An Introduction to the Invertebrates, Second Edition - tiera.ru

PARASITISM71Box 6.1ParasitismParasitismis a specialised form of predation. A parasite livesinside, or attached tothe outer surface of, the body of a living host of another species, from which itobtains its food.The parasite benefits from the association at the host’s expense;typicallyit harms butdoes not killthe host and thus become deprived of a home.Parasites have evolved in almost every phylum but are particularly prevalentin protista (which are outside the scope of this book), platyhelminths (seeFigure 6.4 for two examples of life cycles), nematodes, acanthocephalansand certain arthropods (see Chapter13 for special attributes of parasitic crustaceans).Parasitesare very widespread andlikely to beimportantin anyecosystem.They are sometimes described as degenerate because they have lost some ofthe elaborations of their free-living relatives, but they are more accuratelyseen as specialised, being highly adapted to their particular way of life.For example, internal parasites have little need for organs of locomotionand food capture, but have particular difficulty in completing their life cycle.The young cannot overcrowd the parental host, so they must find new hosts;many fail and then die, so that a large number of young must be produced.Typically thereis an elaboratelife cycle with a series of larvae (perhapsmultiplyingasexually) as agents of dispersal and host-finding. These larvae frequently findan intermediate host, perhaps one eaten by the primary host, to facilitatetransfer.The life of a metazoan parasite demands special adaptations: an ectoparasitemust be able to attach to the host’s outer surface, an endoparasite mustpenetrate the host, find and perhaps attach itself to a particular tissue andresistthe host’s defences. A gut parasite must resist the host’s digestive enzymes,a bloodparasitemust adaptits surface antigens toresistthehost’simmune system.Such adaptations are not made once for all: there is a continuing ‘arms race’between parasite and host. This demands continual individual variationon both sides, which can be supplied most rapidly by sexual reproduction.The ubiquity of parasitism and the demands of the arms race may indeedbe a powerfulreason for the continuation ofthe apparently wasteful proceduresof sexual reproduction. Sudden changes in parasitic infection may in partbe responsible for the often overwhelming success of species invading a newcountry, because the invaders do not necessarily bring their parasites withthem. Such invasions are increasingly common in these days of rapid transportand may quickly decimate the native fauna.Parasites may even alter the behaviour of their hosts to suit their ownrequirements: for example, trematode flukes parasitising certain snails causethe host to seek the light, bringing them to open places where predation bybirds is much more likely. This brings the flukes to the birds, which are theirnext hosts, but does not at all benefit the snails. This behaviour changehas been attributed to action by the parasite on the host hormones, but thata parasite may directly affect gene expression has been shown in another speciesof trematode-infested snail. At infection this snail suffers a three-fold increasein production of the messenger RNA which codes for a brain neuropeptide,causing the snail to cease all sexual activity and rapidly increase in size: it canthen accommodate a larger number ofthe parasites.