I have spent much of the past three decades studying hydromedusae, jellyfish members of one class of the Cnidaria. In general, hydromedusae are small, and either transparent or lightly pigmented, although some of the deep sea species are deeply colored - usually some version of dark red. (Scyphomedusae, in contrast, are usually large and often highly pigmented and are what most people think of when jellyfish are mentioned.) Hydromedusae are often abundant in coastal habitats, but they are usually seasonal, occurring most commonly from late spring into the early fall. Many species live only a few days and never get more than a few millimeters in size, but others are easily visible, reaching a few centimeters and living several months. Few hydromedusae sting, although the few that do may be quite painful. One called Gonionemus (see below upper left) in the Russian Far-East can send its human victims to the hospital.

Some favorite Pacific Northwest (NE Pacific) hydromedusae:

(COUNTERclockwise from upper left, not to scale.)
Gonionemus vertens (whose habitat in the NE Pacific may be decreasing with human "development")
Polyorchis penicillatus (whose habitat and population numbers are definitely decreasing in some places)
Aglantha digitale (probably a living densitometer)
Sarsia princeps (now occupies the path of human migration from Siberia to the New World)
Photographic image copyright 1998 Claudia E. Mills.

Most coastal hydromedusae are asexually budded off their single-sexed parent hydroids. You can see a very nice little video of the hydrozoan species Dipurena reesi, filmed by my Brazillian colleague Alvaro Migotto, which labels the various parts of a hydroid and then shows feeding and growth of the hydroid colony in the laboratory, and then the production of medusa buds on this hydroid and their eventual release as tiny medusae. Each hydroid colony that produces medusae, generates only male or female medusae, but not both. The female or male medusae then produce eggs and sperm which are free-spawned into the sea; the fertilized eggs develop into new hydroids, which are usually benthic (bottom-living). The hydromedusae therefore represent only part of the life cycle of each animal, and it is important to realize that their usually-attached polyps/hydroids must also be considered when one thinks about the ecology of these organisms. Some open ocean hydromedusae also have hydroids, which may live deep on the sea floor, but some of these oceanic hydroids have found highly specialized substrates to live on, such as little floating clumps of algae, the skin of fishes, or the shells of pelagic snails. Other hydromedusae (typically oceanic or deep water species) do not have a hydroid, but have a life cycle in which the fertilized eggs produced by medusae instead develop directly into the next generation of medusae. Such species are sometimes described as "holoplanktonic" - carrying out their entire life cycle in the plankton.

In the early 1970s when I first got interested in jellyfish, I fully intended to approach this group from a holistic point of view and remain familiar with the entire life cycle of these lovely little animals. The fact that one part of the life cycle, the polyp (this is the same as the hydroid), nearly always lives in the benthos (bottom), attached to rocks, docks, and occasionally other animals or plants, and the other phase, the medusa, usually lives up in the plankton, made this intention pretty difficult to uphold. Field work usually takes one either to the intertidal or subtidal bottom, or to the water column, but rarely to both on the same trip. Like virtually everyone else who works on the Hydrozoa, I know a lot more about one part of their life cycle than the other - in my case, it is the medusae; for most researchers, it is the polyp or hydroid form. A good proportion of the scientific papers that I have written report on hydromedusae in one way or the other; the abstracts for all of my papers are elsewhere on this website and although rather technical, provide quite a bit of information if you read through them. I have had really wonderful opportunities to study these animals, first at the Bodega Marine Laboratory, but primarily at the Friday Harbor Laboratories, with occasional trips out onto or into the high seas, peering at the deep denizens from the windows of a submersible or the camera on an unmanned sub (ROV), or occasionally from behind a snorkeller's mask. The simplest tool for collecting small jellyfish from a boat or off a dock is a cup-on-a-stick type jellyfish catcher: several designs are illustrated - anyone could put one together in order to take a look at the lovely planktonic animals near the closest dock.

This is a photo of the little-known trachymedusa Voragonema (was Benthocodon) pedunculata, collected by submersible at about 2700' deep in the Bahamas, and then photographed in the ships' laboratory in an aquarium. The bell is about 2 cm in diameter. This species is usually found near the bottom in very deep water and may be worldwide in distribution. Based on its morphology, I suspect that it may use the myriad of tiny tentacles to feed on small bottom-living crustaceans, but such a behavior hasn't been seen. It is difficult to get near these animals with an illuminated submersible or ROV without disturbing them and causing them to swim, thus losing the ability to see what they are doing when they sit there.
Photographic image copyright 1998 Claudia E. Mills.

In the twenty-first century my work has become mostly synthetic, putting together a more general and global picture from the many experiences I have had. The world's oceans are showing significant change just in the time that I have been working on medusae. I am now particularly concerned with marine conservation issues, some of which involve even these primitive organisms, indeed in some cases are highlighted by them. About ten years ago I put together a rather technical list of those Cnidarian species, mostly Hydrozoa, believed to be non-indigenous but established, in Puget Sound, where I live and work. Movement of ballast, both solid rock ballast in past times, and now ballast water in enormous quantities, has facilitated the movement of many species (including jellyfishes) out of their native biogeographic ranges into new areas, where their populations sometimes explode and cause havoc with the existing marine ecosystem. A number of the jellyfish blooms now documented worldwide are due to non-indigenous jellyfish species getting a toehold in a new ecosystem (although most of these problem jellyfish blooms are due to the generally-larger scyphomedusae, not hydromedusae).

Many species of hydromedusae (as well as siphonophores, scyphomedusae, ctenophores and many other kinds of marine organisms) are bioluminescent. Such bioluminescence does not appear as an all-over glow, but is typically localized in a species- or genus-specific pattern. Some cnidarian species emit their luminescence through a green fluorescent protein (GFP) that causes the luminescence to appear green (rather than its natural blue color); some other colors of fluorescence can also be found naturally in the sea. Research using hundreds of thousands of harvested Aequorea victoria hydromedusae between the 1960s and late 1980s in Friday Harbor, Washington, resulted in synthetic production of two components of the luminescent system of this jellyfish, a luminescent protein aequorin and GFP. Because these two substances have become increasingly prominent in biomedical research and teaching, I have tried to clear up some of the mythology about bioluminescence by the hydromedusa Aequorea victoria on another page.

If you are interested in hydromedusae, you might consider learning a little about the siphonophores, too. Siphonophores are rather exotic, colonial pelagic animals, closely related to hydromedusae, and like hydromedusae, can be found from ocean surface waters to the deep sea. Recent genetic and molecular studies show, in fact, that the siphonophores are just a group of rather modified hydrozoa that belong, phylogenetically, embedded within and alongside the rest of the hydromedusae. Most siphonophores are composed of one or more co-ordinated swimming bells, which are a lot like hydromedusae without tentacles, but they then have a long trailing stem, which carries many stomachs and feeding tentacles arranged as many repeating units; these would be carried inside and around the bell margin in the better-known jellyfish body plan. Casey Dunn has been studying the evolution and development of siphonophores and has put together a very informative web page with both text and images. His siphonophore website also contains a lot of information on coloniality as a concept, and on the collection techniques that we use to study these and other planktonic animals in the open ocean and in the deep sea.


It should be noted that there is little reason for me to provide the kind of taxonomic coverage on this webpage for hydromedusae that I have attempted on two other pages for the ctenophores and the stauromedusae. The hydromedusae have been well organized and well-documented over the past 50 years by a number of other scientists and basic systematic information can easily be extracted from a small number of published sources. The best place to start is P.L. Kramp's (1961) Synopsis of the Medusae of the World, which is volume 40 of the Journal of the Marine Biological Association of the United Kingdom, all of which is available online at no charge. Volume 40 has no illustrations, but one can identify most of the jellyfishes in the world by reading carefully. A trio of less-easily accessed, illustrated companion volumes by Kramp, published as Dana Reports (1965, Dana Report #63 and 1968, Dana Report #72 = Hydromedusae of the Pacific and Indian Oceans, and 1959, Dana Report #46 = Hydromedusae of the Atlantic Ocean and Adjacent Waters), are indispensible for serious study of diversity of the group. The scientific names in those volumes are now 50 or more years old, but most still hold and the keys and illustrations are extremely useful for making identifications. A more recent, but more difficult to find, classification is J. Bouillon's (1985) Essai de classification des Hydropolypes-Hydroméduses (Hydrozoa-Cnidaria) in volume 1 of the journal called Indo-Malayan Zoology, pp. 29-243. Also obscure, but now available at no cost online, is Bouillon and Boero's (2000) Phylogeny and Classification of Hydroidomedusae in Vol. 24 of Thalassia Salentina (download both pdf articles for the complete volume), a journal out of the University of Lecce, which attempts to classify all non-siphonophoran pelagic hydrozoa described through 1999 or so (Like Kramp 1961 this has no illustrations), but it seems to have been perhaps too-hastily published and it unfortunately contains many small errors; still, it is indispensible for putting together all species described in the nearly 40 years following Kramp's 1961 Synopsis. For assigning the most accurate and updated names to any jellyfish, one must consult a broadly-dispersed scientific literature spread through various journal titles from all over the world. A large number of papers on hydromedusae have been published in Scientia Marina, which is presently open access online. See especially volumes from 1992 (vol. 56, no. 2-3 and vol. 56, no. S1), 1996 (vol. 60, no. 1) and 2000 (vol 64, no. S1) for compilations of papers on hydrozoans. Many scientists also provide access to the pdf versions of their published papers on their personal websites.

If you are very interested in jellyfish and would like to read or just look at one of the great classic scientific studies of the group, you can access online and at no charge, the two volume, beautifully illustrated set of books Medusae of the British Isles by the late Sir Frederick Russell: Volume 1 (1953) is primarily about Hydromedusae, and Volume II (1970) covers Scyphomedusae, with some additional pages on hydromedusae at the end. The Marine Biological Association of the United Kingdom has also made available online at no charge, all of the papers that F.S. Russell published in their journal from 1925 to 1978. Many of these will be useful over time to the serious student of the hydromedusae. I was very lucky to have a several year correspondence with F.S. Russell when I was starting and he was retiring from the field.


With many thanks to those who have provided some of the great opportunities (perhaps in order of appearance): the Mills family, my eighth grade science teacher Consuela Shaw, Ralph Smith, Cadet Hand, Richard Mariscal, George Mackie, Tom Schroeder, Richard Harbison, Jim Carlton; and also to Ric Brodeur, Jim Childress, Charles Greene, Steve Haddock, Richard Harbison, Philippe Laval, Erik Thuesen and Edie Widder for inviting me along on their oceanographic cruises; and to all of my many friends and acquaintances at the Friday Harbor Laboratories.

One of my first marine illustration efforts, 1955. They were octopus, actually, but they sure
look like jellyfish. It takes a lot of practice to get good images and these went on for a full page.

This site is maintained by C. E. Mills and all photographs are copyrighted by the author
** This page was established March 1998; last updated 7 September 2012 **

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