Aquatic ecology and Biodiversity

Unit 8- Animal associations and interactions

8.7. Parasitism

In this relationship, one animal or species (the parasite) benefits at the expense of the other (the host). The benefited partner is called as parasite and the affected partner is a host.
I. Parasite
A parasite is an organism which exploits another organism for the purpose of nourishing from its tissues. Some parasitic relationships are harmless, while in other cases a parasite can damage or even kill its host. It lives in or on the body of the host. A number of organisms also go through a parasitic stage at some point during their lives and afterwards they lead a different life. Hence, based on the duration of parasitic mode of life, parasites are classified as permanent and temporary parasites.
a. Temporary parasites
Those organisms which spend only a portion of their life as parasites on the host are called temporary parasites. E.g. Lamprey (Petromyzon) and marine fish. The lamprey sucks the blood from the large fishes with its sucker and leaves the host. This type of parasitic forms is also known as facultative parasites. The glochidium larva of freshwater mussel attaches to the host fish by its hooks till it attains a young stage and leads a permanent benthic life.
b. Permanent parasites
Those organisms which spend their entire life as parasites are called permanent parasites. The permanent parasites of plants are called the phytoparasites and those of animals as zooparasites. These are also called as obligatory parasites, which must need a host.
The permanent parasites can be divided into two categories, viz. ectoparasites and endoparasites, the former referring to external parasites, and the latter internal parasites. The endoparasites are further divided into intracellular parasites and intercellular parasites. Parasites which live inside the host cell are called intracellular parasites (e.g. Trypanosoma) and those which live in between the cells of the host are intercellular parasites (e.g. flukes, isopods, etc.). Another marine example is that the male angler fish of very small size is attached to the large sized female fish, Antennarius hispidus. This attachment of male fish with female is to get nourishment as well as for reproduction. This type of parasitism is also termed as sexual parasitism.
2. Hyperparasitism
Sometimes, parasites themselves are parasitised by other organisms. Such parasites are known as hyperparasites. For example, Nosema notabilis is a hyperparasite on myxosporidian, Sphaerospora polymorpha, a common parasite of the urinary bladder of the toadfish.