Turning inducible defenses on and off: adaptive responses of Daphnia to a gape‐limited predator

Abstract

The use of inducible defenses is a common strategy to reduce predation while minimizing associated costs for prey. The most effective use of these defenses, however, may involve turning them on and off at different stages of ontogenetic development, with the timing dependent on prey body size and the nature of the predation environment. We develop a model based on the strike efficiency of a size-selective predator that examines the interaction between induced morphological defenses and prey body size, including the consequences of this interaction for the optimal development of the defenses during the prey's ontogeny. We then examine this model with respect to a model system of inducible defenses: neck spine induction in the water flea Daphnia in response to predatory larvae of the phantom midge Chaoborus. In accordance with predictions of the model, the body size and timing of neck spine acquisition during Daphnia development are related to the relative sizes of the Daphnia and Chaoborus species interacting in a pond or lake. The Daphnia species examined first acquire neck spines in either the first, second, or third juvenile instar, at body lengths that range from 0.58 to 0.83 mm. Neck spine formation is initiated at larger Daphnia body sizes when these prey are subject to predation by a larger Chaoborus species (C. trivittatus) and at smaller sizes when exposed only to a smaller predator (C. americanus). Induction of these morphological defenses in Daphnia occurs later in juvenile development in the smaller of the two species we examined (D. minnehaha) than in the larger (D. pulex). Delayed acquisition of neck spines also occurs when Daphnia are exposed to predation by larger Chaoborus. The close match between model predictions and the patterns observed in nature suggests that these patterns are adaptive developmental responses to different predator environments.

Changes in water chemistry can disable plankton prey defenses

“…2), a pattern that is common for D. pulex (13,15,17). Most instar 1 individuals produced only small neck spines and there were no significant differences in the development of these structures among the experimental calcium treatments (Fig.…”

“…S1B). The small and large neck spine categories used here are equivalent to the "weak" and "strong" neck spines, respectively, described by Riessen and Trevett-Smith (13).…”

“…Daphnia body size plays a critical role in its vulnerability to Chaoborus, because prey whose size exceeds the mouth gape diameter of this predator are difficult to ingest (10). This results in a linear decrease in Chaoborus strike efficiency (probability of ingestion of prey following an attack) with increasing Daphnia body length (11)(12)(13). The formation of neck spines (also referred to as neckteeth) in juvenile instars of D. pulex also substantially reduces the ability of Chaoborus to successfully manipulate and ingest these prey, greatly decreasing their vulnerability (13)(14)(15).…”

“…This results in a linear decrease in Chaoborus strike efficiency (probability of ingestion of prey following an attack) with increasing Daphnia body length (11)(12)(13). The formation of neck spines (also referred to as neckteeth) in juvenile instars of D. pulex also substantially reduces the ability of Chaoborus to successfully manipulate and ingest these prey, greatly decreasing their vulnerability (13)(14)(15). This defensive structure, which is an extension from the dorsal margin of the head that projects into one or more pointed "teeth" (Fig.…”

Phenotypic plasticity in threeDaphniagenotypes in response to predator kairomone: evidence for an involvement of chitin deacetylases

“…In line with this, induction of neck-teeth occurs only in those instars that are vulnerable to gape-limited predation by Chaoborus sp. (Riessen and Trevett-Smith, 2009).…”