''Hydrichthys'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of colonial marine

hydrozoa Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specializ ...

ns formerly placed 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 ...

Hydrichthyidae but is now included in the family

Pandeidae Pandeidae is a family of hydroids in the class Hydrozoa. Like other jellyfish there is usually a mature medusa form which is pelagic and reproduces sexually and a hydroid or polyp form which is often benthic and reproduces asexually by budding ...

. The polyps of members of this genus are

parasitic 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 c ...

. The polyp attaches itself to a fish, and in one species exhibits

hyperparasitism A hyperparasite, also known as a metaparasite, is a parasite whose host, often an insect, is also a parasite, often specifically a parasitoid. Hyperparasites are found mainly among the wasp-waisted Apocrita within the Hymenoptera, and in two othe ...

by attaching itself to a

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...

, itself the parasite of a fish.

Parasitism

The life history of hydrozoans typically has a larval, polyp stage and a bell-shaped

medusa In Greek mythology, Medusa (; Ancient Greek: Μέδουσα "guardian, protectress"), also called Gorgo, was one of the three monstrous Gorgons, generally described as winged human females with living venomous snakes in place of hair. Those ...

stage. In ''Hydrichthys'', the polyp has no tentacles but develops a root-like stolon which it thrusts through the skin of its host, usually a fish, to suck the blood and body fluids. During the medusa stage, ''Hydrichthys'' lives independently in the ocean. In one species, '' Hydrichthys sarcotretis'', parasitism is taken a stage further when the hydrozoan attaches itself to the

'' Cardiodectes medusaeus''. This is itself an ectoparasite of

lanternfish Lanternfishes (or myctophids, from the Greek μυκτήρ ''myktḗr'', "nose" and ''ophis'', "serpent") are small mesopelagic fish of the large family Myctophidae. One of two families in the order Myctophiformes, the Myctophidae are represente ...

in the family Myctophidae. The copepod attaches itself to the

bulbus arteriosus In the circulatory system of fish, the bulbus arteriosus is a pear shaped chamber that functions as a capacitor, maintaining continuous blood flow into the gill arch Branchial arches, or gill arches, are a series of bony "loops" present in fish, ...

of the fish. Such a parasitic chain is known as

. The actions of the copepod castrate its fish host. Both male and female fish do not reproduce and seem to grow faster when attacked by the copepod and it seems to have a negligible energy demand from them. The hydrozoan parasite castrates the copepod, a process called hypercastration.

Species

The World Register of Marine Species recognises the following species: *'' Hydrichthys boycei'' Warren, 1916 *'' Hydrichthys cyclothonis'' Damas, 1934 *'' Hydrichthys mirus'' Fewkes, 1887 *'' Hydrichthys pacificus'' Miyashita, 1941 *'' Hydrichthys pietschi'' Martin, 1975 *'' Hydrichthys sarcotretis'' (Jungersen, 1913)

References

Parasitic cnidarians Pandeidae Hydrozoan genera {{anthoathecata-stub