The third symbiotic relationship we recognize is parasitism: one symbiont gains food (or other resources) at the expense of the other. Once again the boundaries of definition are blurred, since parasitism as we have defined it can also be considered a form of predation, but the idea is that the parasitic symbiont lives out all or most of its life on or within the host symbiont (sometimes several hosts). Disease-causing bacteria are familiar examples, as are various worm infections. Among plants, one example of parasitism is that of dodder, a parasitic member of the morning glory family that obtains most of its carbohydrates from host flowering plants, having very little chlorophyll of its own.

Parasitism of chinese liver fluke on humans.

The Chinese liver fluke, Clonorchis sinensis, is typical of the large number of flukes that cause parasitic disease in humans and domesticated animals. Flukes spend their adult lives in the gut of their primary hosts such as intestines, lungs, pancreatic ducts, and bile ducts of the liver. Adults of Clonorchis sinensis reside in the bile ducts of their human primary hosts, where they attach to the duct walls by means of suckers to avoid being washed downstream. Here they feed on host tissues and produce enormous numbers of eggs. The eggs leave the bile ducts with the bile and are carried into the intestine of the primary host. They are voided with the host's feces. As long as the feces are voided in water, the fluke eggs will reach their next host, for they can live in the water several weeks but will not hatch until they are ingested by a water snail. A ciliated larval stage, the miracidium, develops in each egg. When the egg is consumed by a snail, the miracidium hatches and makes its way to the snail's body tissues. There it loses its cilia and transforms into a second larval stage, the hollow sporocyst. Inside each sporocyst, germinal cells give rise to clumps of cells, or embryonic masses. Each embryonic mass develops into a third larval stage, the redia (plural: rediae), an elongate larval form with a gut tract, a nervous system, an excretory system, and many germinal cells. Rediae are motile and each makes its way to the digestive gland of the snail; here the redia's germinal cells rapidly give rise to many cercariae (singular: cercaria), which are swimming tadpole-shaped organisms. So far, this complex life cycle has allowed a single egg to give rise to many individuals by asexual reproduction. It is a successful kind of life cycle common to several thousand species of flukes. Examine the figure in the Animal Life Cycle Lab.

The tadpole-like cercariae escape into the water and swim until they encounter a fish of the carp family. The cercariae bore into the body of the fish, lose their swimming tails, and form cysts called metacercariae. If a human (or other mammal) eats the raw fish meat and the metacercariae with it, the cyst walls dissolve in the intestine of the new host and the metacercariae develop into adult flukes complete with reproductive systems. There they may live many years unless the infection is very large, in which case there is too much destruction of liver tissue for the host to withstand. Then the host, with its fluke parasites, dies.

In this system, the human is known as the primary host of the parasite because it is in the human that the parasite develops reproductive organs and reproduces sexually. The snail host is the first intermediate host and is the site of asexual reproduction. The fish host is the second intermediate host and is the second intermediate host and is the site of the larval maturation stage.

Microscopic Examination of Stages in the Life Cycle of the Chinese liver fluke (Clonorchis sinensis ).

Examine the image of an adult fluke. The worm, which may be 10-20 mm long, is flat and leaf-like and, in fact, belongs to the phylum Platyhelminthes. One end of the worm is more pointed than the other end; the pointed end has a sucker and is the anterior end of the worm. The rounded end of the worm is the posterior end; you may be able to see a pore there, which is the excretory pore.

Flukes are specially equipped to attach firmly to their hosts, to feed copiously on their hosts' mucous and body fluids, and to produce enormous numbers of eggs, so our attention will be directed particularly to the adhesive organs, the digestive tract, and the reproductive tract. There are two adhesive organs, the sucker at the anterior end called the oral sucker because it surrounds the mouth, and the ventral sucker a short distance posteriorly. The digestive tract begins with the mouth, which opens immediately into a muscular pharynx. The pharynx pumps cells, cell fragments, tissue fluids, mucous, and blood of the host into the fluke's intestine, which can be seen as a pair of long blind sacs.

Flukes are hermaphrodites, meaning each worm has both ovaries and testes. Probably cross fertilization is the rule, but self fertilization is certainly a possibility. In any case, it means that every individual is capable of producing fertilized eggs, certainly an advantage in species in which a high reproductive output is required. Locate the two highly branched testes in the image. Sperm pass from each testis out a sperm duct. The two sperm ducts unite to form the vas deferens, which widens to become a seminal vesicle, which in turn opens to the exterior via a gonopore, located just in front of the ventral sucker. There is a single rounded ovary. It receives yolk for the eggs from the vitellaria or yolk glands on each side of the worm. The yolk arrives from the vitellaria via the vitelline ducts. Sperm which have been received from another worm are stored in the seminal receptacle, and fertilization occurs. The fertilized eggs with their yolk are provided with shells from glands in the wall of the reproductive tract, shells that will be impermeable to the digestive juices of the host. The most visible structure in the image is the uterus packed with the eggs, ready to be extruded.