danger: arginine as an olfactory cue of reduced predation risk

“…Incandescent bulbs (General Electric Corp. Daylight Ultra, Cleveland, Ohio, USA) were placed within baffled housings to provide diffuse overhead lighting. The mean photon flux (20·moles·m -2 ·s -1 ), spectral composition (intensity peaks between 435 and 543·nm), and angular light distribution (sharp decline in intensity between 40° and 50° relative to the zenith) in each bioassay chamber were similar to morning sunlight within natural stream habitats (Zimmer et al, 2006). Lighting was completely confined by the blinds, and the surrounding laboratory was maintained in darkness to reduce further disturbances to test animals.…”

“…response Newt larvae swim in response to trace concentrations of TTX (Zimmer et al, 2006). The presence of TTX-sensitive cells within larval olfactory epithelium was revealed by electrophysiological recordings.…”

“…In the first R. P. Ferrer and R. K. Zimmer experiment, larvae were presented with solutions of 10 -7 ·mol·l -1 TTX, 10 -7 ·mol·l -1 TTX plus 10 -7 ·mol·l -1 arginine, 10 -7 ·mol·l -1 arginine, and dechlorinated tapwater (control). These conditions were chosen to simulate natural arginine and TTX fluxes from tissues of injured worm prey and from skin surfaces of adult cannibals, respectively (Zimmer et al, 2006;Ferrer and Zimmer, 2007). Actual experimental input rates were 100-to 1000-times lower than those occurring naturally, compensating for dilution in turbulent stream flows.…”

“…Larvae escape cannibalism by detecting the poison tetrodotoxin (TTX) in adult conspecifics, where it functions as a chemical defense (Zimmer et al, 2006). By contrast, the toxin is not found in the larval stage.…”

“…The presence of TTX-sensitive cells within larval olfactory epithelium has been confirmed by electrophysiological recordings. Concentrations of 10 -7 to 10 -9 ·mol·l -1 TTX in natural, adult-scented stream water stimulate effective predator-avoidance and refuge-hiding behaviors in larvae (Zimmer et al, 2006). Once a refuge is detected visually, larvae move rapidly and on a linear trajectory to the hiding place.…”