There are only about 200 species of jellyfishes¹ throughout the world's² oceans, but their large size, abundance³, and stinging tendencies belie their small taxonomic representation.  The total includes about 50 species of sessile jellyfishes. Some 28 species of scyphozoans are known from Alaska to southern California (4 from San Juan Island, Washington), with Chrysaora fuscescens apparently being one of the most common semaeostome. Larson 1990 Bull Mar Sci 47: 546; Mills 1981 The Wasmann J Biol 39: 6; Reum et al. 2010 Northwest Sci 84 (1): 131.

NOTE¹  in the ODYSSEY the term “jellyfish” is used exclusively for the medusa stage of Class Scyphozoa, also known as scyphomedusae.  This may be a bit old-fashioned, as it goes against a trend in the scientific literature to include the medusae of Class Hydrozoa also as “jellyfishes”.  There is nothing right or wrong about either classification, but it should be kept in mind.  In the ODYSSEY the medusae of hydrozoans are referred to as hydromedusae

NOTE²  most species of semaeostome jellyfishes have worldwide distributions, and a species like Aurelia aurita may be found in such diverse regions as Europe, Japan, Polynesia, North America, and Russia, and is common in brackish-water environments such as the Baltic and Black Seas but does not, apparently, inhabit the west coast of North America. The common species here is Aurelia labiata with, based on recent molecular evidence, the occurrence of a cryptic species tentatively named A. limbata in southern California. There are many scientific works on the distribution and biology of these cosmopolitan jellyfishes, but few done on west-coast populations.  It has been difficult to winnow out which of these publications would be useful inclusions in the ODYSSEY, so be mindful of this as you read through the selections. Dawson & Jacobs 2001 Biol Bull 200: 92.

NOTE³  a phenomenon of increasing interest is the worldwide proliferation of jellyfish, including the displacement of local species by non-indigenous species. Many scientists are concerned that jellyfish blooms may be occurring in response to the cumulative effects of anthropogenic changes in ocean conditions. Examples of west-coast blooms include those of jellyfishes Chrysaora melanaster and, to a lesser extent, Cyanea capillata in the Bering Sea, and invasions of several species of indigenous Black-Sea hydromedusans into San Francisco Bay. Blooms of Aurelia spp. are common in many locations where they may interfere with water flow into coastal power plants and with net fishing. Our knowledge of the population ecology of jellyfishes, hydromedusae, and ctenophores is so scanty that scientists cannot presently distinguish between natural short-term fluctuations and possible long-term irreversible changes. Jellyfishes may be phylogenetically low, but they feed high on the food chain, and in many instances compete directly with fishes for food. See review by Mills 2001 Hydrobiologia 451: 55.

Seasonal bloom of jellyfishes Aurelia
labiata in Indian Arm, British Columbia 0.3X

Phylum Cnidaria (lit. “nettle-bearing” G.), referring to the nematocysts possessed by all members of the phylum; includes sea anemones, corals, sea pens, gorgonians, jellyfishes, hydroids

Class Scyphozoa  (lit. “cup animal” G.), including jellyfishes.  There are four Orders of jellyfishes, of which two are considered in the ODYSSEY

Order Stauromedusae (lit. “cross jellyfish” L.), including sessile jellyfishes such as Haliclystus and Manania

Order Semaeostoma (lit. “standard/streamer mouth” G.), referring to the long frilly lobes that extend from the mouth region in many or most species; includes most of the typical large jellyfishes such as Aurelia labiata, Cyanea capillata, and Chrysaora spp.

NOTE the common moon jelly, long thought to be represented on the west coast by A. aurita, has been identified as A. labiata by a researcher at the Cabrillo Marine Aquarium, California.  Chief differences are, for A. labiata:  a broader manubrium, shorter oral armsarising from the base of the manubrium, and planulae brooded on the manubrium itself rather than on the oral arms, but other differences exist, such as in the complexity of the radial-canal systems (see drawings).  For A. aurita, major differences are:  manubrium is inconspicuous and oral arms meet in the middle.  Morphology of A. labiata varies considerably along the west coast as shown in the accompanying drawings. The author also describes a possible third west-coast species (or alternatively, a fourth morph of A.
labiata), Aurelia limbata, found primarily in Arctic regions (see photograph below. A handy key to all 5 types is provided.  Gershwin 2001 Biol Bull 201: 104; see also Greenberg et al. 1996 Mar Biol 125: 401 for other considerations of west-coast Aurelia spp.    

"Southern morph": California Bight
"central morph": Santa Barbara CA
"northern morph": Puget Sound, WA
to Alaska