''Microphallus piriformes'' is a
parasitic trematode (fluke). It belongs to the
Plagiorchiata, a large
suborder
Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...
of the
digenea
Digenea (Gr. ''Dis'' – double, ''Genos'' – race) is a class of trematodes in the Platyhelminthes phylum, consisting of parasitic flatworms (known as ''flukes'') with a syncytial tegument and, usually, two suckers, one ventral and one oral. ...
n fluke
order
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...
Plagiorchiida. ''M. piriformes'' is unusual among the flukes in having only one
intermediate host
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
rather than two, and no free-swimming
cercarian stage.
[McCarthy ''et al.'' (2000)]
It is most notable, however, for modifying the
behaviour
Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as wel ...
of its intermediate host in a way that increases its chance of transmission to the
primary host
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
. This has been researched by
Helen O. McCarthy and her team of
University of Ulster scientists both in the
laboratory and at
Muck Island in
Scotland.
Ecology
The
life cycle of ''M. piriformes'' requires two hosts: the
rough periwinkle
''Littorina saxatilis'', common name the rough periwinkle, is a species of small sea snail, a marine gastropod mollusc in the family Littorinidae, the winkles or periwinkles. First identified in the 1700s, it has been misidentified as a new spec ...
''Littorina saxatilis'' (a
littoral snail) and the
herring gull ''Larus argentatus'' (and perhaps other members of the herring gull complex). The immature flukes live in the snail, and the adult flukes live in the
bird.
Normally, rough periwinkles are rarely eaten by herring gulls. This is due to two reasons: First, the birds forage in the snails' rocky shore
habitat mainly during the breeding season (which lasts about 4 months each summer) but not at other times. Second,
periwinkles move with the
tides to stay at the water line. Thus, even when the gulls forage in periwinkle habitat, the snails tend to stay out of easy reach of the birds.
It is not precisely known how the snails are infected; probably they eat the eggs directly from the gulls'
feces
Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...
. The eggs hatch into a
miracidium
The Miracidium is the second stage in the life cycle of trematodes. When trematode eggs are laid and come into contact with fresh water, they hatch and release miracidium. In this phase, miracidia are ciliated and free-swimming. This stage is comp ...
stage, which develops into multiple
sporocysts. Rather than further developing into a cercaria which leaves the snail to infect the second intermediate host as usual for trematodes, they directly develop into
encysted metacercaria
Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
e in the snail. Infection
castrate
Castration is any action, surgical, chemical, or otherwise, by which an individual loses use of the testicles: the male gonad. Surgical castration is bilateral orchiectomy (excision of both testicles), while chemical castration uses pharmaceut ...
s the periwinkles;
brooding ''L. saxatilis'' generally move only to avoid being left above water by the tide while non-brooding (including castrated) periwinkles are more active.
As soon as the flukes have reached the metacercarian stage, they alter the periwinkles' behaviour: Rather than moving to stay near the water line, the infected snails develop a pronounced tendency to move upwards. This becomes most pronounced shortly before
high tide, and thus the periwinkles carrying mature metacercariae are left a considerable distance above water when the tide recedes. This makes them far more accessible to the foraging gulls than uninfected
conspecifics, which will often move up when the tide is highest to feed on
algae which they cannot otherwise access, but return underwater as soon as the tide recedes to avoid
desiccation
Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container.
...
.
Once the snails are eaten by herring gulls, the metacercariae hatch into mature flukes. These mate and produce eggs which are shed with the bird's feces. Their adult lifespan is a mere two weeks; several
generations are thus produced each summer. It is unknown whether ''M. piriformes'' spends the winter as resting eggs in the habitat, or as sporocysts in already-infected periwinkles. In any case, snails harboring metacercariae generally die of desiccation before winter if they are not eaten.
See also
Other
parasites that modify intermediate host behavior to facilitate transmission:
* ''
Dicrocoelium dendriticum
''Dicrocoelium dendriticum'', the lancet liver fluke, is a parasite fluke that tends to live in cattle or other grazing mammals.
History of discovery
Much of what is presently known about ''Dicrocoelium dendriticum'' is the result of the wor ...
'', a fluke
* ''
Corynosoma constrictum'', a
thorny-headed worm
* ''
Gynaecotyla adunca'', a fluke
* ''
Hymenolepis diminuta
''Hymenolepis diminuta'', also known as rat tapeworm, is a species of '' Hymenolepis'' tapeworm that causes hymenolepiasis. It has slightly bigger eggs and proglottids than '' H. nana'' and infects mammals using insects as intermediate host ...
'', a
tapeworm
* ''
Polymorphus paradoxus'', a thorny-headed worm
Footnotes
References
* (2000): A transmissible trematode affects the direction and rhythm of movement in a marine gastropod. ''Animal Behaviour'' 59(6): 1161–1166.
(HTML abstract)
{{Taxonbar, from=Q6839864
Molluscs described in 1983
Parasites of molluscs
Parasites of birds
Plagiorchiida