Parasites Deliver Omega-3 to Aquatic Systems

Wormlike parasites cause crickets to jump into streams, where the fatty insects nourish endangered fish. Ayano Medo and Takuya Sato sought to characterize the flow of eicosapentaenoic acid (EPA)—a nutritious long-chain omega-3 fatty acid—through aquatic and terrestrial ecosystems. EPA in human diets often comes from fish. Aquatic algae synthesize significant amounts of EPA, and it was thought that fish acquired most of their EPA from aquatic invertebrates that eat the algae. However, from the middle of August to late October, nematomorph parasites induce terrestrial crickets to jump into water, where the parasite completes its life cycle and where salmonid fish, such as an endangered species of charr (Salvelinus leucomaenis japonicus), gorge themselves on these substantial terrestrial insects. Previous work shows infected crickets and katydids account for 60% of the annual energy intake of an endangered charr population in a temperate forested stream in Japan. The authors measured the EPA levels in camel crickets, which are known to scavenge weak and dead aquatic insects and found that camel crickets have around 4–17 times more EPA than known aquatic invertebrates, and nematomorph infections did not reduce EPA. According to the authors, as nematomorph parasites are common globally, parasite-mediated movements of land insects to water may be a common mechanism by which EPA flows from terrestrial to aquatic ecosystems.