Comparing the selectivity between three leaf types of watercress plant (varying in chemical composition and nutritional value) by Gammarus pseudolimnaeus.

Abstract

Herbivory on aquatic macrophytes has been considered unimportant until recently thus, there is little information available on plant-herbivore interactions in freshwater systems. Chemical deterrents, and to some degree, nutritional value, are important factors in plant consumption by herbivores. In this experiment, senesced yellow leaves, fresh green leaves and leached green leaves from the aquatic watercress plant, Nasturtium officinale, were compared in terms of food preference by the herbivore, Gammarus pseudolimnaeus, an aquatic invertebrate. Watercress leaves and Gammarus were collected from Carp Creek, Cheboygan, Michgan, USA. The leaf tissue and Gammarus were divided among ten petri dishes and placed in incubation for 24 hours. Leached green leaves were consumed more than both yellow and fresh green leaves possibly due to a decreased amount of secondary chemical substances and a higher nutritional value present in leached green watercress leaves. These data indicate the following: watercress plants exhibit secondary chemical defenses which are strongest in young green leaves, and when given a multiple choice, herbivores prefer food which has decreased amounts of secondary substances, regardless of its nutritional content.