Life is tough for water fleas. In addition to the usual troubles faced by every organism, Daphnia have to worry about being eaten by many types of predators. To make them less vulnerable, some Daphnia species grow large helmets or protective spines on their heads and tails. But at least one predator, the stickleback, only fancies a Daphnia diet at certain times of year, so it would be inefficient, maybe even paranoid, for a tiny Daphnia to always carry around its cumbersome armor. The solution to this problem is to develop armor only when danger lurks.
Daphnia sense this danger through odor cues associated with predation. But what is the source of the armor-inducing odors? Although some animals respond to odors released by injured conspecifics, previous research has clearly established that such cues do not play a role here; instead, the prevailing belief has been that Daphnia are responding to odors released by the predators themselves. But Daphnia and sticklebacks can inhabit the same lakes, and clearly Daphnia do not arm themselves year-round. In a new paper, Ole Stabell and his colleagues show that the formation of anti-predator spines in Daphnia galeata is induced by the smell of predators that have been eating Daphnia. That is,crushed Daphnia bodies release `latent alarm signals', after passing through a predator's digestive system, that other Daphnia can smell and respond to.
To demonstrate this remarkable phenomenon, Stabell and his colleagues used a native predator, the three-spined stickleback, and a non-native predator,the Malawi cichlid, as odor sources. Both types of fish were fed earthworms for ten weeks to clean out any lingering `eau de Daphnia'. Then they were fed either earthworms or Daphnia for six days. When Daphnia were exposed to water from the fish tanks, they only developed anti-predator defenses in response to odors from the fish that had dined on Daphnia. To further investigate the source of the odor cues,the investigators crushed either Daphnia or earthworms together with fish intestines (a rich source of bacteria) or fish livers (a source of enzymes). Again, the odors associated with Daphnia induced the crustaceans' morphological defenses, but earthworm odors did not, indicating that the odor cues do not simply arise from the predators' digestive systems. Finally, the authors crushed Daphnia in zooplankton medium. After a few hours, odors from crushed Daphnia stored at room temperature led to morphological changes, but Daphnia exposed to similar medium that had been frozen did not undergo changes. Because the medium probably contained bacteria, Stabell and co-workers interpret their results as indicating that odor cues are released from Daphnia after exposure to enzymes, either from bacteria or from predators' digestive organs.
Other research has shown that Daphnia develop different defenses in response to cues from different predators. The work by Stabell and colleagues provides the basis for further investigation, such as uncovering the means by which latent alarm signals interact with a predator's digestion to produce specific cues that identify the awaiting threat.