''Homalometron pallidum'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of marine

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 ...

in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Apocreadiidae Apocreadiidae is a family of parasitic worms in the class Trematoda 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 ...

. It is an

endoparasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

of the

mummichog The mummichog (''Fundulus heteroclitus'') is a small killifish found along the Atlantic coast of the United States and Canada. Also known as Atlantic killifish, mummies, gudgeons, and mud minnows, these fish inhabit brackish and coastal waters in ...

, ''Fundulus heteroclitus'', a small fish found in

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estua ...

along the east coast of the United States and Canada. It has a complex life cycle and lives inside several different host species at different stages.

Description

The original description of ''Homalometron pallidum'' was given by Linton in 1899 and reads:

"Body very minutely spinose, white, translucent; acetabuhuni and oral sucker about same size; outline of body, long oval; neck short, continuous with body; greatest breadth in region of testes, near posterior end; ecaudate; acetabuhum sessile; ranii of intestines simple, elongate ; esophagus as long as pharynx; testes, two, in median line behind uterus; seminal vesicle dorsal to ovary and posterior border of acetabulum; ovary between acetabulum and testes, on right side; pharynx, subglobular; genital aperture in front of acetabulum, on median line; vitelline glands lying at posterior end and along sides of body as far as acetabulum; ova few, relatively large."
"Dimensions of specimen in formalin, given in millimeters: length, 2.72; breadth, anterior 0.43, at acetabulum 0.89, middle 1.07; near posterior 0.36 ; diameter of oral sucker, 0.26; diameter of acetabulum, 0.29; diameter of ovary, 0.21; diameter of testes, 0.33 and 0.39; ova, 0.11 and 0.07 in the two principal diameters."

Stafford used these diagnostic characteristics when he formally described the species in 1904. He was criticized for doing so in 1907 by the German parasitologist

Arthur Looss Arthur Looss (16 March 1861 – 4 May 1923) was a German zoologist and parasitologist. Looss was born in 1861 in Chemnitz, and was educated both there and in Łódź, Poland. Thereafter, he studied at the University of Leipzig, where he received ...

who considered the description insufficient, pointing out that it was not possible to use this description to distinguish this species from other similar parasitic trematodes.

Distribution

''Homalometron pallidum'' is found on the eastern seaboard of Canada and the United States. Its range extends from

Prince Edward Island Prince Edward Island (PEI; ) is one of the thirteen Provinces and territories of Canada, provinces and territories of Canada. It is the smallest province in terms of land area and population, but the most densely populated. The island has seve ...

and the

Gulf of St. Lawrence The Gulf of St. Lawrence () is the outlet of the North American Great Lakes via the St. Lawrence River into the Atlantic Ocean. The gulf is a semi-enclosed sea, covering an area of about and containing about of water, at an average depth of . ...

southwards to the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an oceanic basin, ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of ...

. It is found in the tidal creeks, estuaries and salt water marshes where its host species occur.''Fundulus heteroclitus heteroclitus ''

FishBase FishBase is a global species database of fish species (specifically finfish). It is the largest and most extensively accessed online database on adult finfish on the web.

. Retrieved 2012-03-06.

Life cycle

''Homalometron pallidum'' has a complex

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...

involving several

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

species. The adult worm lives in the gut of a small fish, the mummichog (''Fundulus heteroclitus'').

Eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

are ejected from the host fish in its faeces and are ingested by a small aquatic snail, '' Hydrobia minuta''. This acts as a secondary host. When the eggs hatch, they develop in to rediae. These undergo

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 ...

and multiply before becoming motile

cercaria A cercaria (plural cercariae) is the larval form of the trematode class of parasites. It develops within the germinal cells of the sporocyst or redia. A cercaria has a tapering head with large penetration glands. It may or may not have a long sw ...

e. The cercariae are few in number and large in size, in comparison to their small gastropod host. They emerge from the snail into the water at night and become free-living. They swim tail first with their body bent round and their tail lashing. They can also crawl with the use of two ventral suckers. They have penetration glands and make their way into an intermediate transfer host. Such hosts can include the

amethyst gem clam The amethyst gem clam (''Gemma gemma'') is species of very small saltwater clam, a marine bivalve mollusk in the family Veneridae, the Venus clams. It is a small species, reaching a length of only 5 mm. The shell color is whitish or grayish ...

''Gemma gemma'', another mummichog, or certain

polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...

worms, but not other species of fish. In their new host, the cercariae become metacercariae by encysting in the tissues. In this inactive state, they wait to be ingested by a mummichog. In its gut, they develop into adult worms ready to undergo sexual reproduction and start the cycle again. Other fish have been identified as also acting as a host to the adult worms, including ''

Menticirrhus saxatilis ''Menticirrhus saxatilis'', the northern kingfish or northern kingcroaker, is a species of marine fish in the family Sciaenidae (commonly known as the "drum" or "croaker" family). It lives in the shallow coastal waters of the western Atlantic Oc ...

'', ''

Morone americana The white perch (''Morone americana'') is not a true perch but is a fish of the temperate bass family, Moronidae, notable as a food and game fish in eastern North America. In some places it is referred to as "Silver Bass". The name "White perc ...

'', ''

Pseudopleuronectes americanus The winter flounder (''Pseudopleuronectes americanus''), also known as the black back, is a right-eyed ("dextral") flatfish of the family Pleuronectidae. It is native to coastal waters of the western north Atlantic coast, from Labrador, Canada to ...

'', ''

Tautoga onitis The tautog (''Tautoga onitis''), also known as the blackfish, is a species of wrasse native to the western Atlantic Ocean from Nova Scotia to South Carolina. This species inhabits hard substrate habitats in inshore waters at depths from . It is ...

'', and '' Bairdiella chrysura''. However, in some of these cases, the worm species could have been improperly identified.

References

{{taxobar, from=Q2632864 Plagiorchiida Trematodes parasiting fish Animals described in 1904 Endoparasites