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Life cycles of the White Sea Trematodes


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Molluscan Comparative Immunology (MCI) Group

The Life Cycles of the White Sea Trematodes

The most ancient trematode life cycle is dixenic. In this case final hosts became infected with hermaphroditic stage (marita) after swallowing of free-swimming larvae produced by parthenogenetic stages (parthenitae) in intermediate host - mollusk. In modern trematodes such archaic cycles disappeared.
The further evolutionary transformations of dixenic life cycle were connected with changes in biology of dispersive stages. The larvae gained ability to encystment in the environment, and the adolescaria stage was added in the cycle (Notocotylidae).
The next step is appearance of trixenic cycles; in most cases it was connected with replacing of free-living adolescaria by parasitic metacercariae (Echinostomatidae, Heterophyidae, Renicolidae, Microphallidae).
The further transformations of the life cycles were connected with decreasing in host number. Passage back to dixenic cycles took place in the case a mollusk may act as both the first and the second intermediate hosts (Microphallidae). Monoxenic life cycles in hemiurids resulted from the progressive decrease in number of the hosts in the typical trixenic cycles or dixenic ones where molluscs combined functions of two hosts. (See details in Galaktionov & Dobrovolsky, 2004)

Originally dixenic life cycle
Paramonostomum alveatum, Notocotylus sp. (Notocotylidae)
Paramonostomum alveatum
Typical trixenic life cycles with rediae act as second and third parthenogenetic generation
- "redioid trematodes".
Cryptocotyle lingua & C. concavum (Heterophyidae)
Himasthla elongata (Echinostomatidae)
Cryptocotyle lingua, C.concavum
Himasthla elongata
Typical trixenic life cycles with sporocysts act as second and third parthenogenetic generation
- "sporocystoid trematodes".
Maritrema subdollum, Microphallus claviformes (Microphallidae)
Cercaria (Renicola) parvicaudata (Renicolidae)
Maritrema subdollum, Microphallus claviformes
Renicola parvicaudata
Secondary reduced dixenic and monoxenic life cycles
Microphallus piriformes, M. pseudopygmaeus, M. triangulatus (Microphallidae)
Bunocotyle progenetica (Hemiuridae)
Microphallus piriformes, M. pseudopygmaeus, M. triangulatus
Bunocotyle progenetica

 

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Last update: July 31, 2011