The Hydrozoa are mostly inconspicuous, both in the polyp and in the medusa stage, and are generally overlooked. The famous treatise by Tremblay, describing transplants in Hydra, inspired the fantastic novel Frankenstein by Mary Shelley. The illustrations of some monographs, especially those on medusae by Haeckel, are renowned for their beauty. The modern music composer, Frank Zappa, wrote a song on a hydromedusa that has been named after him: Phialella zappai.
"White weeds" (the colonies of hydroids of the genera Hy-drallmania and Sertularia) had been used as decoration before the sharp reduction of their populations. Some Hydrozoa are used as laboratory animals for experimental biology; Hydra is the most popular one, but others include Aequorea victoria (for the production of the labeling enzyme aequorein), Hydractinia spp., Laomedea spp., and Tubularia spp.
Hydroids are important members of fouling communities, inhibiting the functioning of power plants by clogging their pipes and reducing the velocity of ships by settling on their hulls. Some species have been reported as pests in aquaculture, feeding on the larvae of the reared species or on their food. Polypodium hydriforme is a threat to the production of caviar, being a parasite of sturgeon eggs.
Some species of medusae (e.g., Gonionemus) can inflict severe stings on humans, as do some hydroid colonies such as the species of Millepora (fire corals) and some aglaopheniids. When present in swarms, even small medusae like those of Clytia can inflict slight stings on swimmers. The most important threat to human activities is the predation of some medusae (e.g., Aequorea victoria) and floating hydroids (e.g., Clytia gracilis) on the eggs and larvae of commercially exploited fish. This kind of predation can reduce the success of fish recruitment, reducing the yield of fisheries.
1. Eudendrium glomeratum polyp; 2. E. glomeratum medusa; 3. Immortal jellyfish (Turritopsis nutricula) polyp; 4. T. nutricula medusa; 5. Hy-drichthys mirus medusa; 6. Hydractinia echinata polyp; 7. Liriope tetraphylla medusa; 8. H. mirus polyp; 9. Distichopora violacea polyp; 10. Solmundella bitentaculata medusa; 11. Aglantha digitale; 12. Halammohydra schulzei polyp. (Illustration by Emily Damstra)
1. Aequorea victoria medusa; 2. Obelia dichotoma medusa; 3. A. victoria polyp; 4. O. dichotoma polyp; 5. Zappa's jellyfish (Phialella zappai) polyp; 6. P. zappai medusa; 7. Laingia jaumotti medusa; 8. Aglaophenia pluma polyp. (Illustration by Emily Damstra)
1. By the wind sailor (Velella velella) polyp; 2. V. velella medusa; 3. Polyorchis penicillatus medusa; 4. Hydra vulgaris polyp; 5. Corymorpha nutans medusa; 6. Fire coral (Millepora alcicornis) polyp; 7. M. alcicornis medusa; 8. Paracoryne huvei polyp; 9. C. nutans polyp. (Illustration by Emily Damstra)
1. Portuguese man of war (Physalia physalis) polyp; 2. Craspedacusta sowerbyi polyp; 3. Apolemia uvaria polyp; 4. C. sowerbyimedusa; 5. Olin-dias phosphorica polyp; 6. O. phosphorica medusa. (Illustration by Emily Damstra)
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