At least 60 species of hydromedusae inhabit the nearshore environs of Washington and British Columbia.  Most live in surface waters where the presence of pycnoclines, commonly caused in this geographical area by surface freshwater overlying higher salinity water, may present challenges to maintenance of proper buoyancy or to movements through the salinity layers.  Research at the Pacific Biological Station, British Columbia on the hydroid species Clytia gregaria (Phialidium gregarium) and Sarsia tubulosa reveal that their medusae are capable of sensing salinity discontinuities down to 2‰ in magnitude.  That the discontinuities do not act as a physical barrier is shown by making adjacent salinity layers equal in density by addition of sucrose to the less saline solution. The medusae easily pass through the boundary.  Arai 1973 J Fish Res Bd Can 30: 1105; Arai 1976 p. 22 In, Coelenterate biology and behavior (Mackie ed.) Plenum Press, NY.

NOTE  a rapid change of water density with depth usually associated with gradients of temperature, known as a thermocline, and salinity. known as a halocline

NOTE  created by carefully layering water of lower salinity on top of water of higher salinity.  Dye experiments show that the interface between different salinity waters is stable for several hours if not disturbed

Collections of medusae of Sarsia tubulosa in waters around Friday Harbor Laboratories, Washington show that most individuals are less dense than seawater.  By cutting off the tentacles and manubrium, the author shows that it is the bell that is the buoyant structure, as the cut-off parts sink.  If placed in dilute seawater down to about 20‰ the medusae are able to attain neutral or positive buoyancy within a few hours, possibly by exclusion of sulphate ions.  Leonard 1980 Helgoländer Meeresunters 34: 55.

The effect of salinity gradients on buoyancy of 9 hydromedusa species is tested at Friday Harbor Laboratories, Washington.  Test specimens are placed in beakers containing different salinity seawater over a range of 19-38‰ and observed for up to 48h.  After this, whether buoyancy acommodation has occurred or not, the medusae are returned to natural seawater at approximately 30‰ and again observed.  In one experiment, specimens of Aequorea victoria are placed in 5 salinities ranging from 75-127% for 24h, and tissue samples taken for analyses of mesogleal osmolalities. Results show general osmoconformation over the salinity range tested, but with a tendency for a slight, but probably non-significant, hyposmoticity (see graph ). 

The cartoon on the Left shows that specimens moved from normal seawater into more dilute seawater sink initially because of their greater density and then, after adjusting osmotically and regaining their equilibrium buoyancy, begin to swim freely in their beakers.  Conversely, specimens transferred to higher salinity water initially float to the surface, but then eventually regain their equilibrium density and also swim freely. The lowest salinity waters used (19‰) are lethal to most species, while osmotic adjustment in the other test salinities is attained by most species within 1-10h.  Based on these and other results the author suggests that medusae may not be able to cross sharp density gradients but, if they do, their abilities to adjust will allow then to resume normal acitivities within a short time. Mills 1984 Biol Bull 166: 206. 

NOTE  these include Aequorea victoria, Aglantha digitale, Bougainvillia principis, Gonionemus vertens, Clytia gregaria (Phialidium gregarium), Polyorchis penicillatus, Proboscidactyla flavicirrata, Sarsia tubulosa, and Stomotoca atra

A related study by the same research group tests whether exclusion of sulphate¹ ions is involved in regulation of buoyancy and, hence, in vertical migration² of several species of hydromedusans found around Puget Sound, Washington and Barkley Sound, British Columiba.  The researchers find considerable differences in concentrations of sulphate ions in the different species tested, but no evidence of significant day/night changes in ion³ concentrations, suggesting that vertical migration may be accomplished solely by swimming.  Mills & Vogt 1984 Biol Bull 166: 216.

NOTE¹  exclusion of heavy sulphate ions is shown to be a factor in regulation of buoyancy in certain jellyfishes, including Aurelia aurita, and the liklihood exists that it is also be found in hydromedusae; see LEARN ABOUT JELLYFISHES: LOCOMOTION & ORIENTATION

NOTE² one species used in the study, Aglantha digitale, is known to migrate vertically as much as 100m each way daily

NOTE³  the experiments involve injecting and monitoring radio-labelled sulphate, as well as simultaneously monitoring levels of several cations (sodium, calcium, potassium, and magnesium)

The increasing incidence of hypoxic conditions in marine environments prompts researchers from The Evergreen State College, Washington to investigate metabolic performance and survival in hypoxic conditions in 12 species of Puget-Sound hydro- and scyphomedusae.  Results show that half the species are oxyregulating over a range of oxygen saturation from 0-80%, while the other half are oxyconforming (see examples for 2 species of hydromedusae Aequorea victoria and Euphysa flammea in the accompanying graphs).  Note for the oxyregulating A. victoria the abrupt decline in respiration rate at oxygen concentrations less than about 10% saturation.  Survival in conditions of anoxia is, as expected, significantly greater in the oxyregulating species as compared with the oxyconforming species.  For example, of the 2 species shown in the graphs, survival is 100% for the oxyregulating A. victoria and 25% for the oxyconforming E. flammea.  Interestingly, all of the oxyconforming species have narrow velar apertures that tend to restrict exchange between the subumbrellar cavities and the surrounding seawater.  As a consequence, the subumbrellar volume becomes quickly depleted of oxygen.  Most of these oxyconforming species are also more quiescent in the respiration flasks than are the oxyregulating ones, thus exacerbating the effect.  A correspondence in poor tolerance to anoxia in several Puget Sound species with related Adriatic Sea species that have disappeared possibly as a result of increasing anoxia in their habitats, suggests that similar hypoxic events may be regulating distributions of our local species.  Rutherford & Thuesen 2005 Mar Ecol Progr Ser 294: 189. Photograph of Euphysa flammea courtesy Arctic Ocean Diversity.

NOTE  as 9 of the 12 species are hydromedusae, this study is considered in this Learn-About Hydroid section

NOTE  tests in severe hypoxic conditions last about 20h

Little is known about the ecology of the polyp phases of west-coast hydroids, and even less about those of non-native species.  This is addressed by researchers at the University of California, Davis using 3 species at 5 sites in the San Francisco Estuary.  The authors use settling plates to record settlement and distribution over a 1yr study period, and monitor several abiotic parameters possibly related to these patterns, including temperature, salinity, depth, water transparency, and dissolved oxygen.  The authors use a canonical-correspondence analysis to explore relationships of these water-quality parameters with recruitment patterns.  Results show species-specific differences in recruitment of polyps of the 3 species, with Moerisia sp. favouring an area of high dissolved oxygen (Montezuma Slough), Blackfordia virginica regions of relatively high salinity, water transparency, and temperature, and greater depths (Petaluma River and Napa River), and Cordylophora caspia areas with low- or mid-levels of these various parameters (Boynton Slough and Suisun Slough; see figure).  The 3 species differ significantly in the correlation of distribution with the environmental variables measured.  The study is interesting and includes a brief consideration of the role that that these non-native hyroids may be playing in declines in abundances of several species of smelt, bass, and shad in the Estuary.  Wintzer et al. 2011 Aquat Ecol 45: 151.

NOTE  these are Blackfordia virginica, Moerisia sp., and Cordylophora caspia, likely introduced into the area in ballast waters as early as the 1920s from the Caspian Sea region

As noted in the foregoing Research Study, a number of invasive hydroid species have arrived at, and successfully colonised, areas of the San Francisco Bay estuary, several originating from the Ponto-Caspian region.  Laboratory experiments at the University of California, Davis to determine the effects of temperature and salinity on population growth of one such species, Cordylophora caspia, suggest that while increase in the former leads to expected population-growth maxima, increase in salinity has little effect.  The authors discuss their results in the context of how climate change may preferentially benefit invasive species of hydroids over native ones.  The species has successfully colonised many areas of the world, including Europe, the Baltic Sea, both coasts of North America and parts of the Great Lakes.  Meek et al. 2012 PLoS ONE 7 (10): e46373.

NOTE  an area enclosing the Black and Caspian Seas

NOTE  from its arrival in the Suisun Marsh region of San Francisco Bay in the 1920s, densities of hydranths may now exceed 600,000 per square meter