SUMMARY Populations of jellyfish are known to thrive in many low oxygen environments, however, the physiological mechanisms that permit these organisms to live in hypoxia remain unknown. The oxyregulatory abilities of four species of scyphomedusae were investigated, and it was found that Aurelia labiata, Phacellophora camtschatica, Cyanea capillata and Chrysaora quinquecirrha maintain steady oxygen consumption to below 20 hPa oxygen (<10% air saturation). Oxygen content of the mesoglea of A. labiata was measured using a fibre optic oxygen optode, and oxygen profiles through the gel are characterised by a gradient that decreases from just below normoxia at the aboral subsurface to ∼85% air saturation near the subumbrellar musculature. This gradient sustains oxyregulation by scyphomedusae, and it is demonstrated that A. labiata must be using intragel oxygen to meet its metabolic needs. Gel can also be used as an oxygen reservoir when A. labiata moves into hypoxia. Gel oxygen is depleted after about 2 h in anoxia and recovers to 70% of normal after 2.5 h in normoxia. Behaviour experiments in the laboratory showed that Aurelia labiata behaves similarly in normoxia and hypoxia (30% and 18% air saturation). The acute threshold for provoking behavioural changes in A. labiata is somewhere near its critical partial pressure, and oxygen stratification stimulates swimming back and forth across the oxycline. Intragel oxygen dynamics are recognised as a fundamental component of medusan physiology.

Increasing eutrophication and hypoxia in marine environments appear to differentially promote the survival of some medusae species and the disappearance of others. To understand the physiological basis for this phenomenon, respiration rates and critical oxygen tensions (P c ) were measured for 12 species of medusae from Puget Sound (Washington State, USA). Mean mass-specific respiration rates ranged between 0.064 µmol O 2 g -1 h -1 for Aequorea victoria to 0.78 µmol O 2 g -1 h -1 for Cyanea capillata. Six of 12 species studied were oxyregulators; including the scyphomedusae Aurelia labiata, C. capillata, and Phacellophora camtschatica and the hydromedusae A. victoria, Polyorchis penicillatus, and Proboscidactyla flavicirrata. Mean P c s ranged from 5.5 hPa in Muggiaea atlantica to 39.5 hPa in Euphysa flammea. The relationship between mass-specific metabolic rate and P c was significant for oxyregulators but not oxyconformers. An apparent metabolic depression occurred variably within all oxyregulating species and 2 oxyconforming species, Clytia gregaria and Sarsia sp., whereby sub-P c oxygen uptake decreased by 77 to 99% relative to standard aerobic metabolic rate (SMR). Anoxia survival varied from less than 2 h for E. flammea and Eutonina indicans to more than 10 h for A. victoria. The poor low oxygen tolerance of several Puget Sound species in our study was consistent with the historical disappearance of related species in the Adriatic Sea following increased frequency of dysaerobic events. Interspecies variation in aerobic metabolic characteristics and hypoxia and anoxia tolerance may explain why some medusae thrive in lowoxygen conditions, while others disappear.KEY WORDS: Medusae · Oxyregulation · Hypoxia tolerance · Critical PO 2 · Hydromedusae · ScyphomedusaeResale or republication not permitted without written consent of the publisher

Ctenophores are important members of planktonic communities that are often abundant in dysaerobic environments. Previous studies have shown that ctenophores are not adversely affected by extended periods of hypoxia. The three species used in this study, Pleurobrachia bachei, Bolinopsis infundibulum, and Mnemiopsis leidyi, were all able to oxyregulate to very low partial pressures of oxygen (PO2s). These species were found to have mean critical oxygen tensions of 7.7, 10.6, and 7.2 hPa respectively. In general, ctenophores are better oxyregulators than medusae and many species of shrimps, fish and squid. Intragel oxygen was measured using a fibre optic oxygen optode. All these ctenophores have intragel subsurface [O2]s of 5–10% below that of the surrounding seawater. Intragel oxygen measurements of P. bachei showed a gradient of decreasing PO2 from surface tissues to the gut. Specimens of P. bachei over 14 mm in diameter had anaerobic guts. Survival times in anoxia ranged from 0 h for M. leidyi to up to 6 h for P. bachei. Ctenophores rely on aerobic metabolism to tolerate hypoxia.