In aquatic systems, organisms largely rely on chemical cues to perceive information about the presence of predators or prey. Daphnia recognize the presence of the predatory larvae of Chaoborus via a chemical cue, emitted by the larvae, a so-called kairomone. Upon recognition, neckteeth, an alteration of the carapace, are induced in Daphnia that reduce predation rates of Chaoborus. Neckteeth induction was often reported to entail costs. In a previous study, food quantity affected the level of neckteeth induction, with stronger neckteeth induction at low food concentrations and weak induction at high food concentrations. However, reducing neckteeth induction at high food quantities seems to be maladaptive and not in accordance with the concept that inducible defenses are associated with costs.

Here, we hypothesized that weaker neckteeth induction at high food concentrations is caused by increased bacterial degradation of the kairomone. More specifically, we assume that higher algal food concentration is associated with higher bacterial abundances, which degrade the kairomone during the experiment. We tested our hypothesis by treating food algae with antibiotics before providing them as food to Daphnia. Antibiotics reduced bacterial abundances at high and low food concentrations. Reduced bacterial abundances at high food concentrations led to the same level of neckteeth induction as at low food concentrations. A linear regression revealed a significant correlation of neckteeth induction to bacterial abundances. We therefore conclude that differences in neckteeth induction at different food concentrations are not caused by the food quantity effects but by differences in bacterial degradation of the kairomone.

在水生系统中，生物主要依靠化学线索来感知有关捕食者或猎物存在的信息。水蚤通过一种由幼虫发出的化学信号（即所谓的海洛酮）识别潮虫的食肉幼虫的存在。识别后，在水蚤中诱发了颈齿，即甲壳的改变，从而降低了潮虫的捕食率。经常据称诱发颈椎病需要花费费用。在先前的研究中，食物量影响了颈部诱导水平，在低食物浓度下增强了对颈部的诱导，而在高食物浓度下则减弱了对颈部的诱导。但是，减少高食量对颈部牙齿的诱导似乎是适应不良的行为，而不符合诱导防御与成本相关的概念。

在这里，我们假设在高食物浓度下较弱的颈部诱导作用是由于海洛酮的细菌降解增加所致。更具体地说，我们假设较高的藻类食物浓度与较高的细菌丰度有关，细菌丰度在实验过程中会降解海洛酮。我们通过在使用藻类作为水蚤食物之前用抗生素处理藻类来检验我们的假设。抗生素可降低食物浓度高低时的细菌丰度。高食物浓度下细菌丰度降低导致与低食物浓度下相同程度的颈部诱导。线性回归表明，颈部诱导与细菌丰度之间存在显着相关性。因此，我们得出的结论是，在不同食物浓度下领口诱导的差异不是由食物量效应引起的，而是由海洛酮的细菌降解差异引起的。