''Hydrichthys sarcotretis'' is a species 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 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 ...

and attach themselves to a fish. ''H. sarcotretis'' is a species that 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 (living in the water column), some are benthos, benthic (living on the sedimen ...

, itself the parasite of a fish. In ''H. sarcotretis'', parasitism is taken a stage further when the hydrozoan attaches itself to the

Cardiodectes medusaeus ''Cardiodectes bellottii'' is a species of copepods in the family Pennellidae. It is a parasite of fish. It is found in the Atlantic and Pacific Oceans as well as the Mediterranean Sea; specimens from the Pacific were formerly treated as a separ ...

''. This is itself an ectoparasite of the Northern lampfish (''Stenobrachius leucopsarus'') 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 paired bony/ cartilaginous "lo ...

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.

References

Pandeidae Hyperparasites Parasites of crustaceans Parasitic cnidarians Animals described in 1913 {{anthoathecata-stub