''Hydrichthys'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of colonial marine

hydrozoa Hydrozoa (hydrozoans; from Ancient Greek ('; "water") and ('; "animals")) is a taxonomy (biology), taxonomic class (biology), class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline wat ...

ns formerly placed in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Hydrichthyidae but is now included in the family

Pandeidae Pandeidae is a family (biology), family of Anthomedusae, hydroids in the Class (biology), class Hydrozoa. Like other jellyfish there is usually a mature Medusa (biology), medusa form which is pelagic and reproduces sexually and a hydroid or polyp ...

. The polyps of members of this genus are

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The ent ...

. The polyp attaches itself to a fish, and in one species exhibits hyperparasitism 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 (living in the water column), some are benthos, benthic (living on the sedimen ...

, 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 (; ), also called Gorgo () or the Gorgon, was one of the three Gorgons. Medusa is generally described as a woman with living snakes in place of hair; her appearance was so hideous that anyone who looked upon her wa ...

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 ''Hydrichthys sarcotretis'' is a species of colonial marine hydrozoans now included in the family Pandeidae. The polyp (zoology), polyps of members of this genus are Parasitism, parasitic and attach themselves to a fish. ''H. sarcotretis'' is a s ...

'', parasitism is taken a stage further when the hydrozoan attaches itself to the

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

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

in the family Myctophidae. The copepod attaches itself to the bulbus arteriosus of the fish. Such a parasitic chain is known as hyperparasitism. 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 ''Hydrichthys'' is a genus of colonial marine hydrozoans formerly placed in the Family (biology), family Hydrichthyidae but is now included in the family Pandeidae. The polyp (zoology), polyps of members of this genus are Parasitism, parasitic. T ...

'' Fewkes, 1887 *''

Hydrichthys pacificus ''Hydrichthys'' is a genus of colonial marine hydrozoans formerly placed in the family Hydrichthyidae but is now included in the family Pandeidae. The polyps of members of this genus are parasitic. The polyp attaches itself to a fish, and in one ...

'' Miyashita, 1941 *''

Hydrichthys pietschi ''Hydrichthys'' is a genus of colonial marine hydrozoans formerly placed in the family Hydrichthyidae but is now included in the family Pandeidae. The polyps of members of this genus are parasitic. The polyp attaches itself to a fish, and in one ...

'' Martin, 1975 *''

'' (Jungersen, 1913)

References

Parasitic cnidarians Pandeidae Hydrozoan genera {{anthoathecata-stub