The gelatinous capsule of Holopedium gibberum is variable in size, and seasonal size development shows different courses in different lakes. Reasons for this variation were experimentally studied. Differences in density of invertebrate predators rather than abiotic differences were responsible. In experiments, adaptive change in capsule size took place without any physical contact between Holopedium and the predator Chaoborus obscuripes, suggesting a chemically triggered mechanism. The specific receptor in Holopedium and the key substance released by the predator are not yet known. The experiments showed that the same Holopedium individual that received the stimulus reacted by a direct phenotypical change. The reduction of capsule size in response to reduced predator density suggests a cost in maintaining this antipredator armor. A larger capsule leads to a higher cost for food gathering because of an increased drag in the water. The need for better armor may postpone the start of reproduction in Holopedium.
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Jan A. E. Stenson (1987) studied this question.