Inducible Defences in Daphnia Depend on Latent Alarm Signals from Conspecific Prey Activated in Predators

Abstract

Some water fleas (Daphnia spp.) undergo phenotypic changes when exposed to chemical signals from predators. The chemical signals have been assumed to be of predator origin (i.e. kairomones), since juices of crushed Daphnia have been found ineffective. We speculated that latent alarm signals could be present in Daphnia, to be activated in predators following ingestion. Accordingly, fish predators were fed earthworms for 10 weeks to remove Daphnia remains from their gastro-intestinal tracts. Following another 6 days of earthworm feeding, water conditioned by fish induced no morphological changes in D. galeata. When fish were alternatively fed Daphnia for 6 days, changes were induced with fish-conditioned water. Extracts made from intestines of earthworm-fed fish, homogenized with earthworms, gave no morphological changes, but intestines of the same origin homogenized with Daphnia did. Similar results were found when earthworms and Daphnia were homogenized with fish liver. Freshly frozen extracts of homogenized Daphnia gave no detectable changes at first instar stage in test animals, whereas extracts of Daphnia that had been kept at room temperature did induce such changes. Our results suggest that Daphnia respond to latent conspecific alarm signals (i.e. 'dormant' pheromones) that are activated by intestinal or bacterial enzymes in predators or in the water.

Challenging fear: chemical alarm signals are not causing morphology changes in crucian carp (Carassius carassius)

“…The tissue content of these particles may normally be metabolized by bacteria, either in the intestine of predators or in the water, resulting in water soluble cues. Such latent chemical signals, inducing morphology changes with the purpose of prey defences, have previously been found in other aquatic organisms, but have hitherto not been demonstrated in fishes (Hagen et al 2002;Stabell et al 2003;Jacobsen and Stabell 2004;Stabell 2005). Further studies are now underway to clarify the true origin of the chemical signals inducing morphology changes in crucian carp.…”

Invited Review - Fight on Plankton! or, Phytoplankton Shape and Size as Adaptive Tools to Get Ahead in the Struggle for Life

“…Improved standardization by novel infochemical extraction methods looks promising, and by far the most progress in this area has been made for the Daphnia infochemical (van Holthoon et al 2003). Herbivore information chemicals are released during the grazing process, probably by digestive processing of the algae or by release of latent alarm substances by the algae (Stabell et al 2003). Colony formation in various strains of Scenedesmaceae can be induced by infochemicals produced by herbivorous zooplankton that have been grazing on other strains of Scenedesmaceae or even other Chlorophytes (Lürling1998, .…”

“…An interesting question remains whether the algae respond to some general algal chemicals or to species-specific latent alarm pheromones released by the algae (cf. Stabell et al, 2003). All pelagic zooplankton taxa are exposed to similar environmental constraints, which may have selected for similar chemical pathways, and the release of similar infochemicals.…”

Inter-genotype variation in reproductive response to crowding among Daphnia pulex

“…Much research has been conducted on the effects of fish-inhabited water on Daphnia, with effects attributed to fish kairomones (e.g., Boersma et al, 1998;Weetman & Atkinson, 2002). However, fish were almost always fed Daphnia, and evidence suggests it is not fish kairomones but latent daphnia alarm cues activated by bacteria and/or enzymes in fish digestive tracts to which daphnids respond (Stabell et al, 2003). A similar mechanism may be responsible for observed daphnid responses to water exposed to invertebrate predators (e.g., Pijanowska & Kowalczewski, 1997).…”