''Bucephalus polymorphus'' is a type of

flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...

. This species is within the

Bucephalidae Bucephalidae is a family of trematodes that parasitize fish. They lack suckers, having instead a muscular organ called a "rhynchus" at the front end which they use to attach to their hosts. The characteristics of the rhynchus are used to help def ...

family of

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. ...

, which in turn is a subclass of

Trematodes Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host ...

within the phylum Platyhelminthes (i.e. flatworm). It is characterized by having a mouth near the middle of its body, along with a sac-like gut. The mouth opening is located in the centre of the ventral surface. This is a specific body type of cecaria known as a gastrostome. The adults occur in the gut of marine and fresh-water fish. The metacercariae encyst in smaller fish, sometimes in the nervous system. These parasitic flatworms are dorso-ventrally flattened animals characterized by a bilaterally symmetrical body enclosed within a syncytial tegument. They have a distal, anucleate later (distal cytoplasm). The distal cytoplasm contains vesicular inclusions that are Golgi derived. The adults of these

acoelomate The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it r ...

worms are common in the digestive tract, but are also found in other organs of vertebrates. The adult parasite attaches via a characteristic anterior adhesive organ with tentacles. This organ is recognized as a holdfast, which helps Bucephalus to stay attached to the host's organs. Bucephalus are native to North American fresh waters that parasitize freshwater

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

.Malloy et al.: "Survey of the Parasites of Zebra Mussels (Bivalvia: Dreissenidae) in Northwestern Russia, with Comments on Records of Parasitism in Europe and North America." J. Helminthol. Soc. Wash. 63(2), 1996, pp. 251-256

Taxonomy

The genus ''Bucephalus'' was based on this species, which was the earliest known, initially described by Baer (1827) from its cercaria. Von Siebold (1848) believed that the adult bucephalid he named ''Gasterostomum fimbriatum'' represented an adult form of the same bucephalid, but this identity has never been proven.

Life cycle

Dignetic flatworm species require more than one host to complete a full life cycle. ''Bucephalus polymorphus'' requires three hosts.

Dreissena polymorpha The zebra mussel (''Dreissena polymorpha'') is a small freshwater mussel. The species originates from the lakes of southern Russia and Ukraine, but has been accidentally Introduced species, introduced to numerous other areas and has become an inv ...

, a small freshwater mussel, is the first intermediate host parasitized by the hatching

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 com ...

. Within the visceral mass of

Dreissena ''Dreissena'' is a genus of small freshwater mussels in the family Dreissenidae in the class Bivalvia. They are found attached to firm substrates by threads from underneath the shells and are the only freshwater bivalves to attach to hard subst ...

, the miracidium transforms into a mother (primary) sporocyst.

Asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...

produces many daughter (secondary) sporocysts which are called cecariacysts, that eventually release

cercariae 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 ...

. Unlike most digenetic trematodes, Bucephalus Polymorphus lacks a redial stage and thus emerges as a cecaria directly from the sporocyst stage. Rapid proliferation of sporocysts results in a knotted white mass of tubules, which is found primarily in the gonads of the mussel. Released from the infected mussels, cercariae attach to fish (second intermediate host), encyst, and transforms into metacercariae. The third (definitive) hosts are predatory fish that consume the infected foraging fish. It has been shown experimentally that cercarial emergence exhibits a

circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogeno ...

of shedding with a peak in the dark period of a light:dark 12:12 h photoperiod.

Influence on Host

''Bucephalus polymorphus'' is a parasite residing exclusively in host connective tissues. The gonads of its first intermediate host, ''Dreissena'', is the primary target of infection and sporocyst proliferation. As infection intensifies, the sporocyst develops branches through connective tissue passages, emerges from the gonads, and can spread into other body regions. Such secondary sites of infection have been previously reported to occur in the digestive glands, the gills, the bundles of adductor muscle, and the mantle epithelium lining the interior of the shells. The study found that the digestive glands of infected bivalves appeared to be relatively normal when compared to full bodied, uninfected specimens. Within the sporocyst, the cercariae existed within a wide range of developmental stages, indicating that its development is asynchronous. It was also observed that heavy infection of the parasite led to host

castration 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 ...

, which left the entire gonadal space often occupied by the sporocysts. By limiting the infection almost exclusively to the gonads, the parasites have developed an interesting strategy to only use the reproductive energy of their hosts, thereby minimizing the risk of host mortality. Extending the life of a bucephalus infected host is important because this allows the parasite to proliferate continuously from year to year, since infected gonads produce cercariae instead of gametes. The location of the sporocyst (primarily in gonads), its overall shape, irregular branches, and the morphology of its cercariae with a bifurcated tail, distinguish ''B. polymorphus'' from other trematode parasites of zebra mussels in histological sections.

Prevalence

Infection of ''B. polymorphus'' is geographically widely distributed. However, prevalence of infection in zebra mussel population is not common. A study by Lajtner ''et al.'', which surveyed the zebra mussel population in the Drava River in Croatia, found a prevalence of 21.3%. In the most extensive study conducted to date, a prevalence of 73% was recorded in zebra mussel in South-Eastern France. Low rates of prevalence were also recorded: 1% (Kuperman ''et al.'' 1994), 1-4% (Baturo 1977),Baturo B.: "Bucephalus polymorphus Baer, 1827 and Rhipidocotyle illense (Ziegler, 1883)(Trematoda, Bucephalidae): morphology and biology of developmental stages." Acta Parasitologica Polonica, 24, 203-220, 1977. 2-5% (Smirnova and Ibrasheva 1967),Smirnova and Ibrasheva: "Larval trematodes from freshwater molluscs in the western Kazakhstan." Trudy Instituta Zoologii Akademii Nauk Kazakhskoy SSR, 27, 53-87, 1967. 9% (Malloy ''et al.'' 1996), and 13-28% (de Kinkelin ''et al.'' 1968).de Kinkelin et al.: "Etude epizootiologique de la bucephalose larvaire a Bucephalus polymorphus (Baer 1827) dans le peuplement piscicole du bassin de la Seine." Recherches Veterinaires, 1, 77-98, 1968 Therefore, the prevalence of ''Bucephalus polymorphus'' can vary widely depending on the ecosystem.

References

{{Taxonbar, from=Q2927485 Plagiorchiida