SummarySome photosynthetic organisms benefit from elevated levels of carbon dioxide, but studies on the effects of elevated PCO2 on the algal symbionts of animals are very few. This study investigated the impact of hypercapnia on a photosynthetic symbiosis between the anemone Anthopleura elegantissima and its zooxanthella Symbiodinium muscatinei. Anemones were maintained in the laboratory for 1 week at 37 Pa PCO2 and pH 8.1. Clonal pairs were then divided into two groups and maintained for 6 weeks under conditions naturally experienced in their intertidal environment, 45 Pa PCO2, pH 8.1 and 231 Pa PCO2, pH 7.3. Respiration and photosynthesis were measured after the 1-week acclimation period and after 6 weeks in experimental conditions. Density of zooxanthellal cells, zooxanthellal cell size, mitotic index and chlorophyll content were compared between non-clonemate anemones after the 1-week acclimation period and clonal anemones at the end of the experiment. Anemones thrived in hypercapnia. After 6 weeks, A. elegantissima exhibited higher rates of photosynthesis at 45 Pa (4.2 µmol O2 g−1 h−1) and 231 Pa (3.30 µmol O2 g−1 h−1) than at the initial 37 Pa (1.53 µmol O2 g−1 h−1). Likewise, anemones at 231 Pa received more of their respiratory carbon from zooxanthellae (CZAR  = 78.2%) than those at 37 Pa (CZAR  = 66.6%) but less than anemones at 45 Pa (CZAR  = 137.3%). The mitotic index of zooxanthellae was significantly greater in the hypercapnic anemones than in anemones at lower PCO2. Excess zooxanthellae were expelled by their hosts, and cell densities, cell diameters and chlorophyll contents were not significantly different between the groups. The response of A. elegantissima to hypercapnic acidification reveals the potential adaptation of an intertidal, photosynthetic symbiosis for high PCO2.

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.

In southern Puget Sound, large numbers of megalopae and juveniles of the brachyuran crab Cancer gracilis and the hyperiid amphipod Hyperia medusarum were found riding the scyphozoan Phacellophora camtschatica. C. gracilis megalopae numbered up to 326 individuals per medusa, instars reached 13 individuals per host and H. medusarum numbered up to 446 amphipods per host. Although C. gracilis megalopae and instars are not seen riding Aurelia labiata in the field, instars readily clung to A. labiata, as well as an artificial medusa, when confined in a planktonkreisel. In the laboratory, C. gracilis was observed to consume H. medusarum, P. camtschatica, Artemia franciscana and A. labiata. Crab fecal pellets contained mixed crustacean exoskeletons (70%), nematocysts (20%), and diatom frustules (8%). Nematocysts predominated in the fecal pellets of all stages and sexes of H. medusarum. In stable isotope studies, the δ 13 C and δ 15 N values for the megalopae (-19.9 and 11.4, respectively) fell closely in the range of those for H. medusarum (-19.6 and 12.5, respectively) and indicate a similar trophic reliance on the host. The broad range of δ 13 C (-25.2 to -19.6) and δ 15 N (10.9 to 17.5) values among crab instars reflects an increased diversity of diet as crabs develop.The association between C. gracilis and P. camtschatica is unusual because of the ontogenic switch of the symbiont from a primarily kleptoparasitic association to a facultative cleaning association. It is suggested that P. camtschatica incidentially concentrates H. medusarum in its oral arms as the symbionts transfer from its gelatinous prey. Metabolic studies suggest that by riding medusae crabs may be able to develop faster through transport into warmer surface waters while reducing the energetic costs associated with locomotion.KEY WORDS: Symbiosis · Scyphozoa · Commensal crab · Hyperiid amphipod · Cleaning behavior · Stable isotope · Metabolic rate · Fecal pellet Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 315: [221][222][223][224][225][226][227][228][229][230][231][232][233][234][235][236] 2006 1988, Sorarrain et al. 2001, Gasca & Haddock 2004. During their early stages of development, hyperiid amphipods have been established as obligate parasites of gelatinous hosts (Harbison et al. 1977), and Hyperia medusarum Müller is one hyperiid species that remains associated through adulthood and is recognized as a marine parasitoid (Laval 1980). Although much of the life cycle of H. medusarum has been examined, the mechanisms of its winter survival and the dynamics of fall population outbreaks remain unclear.The symbiotic relationships between cnidarians and brachyuran crabs are less clearly defined. Numerous symbioses of this type have been observed in the field and noted in the literature. Crabs are thought to benefit from enhanced mobility and shelter, however specific interactions in these relationships have remained largely uncharacterized.The scyphozoan jellyfish Phacellophora camtsc...

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