Click the underlined text for definitions of terms.
Fun Facts
- This species causes “Swimmer’s itch” in humans when accidentally try to penetrate a person, instead of a seagull or duck, and the immune system responds resulting in a rash.
- The life cycle was elucidated by researchers at UConn, who discovered that seagulls and ducks serve as the final host for this worm.
Morphology: This is one of the few groups of digeneans with separate male and female worms. The female sits in a groove on the body of the male like a hot dog (female) in a hot dog bun (male); most other digeneans are . Both sexes have two anterior suckers.
Life Cycle
- , involving a seagull (e.g., American Herring Gull) as the and the common mud dog whelk (i.e., marine snail) as the .
- Male and female worms mate in the blood vessels.
- The female produces eggs that move from the blood vessels through the wall of the gut and are released into seawater with the poop of the bird.
- Snails become infected when a free-swimming larva, called a , hatches from an egg, and finds and penetrates the snail.
- The miracidium develops into the next larval stage, called a , which produces multiple larvae called .
- Birds become infected when the free-swimming cercaria are released from the snail into the seawater and find and penetrate the bird, where they migrate to the blood vessels and develop into adults.
Click the question marks for definitions of terms.
Final Host: Also sometimes referred to as the definitive host, this is the host in which the parasite reaches sexual maturity (i.e., develops into the adult stage). This type of host is required for parasites with complex life cycles to complete their development.
Intermediate Host: A host in which a parasite undergoes some development but is not able to reach sexual maturity. Parasites with complex life cycles must pass through one or more intermediate hosts in order to complete their development.
Nematode: Nematodes are round worms. Although many live in soil or water, many others are endoparasites that live in a variety of different organ systems in all major groups of vertebrates and even some invertebrates. They feed on bacteria, blood, or other body fluids, depending on where they live in the host. Some parasitic nematodes have a simple life cycle and parasitize only one species of host throughout their life. Others have a complex life cycle that involves two or more host species. They typically have five larval stages all of which resemble the adult worm but are smaller. Adult parasitic nematodes are either male or female.
Monogenean: Monogeneans are parasitic flatworms that typically live on the external surfaces of fish, sharks, and stingrays. Those that live on the gills feed on blood and gill tissue while those that live on the skin feed on skin cells and mucous. They have a simple life cycle that involves only a single host species. They are hermaphroditic. They have a free-swimming larval stage called an oncomiracidium. Adults have a posterior attachment structure with hooks, clamps, or anchors, which they use to attach to their host.
Oncomiracidium: The oncomiracidium (plural oncomiracidia) is the only larval stage in the life cycle of a monogenean. It hatches from the egg and uses the cilia on its outer surface to swim until it finds its next host. It resembles the miracidium of digeneans but differs in its possession of multiple pairs of hooklets on the posterior region of its body.
Tapeworm: Tapeworms are parasitic flatworms that live in the stomach or intestine of all major groups of vertebrates as adults. They lack all elements of a digestive system and absorb their food through their body surface. They are hermaphroditic. Adult tapeworms have an anterior attachment organ called a scolex followed by a chain of compartments called proglottids that essentially contain only reproductive organs, referred to as a strobila. The scolex is often equipped with structures such as suckers and hooks to help them attach to the surface of the gut of their host. Most have complex life cycles that involve two or more hosts. Different kinds of tapeworms have different kinds of larval stages.
Procercoid: This is the first larval stage in the life cycle of some aquatic tapeworms. It has a simple body with a posterior region that bears three pairs of small hooks. It typically parasitizes the first intermediate host.
Plerocercoid: This is the second larval stage in the life cycle of some aquatic tapeworms. It has a rudimentary scolex and a solid larval body, which unlike the adult body, is not divided into compartments called proglottids. It typically parasitizes the second intermediate host.
Copepod: Copepods are aquatic arthropods. Many are free living, but many others are parasites on the outer surfaces of aquatic vertebrates or inside the body of aquatic invertebrates. Parasitic species often have mouthparts and legs with hooks and claws for attaching to their host. Male and female copepods mate on/in the host and the female produces eggs that are usually housed in a pair of external egg sacs. They have 2 free-swimming larval stages called the nauplius and the copepodid, which molts to become a parasitic copepodid stage when it finds a host.
Nauplius: The nauplius (plural nauplii) is the first larval stage in the life cycle of a copepod. It is the stage that hatches from the egg. This free-swimming larva has only three pairs of simple appendages and its body is unsegmented. It typically bears a single eye.
Copepodid: The copepodid is the second larval stage in the life cycle of a copepod. It is initially free-swimming, has a greater number of appendages than the nauplius, has some body segments and is the larval stage responsible for finding a host. Once on the host it remains parasitic and molts several times, eventually developing into the adult.
Digenean: Digeneans (singular digenean) are parasitic flatworms. They have complex life cycles, that typically involve a mollusk (i.e., snail or clam) and one or two other hosts. Adult worms are endoparasites. They live in most organ systems of all major groups of vertebrates, but specific groups of digeneans are usually restricted to specific organs in specific groups of vertebrates. They feed on blood or other body fluids, depending on where they live in their host. Most have two suckers, one located at the front of their body associated with their mouth and the other located more posteriorly. Almost all are hermaphroditic; the exceptions are those that live in the blood of their host.
Miracidium: The miracidium (plural miracidia) is the first larval stage in the life cycle of a digenean. It is the stage that hatches from the egg. Most are able to swim using cilia on their outer surface. Their lives are very short; they typically have only a few days, or in some species a few hours to find the first host in their life cycle, which is usually a snail. When they find their snail host they burrow into its tissue and take up residence in one of the internal organs where they develop into the sac-like second larval stage.
Sporocyst: The sporocyst (plural sporocysts) is a sac-like stage in the life cycle of a digenean that develops from the miracidium in the mollusk intermediate host. It multiplies asexually, producing large numbers of genetically identical individuals of the next larval stage, which can be a second generation of sporocysts, or redia.
Cercaria: The cercaria (plural cercariae) is a free-swimming larval stage, produced by sporocysts or redia within the mollusk intermediate host of digeneans. It typically escapes from the mollusk and swims until it finds, and penetrates its next host. The anterior portion of the body of most cercariae bears two suckers; the posterior portion consists of a tail that assists in swimming.
Endoparasite: Endoparasites live in any of the internal organs of a host, such as the brain, lungs, liver, stomach, intestine, blood vessels, heart, lymphatic channels, etc. and tend to have smooth bodies.
Ectoparasite: Ectoparasites live on any of the outer surfaces of the body of a host, such as skin, scalp, feathers, fins, eyes, gills, mouth, etc. and typically have a variety of structures modified for attachment to their host.
Simple Life Cycle: Parasites with simple life cycles require only a single host species to complete their development.
Complex Life Cycle: Parasites with complex life cycles require two or more different host species in order to complete their development, and typically undergo extensive body form transformation in each of these host species.
Paratenic Host: This term is used for a host in which a parasite lives but does not undergo any development. Unlike intermediate and finals hosts, paratenic hosts are not required for a parasite to complete its life cycle. Some parasites use this type of host to bridge a gap in the food chain, making it more likely that they will encounter their final host.
Trophic Transmission: Trophic transmission is when the transfer of a parasite between two hosts in its life cycle involves one host being eaten by another host.
Appendage: Appendages are extensions or projections from the body of an organism. Examples are the legs and mouthparts of copepods.
Hermaphroditic: Hermaphroditic individuals possess both male and female reproductive organs.
Gravid: This term is used when the uterus of the organism is full of eggs.
Infective Stage: An infective stage is any stage in the life cycle of a parasite that is able to successfully enter or attach to and survive in or on a host.
Pathology: Pathology is the harm to its host caused by a parasite.
Lesion: Lesions are damage to the tissue caused by parasites at the site of their attachment to the host.
Photograph of adult male of Austrobilharzia variglandis
Line drawing of adult male of Austrobilharzia variglandis
Photograph of adult female of Austrobilharzia variglandis
Line drawing of adult female of Austrobilharzia variglandis
Photograph of adult male of Austrobilharzia variglandis with adult female in gynecophoral canal
Definitive Host (American Herring Gull)
Photo by Sydney Horan