Chemically induced defenses in phytoplankton
Many organisms, including some phytoplankton and zooplankton, are phenotypically plastic, and respond to predators by changing their shapes – a phenomenon known as “inducible defense.” For example, some rotifers grow large spines in the presence of their predators, and some clones of the common crustacean Daphnia develop protective “neckteeth” protrusions when their predator, the insect larva Chaoborus (the phantom midge), is abundant. Many predators and grazers are limited by the size of food they can consume, and individuals with these spines and bumps have reduced predation risk. One of the most interesting aspects of these shape changes is that they are a morphometric response to the presence of compounds released either by the predator itself or by the predator actively feeding. For these inducible defenses, the prey does not exhibit the spines when the predator is absent, but develops the defensive morphology in response to a water-borne cue from the predator. Repeated studies have demonstrated that many changes in shape of zooplankton and phytoplankton do not require the presence of the predator, but can be induced just by adding water from an aquarium or lake where the predator is present. So, the changes we observe in lakes, where zooplankton and phytoplankton often become spinier when a predator becomes abundant, are not necessarily due to the predators eating all the nonspiny individuals and leaving only the spiny members of the population, but also can be caused by inducible shape changes.
KeywordsColony Formation Colony Size Inducible Defense Insect Larva Reduce Predation Risk
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