Broadly defined, a parasite is an organism which lives on or in a second organism (known as the host) and the parasite benefits at the expense of the host. Some parasites are minimally harmful to their hosts while others can cause serious diseases and even death. The Aquatic Parasite Observatory is primarily concerned with the macroparasites found in the amphibians, fishes, birds, and invertebrates of freshwater aquatic ecosystems. Macroparasites include flatworms (trematodes), roundworms (nematodes), tapeworms (cestodes), spiny-headed worms (acanthocephalans), as well as various ectoparasites.
Apart from the parasites that cause diseases in humans and domestic animals, scientists often overlook parasites. This is partially explained by the fact that parasites are small and ‘hidden’ within their hosts. Although individual parasites may seem small and insignificant, their combined biomass can sometimes exceed the biomass of top predators. Therefore, parasites may play a larger role in ecosystem processes than previously appreciated.
There is growing public and scientific interest in gaining a more comprehensive understanding of the biodiversity on Earth, of which ~40% involves parasites. Understanding parasite biodiversity can thus be a useful tool for studying not only patterns of infectious diseases, but also as bioindicators of global change, species biogeography, and even the mechanisms that promote evolutionary diversification.
The life of a parasite is usually very different than the life of a free-living organism. Some parasites need only one kind of host to complete a single generation, while others require a complex series of different kinds of hosts. In this section you’ll be able to familiarize yourself with some generic life cycles before you move on to learn about the life cycles of specific parasites.
Parasites with a direct life cycle live and reproduce all within a single kind of host. These parasites have an infective stage which allows the parasite to be transmitted to a new host individual. The direct life cycle is common among monogenes, nematodes, and protozoans.
Parasites with an indirect life cycle require multiple, different kinds of hosts to complete a single generation. These parasites often undergo sexual reproduction in one kind of host and asexual reproduction in a different kind of host. The indirect life cycle is very typical of trematodes.
Organisms with a heterogonic life cycle actually alternate between parasitic and free-living generations. These parasites reproduce in both the parasitic and free-living generations. Some nematodes have a heterogonic life cycle.
