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The Biology and Evolution of Trematodes
An Essay on the Biology, Morphology, Life Cycles, Transmissions, and Evolution of Digenetic Trematodes

Kirill V. Galaktionov
Zoological Institute of the Russian Academy of Sciences, St. Petersburg, Russia
Andrej A. Dobrovolskij
St. Petersburg State University, Russia

The trematodes are parasitic flatworms of great medical and veterinary importance. An understanding of the evolution of trematodes depends on an interpretation of their complex and diverse life cycles. It is the life cycles in general and the stages that comprise these cycles that are the focus of the detailed analysis presented herein. The book contains a broad scope of modern information on digenetic trematodes, from descriptions of their morphology and development to their behaviour and the structure of their populational groups. The book provides information on all characteristics of trematode organization and biology from an evolutionary standpoint. Possible scenarios of early stages of life cycle formation are discussed as well as a consideration of further evolution in different taxa and ecological groups of trematodes. An original approach to the elaboration of a natural system of these parasites is proposed.

The book is addressed to zoologists and parasitologists, as well as to researchers from a wide range of disciplines interested in understanding the evolution of life cycles and host-parasite interactions. It will be useful as a textbook for undergraduate and postgraduate students.

Kluwer Academic Publishers, Dordrecht
Hardbound, ISBN 1-4020-1634-4
November 2003, 620 pp.
EUR 210.00 / USD 231.00 / GBP 145.00

To order:


The Origin and Evolution of Trematode Life Cycle.

K.V. Galaktionov and A.A. Dobrovolskij

Sankt-Peterburg, "NAUKA", 1998, p.404, ISBN 5-02-026089-4 [in Russian].


The Origin and Evolution of Trematode Life Cycle.

SUMMARY Based on analysis of the original and literature data, the book gives a review of some specific features of morpho-functional organization of trematodes at all stages of their life cycle. The special attention is paid to the parthenogenetic generations (mother sporocysts and their larvae — miracidia, daughter sporocysts and rediae) which drop out the field of vision of modem zoologists and parasitologists investigating evolution of trematodes. It is shown that the main trend of morphological evolution of miracidia is their simplification and miniaturization which is accompanied by the passage from active infection of the first intermediate host with free-swimming larva to the passive one whereby miracidium hatching takes place in the alimentary tract of molluscan host having swallowed the egg. The evolution of parasitic stage of the mother sporocyst development shows apparent trend to the increase of generative function. In the primitive families (Fasciolidae, Paramphistomatidae, Echinostomatidae and others) reproduction of mother sporocysts is practically ending during miracidium development. The increase of mother sporocyst reproduction in the «strigeidid» branch of higher trematodes usually is not accompanied by any essential morphological transformations of the parasitic stages. At the same time, the main direction in the mother sporocyst morphological evolution in the «plagiorchiid» branch is morpho-functional disintegration. As a result, individual parts of the parthenite act as independent organisms. In the most specialized specimens (Microphallidae, Lecithodendriidae, some Plagiorchidae and others) generative units (germinal masses, germ cells, embryos etc.) parasitize independently the host. The morphological simplification is also obvious in the evolution of the daughter parthenogenetic generation and may be demonstrated by reolacing of rediae by daughter sporocysts in the more advanced trematode.

Heterochronies resulting most often in juvenilization and miniaturization of the organism are revealed in the evolution of different stages of hermaphroditic generation, especially in cercariae of the higher orders Strigeidida and Plagiorchiida. The simplification of the definitive organization in the latters permits to purpose energetic sources for the development of rather advanced adaptations (highly-differentiated gland complex, tail with striated musculature, stylet in Xiphidiocercariae, host-finding behavior, etc.) directed to the solution of main task of the larva — infection of the second intermediate host. Formation of these heterochronies is only possible under rather advanced interactions between parasite and the second intermediate host. The effective use of energetic sources of the latter has permitted to carry out development of the definitive structure and preparation to infection of the final host at the stage of metacercaria.

At the post-cercarial stages of the ontogenesis specimens of hermaphroditic generation in Plagiorchiida and Strigeidida demonstrate different ways of their morphological evolution. In the former family the trend to juvenilization and miniaturization covers adults, too, whereas in the latter one «imaginization» (i.e. complication of definitive structure) takes place and, as a result, among trematodes strigeidid adults have the most complex structure. The mentioned evolutionary trends demonstrates two probable ways of the increase of individual fecundity of adults: (1) due to elongation of longevity of adults producing a few large eggs enriched with yolk (Strigeidida) and (2) production of numerous small eggs containing full-formed miracidia during the short term of the adult existence in the final host (Plagiorchiida). The both strategic; provide a successful completion of the parasite life cycles in ecosystems of different sort. Other trematodes demonstrate transitional versions of these strategies. The diversity of the versions reflects a real diversity of ecosystems where the trematode life cycles are completed.

A significant part of the book is devoted to the analysis of adaptations in trematode life cycles (both cycle as a whole and individual stages). The analysis is carried out both at the level of specimen and population. The description of the specific feature in spatial and temporal structure of groups of miracidia infecting actively and passively their hosts, generations of mother and daughter parthenites, cercariae, metacercariz (including adolescariae and mesocercariae) and adults is given separately. The analysis has permitted to propose a scheme of successive stages in biological radiation of trematodes. It was shown that inhabiting of ocean pelagic areas was connected with complication of circulation ways and including one more host providing development of mesocercaria between the first intermediate host and the second one. Just opposite trend take place in the course of trematode penetration into terrestrial and littoi ecosystems. Here secondary dixenic life cycles devoid of free-living larvae have been formed.

The analysis of trematode evolution itself is based on the principle of the posse sing equal rights study of all stages of the life cycles, i.e. according to the opinion that trematode evolution is evolution of their life cycles. From this point of view all cum notions on this topic including widely-distributed D. R. Brooks and his co-workers' ideas (Brooks et al., 1985, 1989) have been critically examined. Searching for I ancestrial («sister») group among modern trematodes is proved to be ineffective even specimens from the so-called «primitive» families (Fasciolidae, Paramphiston tidae, Notocotylidae, Azygiidae etc.) have both archaic and advanced features. Evolutionary development of trematodes is considered as result of complex evolution, transformations of their life cycles, the individual stages and generations composing them are characteristic of different directions of the adaptations.

The authors have attempted to typologization the life cycles of modern trematodes The original scheme of probable ways of their transformations in the course of taxon evolution and its penetration into ecosystems of different sorts is given. An obligatory dixenic life cycle with stable heterogony is considered as an initial one. The infection of the final host was carried out per os by free-swimming adult-like larvae devoid any adaptations to prolongation of longevity in the environment. In modern trematodes such cycles are probably absent.

The further evolutionary transformations of dixenic life cycle were probably connected only with changes in biology of dispersive stages. The better probability of successful infection of the final host is got by two ways. The larvae keeping arc organisation gained ability to encystment in the environment, and the adolescaria stage was added to the onthogenesis of hermaphroditic individuals. Among the modern trematodes such a life cycle may be found in families Fasciolidae, Paramphistomat (and closely-related groups), Notocotylidae, Pronocephalidae. The other trematode group chose cercaria specialization, and their longevity in the environment increased due to improvement of locomotion and appearance of movement of discrete type. In this case final hosts became infected after swallowing of free-swimming larvae. Archaic cycles of this type have also disappeared. Probably, Azygiidae and Bivesiculidae which cercariae infect their final hosts per os are their direct descendants.

The next stage is appearance of trixenic cycles; in most cases it was connected with replacing of free-living adolescaria by parasitic metacercariae. Facultative trixeny appeared independently in Notocotylidae and Echinostomatidae but only in the latter the initial «lodgerment» is replaced by true metacercarial parasitism. Life cycles of this type are characteristic of the most modern trematodes both keeping some archaic characters (Heterophyidae, for example) and the most advanced (Strigeidida, Palgiorchiida). But trixeny arised, probably, several times and by different ways as in all above-mentioned groups (with the exception of Strigeidida, possibly) the second intermediate host is the latest one whereas in trixenic azygiids the latest host is the modern final host. In this case cycle becomes longer due to a «superstructure» of a sort. The initial final host becomes the second intermediate one.

The further transformations of the life cycles were connected with two just opposite trends. The common phenomenon is decreasing in host number — passage to dixenic cycles took place independently in different groups. Sometimes it was not connected with abridgement of the trematode life cycle itself, which maintained all stages. In this case either mollusk may act as both the first and the second intermediate I hosts (Heronimus mollis — Heronimidae, some Echinostomatidae, Microphallidae, etc.), or the final host acts as first as the second intermediate host (genus Opisthioglyphe — Plagiorchiida and others). Dixenic cycles of the other type also results from progenetic development of parasites and are characteristic of disappearance of the final host as the metacercaria gets reproductive maturity. In the case the reduction of circulation ways is accompanied by hypomorphosis combined with disappearance of the adult. The good example is Paralepoderma brumpti (Plagiorchiidae) life cycle. The authors consider that disappearance of marita stage has been resulted in the development of numerous group of blood parasites (Schistosomatidae, Spirorchidae, Sanguinicolidae) and ectoparasites (Transversotrematidae).

Monoxenic life cycles are of different origin. In plagiorchiids and hemiurids they 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. Monoxenic cycles in Azygiids possessing initially the archaic dixenic cycle is of the other origin.

Just opposite trend (increase in number of the animals used in the life cycle) is also revealed in trematode evolution. Sometimes up to four hosts may be recorded (tetraxenic life cycles). This is accompanied by appearance of one more developmental stage — mesocercaria. The origin of tetraxenic cycles is rather unclear. In hemiurids and dydimozoids this phenomenon probably resulted from «superstructure», as trixenic cycle in azygiids did. In Strigea and Alaria it resulted from including of one more step in the chain of carnivores circulated (infected) by the parasites.

The given data demonstrate that evolution of trematode life cycles as any other evolutionary processes are accompanied by numerous appearances of parallelisms. The each case needs a special analysis for adequate evaluation of importance and nature of either phenomenon observed. Creation of the «natural» trematode system reflecting the real phyletic interactions between certain groups requires a thorough accounting of the specific features in structure and biology of all stages of the life cycles. Only this will permit proper identification of plesiomorphic states and adequate estimation of apomorphies and homoplasies.


Chapter 1. Specific features in the structure of specimens from parthenogenetic and hermaphroditic trematode generations.
1.1.  Parthenogenetic generations
1.1.1. The first (mother) parthenogenetic generation  Miracidium Parasitic stage in mother sporocyst development.
1.1.2. The second (daughter) parthenogenetic generation.
1.2. Hermaphroditic generation.
1.2.1. Cercaria.
1.2.2. Metacercaria.
1.2.3. Adult (marita).
Chapter 2. Trematode life cycle as a system of adaptations.
2.1. Mother sporocyst.
2.1.1. Miracidium adaptations. Actively-infecting miracidia. Passively-infecting miracidia.
2.1.2. Parasitic stage in mother sporocyst development.
2.2. Daughter generation of parthenites.
2.3. Hermaphroditic generation.
2.3.1. Cercaria adaptations.
2.3.2. Metacercaria adaptations.
2.3.3. Adult (marita) adaptations.
Chapter3. Typologization of trematode life cycles.
3.1. Trixenic life cycles.

3.2. Trixenic life cycles with two endogenous agglomerations
3.3. Dixenic life cycles.
3.4. Homoxenic life cycles.
3.5. Tetraxenic life cycles.
Chapter 4. Structure specific features in trematode population.
4.1. On the nature of parasite populations.
4.2. Host-parasite interactions and their manifestations at population level.
4.3. Phase analysis of trematode populations.
4.3.1. Hemipopulations of mother sporocyst larvae.
4.3.2. Microhemipopulations of parthenogenetic generations
4.3.3. Cercaria hemipopulations.
4.3.4. Metacercaria hemipopulations.
4.3.5. Adult (marita) hemipopulations.
4.3.6. General notes.
Chapter 5. Main principles and trends in trematode evolution.
5.1. Main trends in morphological evolution of parthenogenetic and hermaphroditic generations.
5.1.1. Parthenogenetic generation.
5.1.2. Hermaphroditic generation.
5.2. Ways of trematodes expansion in ecosystems of different types.
5.3. Evolution of trematode life cycles.

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