OPHIOTAmA PEIf6P/CUA 



LIFE CYCUe 



FIG. 12-2 Life cycle of a snake tapeworm. The eggs are voided 

 into the water with the feces of the snake, where they are 

 ingested by the copepod Cyclops (lower right). A procercoid 

 (middle right) develops in the copepod, from the egg. If the 

 copepod is eaten by a fish, the procercoid changes into a 

 plerocercoid (upper right) and becomes encysted in the liver 

 or mesenteries. When the fish is eaten by a water snake, the 

 mature tapeworm develops (upper left). Other intermediary 

 hosts are tadpoles and frogs (Thomas 1944). 



Host specificity 



Copepods are of all parasites the most ubiqui- 

 tous in their host relationships, being reported from 

 various invertebrate groups and from fish. Most par- 

 asitic genera, however, are adapted to hosts of one 

 phylum only. The acanthocephalans Gracilisentis and 

 Tanarhamphns are. yet more specific, normally found 

 only in the gizzards of shad fish ; Octospinifer is 

 found only in catostomids ; Eocollis, only in cen- 

 trarchids. Each order of birds possesses its own 

 particular species of tapeworms ; this is true even 

 when several orders of birds live in the same habi- 

 tats, as do, for instance, grebes, loons, herons, ducks, 

 waders, flamingoes, and cormorants (Baer 1951). 

 Species of flagellate protozoans that occur in termite 

 alimentary tracts are largely host-specific (Kirby 

 1937). However, considerable caution needs to be 

 exercised in assigning host specificity to protozoans. 

 Many species have invaded more than one taxonomic 

 host group : and often several species of a single 

 genus of Protozoa frequent the same host species. 



Some species of gall wasps attack only one species 

 of oak. Where a single species parasitizes two or 

 more host species, the shape and structure of the 

 gall formed around the egg and larva on both hosts 

 is essentially similar. When several insects are found 

 on the same oak, each kind of parasite produces its 

 own characteristic gall form. Apparently the charac- 

 teristics of the gall that develops depend more on 

 the kind of enzyme secreted by the parasite than on 

 differences of host tissues (Kinsey 1930). 



Restriction of parasites to special niches is demon- 

 strated by species of biting lice restricted to the 

 head and body regions of birds. Some nematode spe- 

 cies are found throughout the body in connective tis- 

 sue, but not in the gut ; some occur only in the 

 digestive tract and associated organs ; certain species 

 occur in the glandular crop of birds, but others only 

 in the caecum : many species occur exclusively in the 

 lungs or in the frontal sinuses. Such fine restriction 

 of parasites to particular hosts or organs is a conse- 

 quence of precise physiological and morphological 

 adaptations that permit the parasite to survive and 

 complete the life-cycle only under very special condi- 

 tions. 



Host-specificity can make the taxonomies of many 

 parasites useful for corroborating phylogenetic rela- 

 tionships of their hosts (Kellogg 1913). The South 

 American bird Cariama cristate has been shifted from 

 one order to another, and was at one time even put 

 into a special order. A study of its helminth parasites 

 disclosed two species of nematodes and two genera of 

 cestodes present which occur together elsewhere only 

 in Eurasian bustards. The occurrence of these forms 

 in groups so far removed geographically from one 

 another could be coincidental or the result of parallel 

 evolution, but for a number of reasons it seems 

 more likely that Cariama and the bustards are 

 derived of a common ancestor which became infected 

 with these parasites, the parasites persisting in spite 

 of evolutionary divergence and geographic separation 

 of hosts. It is interesting to note that this relation- 

 ship of the hosts is sustained by recent taxonomic 

 study of them by ornithologists (Baer 1951). 



Effect on host: disease 



By disease we mean a condition which so affects 

 the body or a part of it as to impair normal func- 

 tioning. Parasites may not cause immediate mortal- 

 ity, but they cause damage to body structures which, 

 should it become excessive, may cause death. We 

 may perhaps better visualize the role parasites play 

 in producing disease by listing some of the more com- 

 mon agents of mortality in organisms, in addition to 

 predators and parasitoids, which will be described 

 beyond. 



1. Worm parasites, such as tapeworms, nema- 

 todes, and acanthocephalans may wander through the 

 host's body doing mechanical injury as well as de- 

 stroying and consuming tissues. The host may re- 

 spond by forming a fibrous capsule or cyst around an 

 imbedded parasite. 



2. Protozoan parasites are especially important 

 in the alimentary tract and in the blood. A sporozoan 

 species of Eimeria damages the walls of the intestine 



1 80 Ecological processes and dynamics 



