Habitat

When considering habitats of parasitic organisms, parasitologists make difference between microhabitats and macrohabitats. The microhabitat of a parasite species is an organ or a tissue in the host inhabited by parasite individuals. In broader sense, the whole host individual also could be recognized as a microhabitat. The macrohabitat of a parasite species is that place or environment where its hosts (final, intermediate, and paratenic) live.

The usual microhabitat of the adult tapeworms is the intestine of vertebrate animals. Only the species of the order Amphilinidea inhabit the body cavity of their final hosts (fishes and turtles). Some species of the order Cyclophyllidea occur in other parts of the digestive system (e.g., Cloacotaenia megalops [Hymenolepididae] lives in the cloaca of ducks, Thysanosoma actinioides [Thysanosomatidae] inhabits the bile ducts of ruminants, and Gastrotaenia cygni [Hymenolepididae] occurs under the horny lining of the gizzard of swans).

The intestine of vertebrate animals is not a homogeneous habitat. Its portions differ from each other by their content of nutrients, range of enzymes, pH, and structure of the intestinal wall. It is known for many cestode species that they can be found only in a certain portion of the intestine, e.g., in the duodenum or in the most posterior part (at the ileo-caecal junction). A detailed study on the distribution of the parasitic worms along the small intestine of grebes in Canada showed that each of four grebe species was parasitized by 9-17 tapeworm species. Some of these occurred in a high proportion of the intestine that could be interpreted as a broad tolerance for conditions along it. A quarter of cestode species, however, occupied portions no longer than 20% of the intestine length.

Tapeworm larvae have diverse locations in intermediate hosts. Species occurring in crustaceans or insects are usually in the body cavity. The members of the family Taeniidae are parasitic in carnivore mammals and humans and their larvae develop also in mammals. Depending on the cestode species, the larvae of taeniids can be situated in the liver, lungs, musculature, body cavity, brain, and mesentery, sometimes even in the eyes. The most typical locations of cestode larvae in fishes are the musculature, body cavity, and gall bladder.

Concerning the range of the final hosts of cestodes, it is not exaggerated to say that they are not recorded only in vertebrate species, which have not been examined. Only the Cy-clostomata (lampreys and hagfishes) have no cestode parasites.

Each tapeworm species is characterized by its host spectrum. This phenomenon is usually marked as "parasite specificity to the host," or simply as "host specificity." Some cestode species are found in one vertebrate species only (oioxenous parasite species). A good example is Dollfusilepis hoploporus (Hymenolepididae). This worm lives in the great crested grebe (Podiceps cristatus) only, even in lakes where this host co-occurs with other grebe species. Other cestode species are found in a small group of related host species belonging to one genus or one family (stenoxenous parasite species) (e.g., Tatria biremis [Amabiliidae] occurs in Eurasia and America in three species of grebes). The majority of tapeworm species are stenoxenous. A few cestodes are euryxenous parasites, i.e., live in unrelated (phylogenetically distant) hosts. In the latter case, a prerequisite for the formation of such parasite-hosts associations is the convergence of the ecology (in terms of similar habitats and diets) of the hosts. An example for this can be Ligula intestinalis (Diphyllobothriidae) occurring as adult in birds of various orders: gulls of Charadriiformes, herons of Ciconiiformes, grebes of Podicipediformes, cormorants of Pelecaniformes, and mergansers of Anseriformes. However, the infective larvae of this parasite live in fishes of the family Cyprinidae only. Therefore, L. intestinalis is eu-ryxenous relative to the final hosts but stenoxenous relative to the second intermediate host.

A few tapeworm species are known that mature in invertebrate animals. Thus, Archigetes sieboldi, Archigetes limnodrili, and Archigetes iowensis (Caryophyllidea) can mature in the body cavity of freshwater annelids, and Diplocotyle olriki and Cyathocephalus truncatus (Spathebothriidea) in amphipod crustaceans. The same invertebrate groups are intermediate hosts of the two cestode orders. Alternatively, all these forms can mature in the intestine of fishes (final hosts). The life cycles with the participation of only one invertebrate host are considered secondarily simplified, allowing shortening time needed for maturation of worms in the final host. However, there are data showing that at least some of these species can live in water basins where appropriate fish final hosts are lacking.

Macrohabitats of tapeworms are diverse, from deep sea to high mountains and from tropical forests to tundra. There are even species living in extreme conditions (e.g., several species parasitizing flamingos, avocets, and plovers live as larvae in brine shrimps [Artemia spp.] in salinas. Such species is Flamingolepis liguloides [Hymenolepididae]).

Essentials of Human Physiology

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