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Recent development of environmental DNA (eDNA) analysis allows us to survey underwater macro-organisms easily and cost effectively; however, there have been no reports on eDNA detection or quantification for jellyfish. Here we present the first report on an eDNA analysis of marine jellyfish using Japanese sea nettle (Chrysaora pacifica) as a model species by combining a tank experiment with spatial and temporal distribution surveys. We performed a tank experiment monitoring eDNA concentrations over a range of time intervals after the introduction of jellyfish, and quantified the eDNA concentrations by quantitative real-time PCR. The eDNA concentrations peaked twice, at 1 and 8 h after the beginning of the experiment, and became stable within 48 h. The estimated release rates of the eDNA in jellyfish were higher than the rates previously reported in fishes. A spatial survey was conducted in June 2014 in Maizuru Bay, Kyoto, in which eDNA was collected from surface water and sea floor water samples at 47 sites while jellyfish near surface water were counted on board by eye. The distribution of eDNA in the bay corresponded with the distribution of jellyfish inferred by visual observation, and the eDNA concentration in the bay was ~13 times higher on the sea floor than on the surface. The temporal survey was conducted from March to November 2014, in which jellyfish were counted by eye every morning while eDNA was collected from surface and sea floor water at three sampling points along a pier once a month. The temporal fluctuation pattern of the eDNA concentrations and the numbers of observed individuals were well correlated. We conclude that an eDNA approach is applicable for jellyfish species in the ocean.

What happens when an animal is injured and loses important structures? Some animals simply heal the wound, whereas others are able to regenerate lost parts. In this study, we report a previously unidentified strategy of self-repair, where moon jellyfish respond to injuries by reorganizing existing parts, and rebuilding essential body symmetry, without regenerating what is lost. Specifically, in response to arm amputation, the young jellyfish of Aurelia aurita rearrange their remaining arms, recenter their manubria, and rebuild their muscular networks, all completed within 12 hours to 4 days. We call this process symmetrization. We find that symmetrization is not driven by external cues, cell proliferation, cell death, and proceeded even when foreign arms were grafted on. Instead, we find that forces generated by the muscular network are essential. Inhibiting pulsation using muscle relaxants completely, and reversibly, blocked symmetrization. Furthermore, we observed that decreasing pulse frequency using muscle relaxants slowed symmetrization, whereas increasing pulse frequency by lowering the magnesium concentration in seawater accelerated symmetrization. A mathematical model that describes the compressive forces from the muscle contraction, within the context of the elastic response from the mesoglea and the ephyra geometry, can recapitulate the recovery of global symmetry. Thus, self-repair in Aurelia proceeds through the reorganization of existing parts, and is driven by forces generated by its own propulsion machinery. We find evidence for symmetrization across species of jellyfish ( Chrysaora pacifica , Mastigias sp., and Cotylorhiza tuberculata ). Significance Animals are endowed with the capacity to repair injuries. In this study, we found that, upon amputation, the moon jellyfish Aurelia aurita rearranges existing body parts and recovers radial symmetry within a few days. This unique strategy of self-repair, which we call symmetrization, requires mechanical forces generated by the muscle-based propulsion machinery. We observed a similar strategy in a number of other jellyfish species. This finding may contribute to understanding the evolutionary pressures governing biological self-repair strategies. Beyond biology, this finding may inspire a mechanically driven, self-organizing machinery that recovers essential geometry without regenerating precise forms.

A large number of humans are stung by jellyfish all over the world. The stings cause acute pain followed by persistent pain and local inflammation. Harmful jellyfish species typically cause strong pain, whereas harmless jellyfish cause subtle or no pain. Jellyfish sting humans by injecting a tubule, contained in the nematocyst, the stinging organ of jellyfish. The tubule penetrates into the skin leading to venom injection. The detailed morphology of the nematocyst tubule and molecular structure of the venom in the nematocyst has been reported; however, the mechanism responsible for the difference in pain that is caused by harmful and harmless jellyfish sting has not yet been explored or explained. Therefore, we hypothesized that differences in the length of the nematocyst tubule leads to different degrees of epithelial damage. The initial acute pain might be generated by penetration of the tubule, which stimulates pain receptor neurons, whilst persistent pain might be caused by injection of venom into the epithelium. To test this hypothesis we compared the lengths of discharged nematocyst tubules from harmful and harmless jellyfish species and evaluated their ability to penetrate human skin. The results showed that the harmful jellyfish species, Chrysaora pacifica, Carybdea brevipedalia, and Chironex yamaguchii, causing moderate to severe pain, have nematocyst tubules longer than 200 μm, compared with a jellyfish species that cause little or no pain, Aurelia aurita. The majority of the tubules of harmful jellyfishes, C. yamaguchii and C. brevipedalia, were sufficiently long to penetrate the human epidermis and physically stimulate the free nerve endings of Aδ pain receptor fibers around plexuses to cause acute pain and inject the venom into the human skin epithelium to cause persistent pain and inflammation.

Chrysaora pacifica, a scyphozoan jellyfish widely distributed in East Asian waters, has recently shown signs of range expansion along the coasts of Korea, Japan, and China. However, ecological information on its early planktonic stage, the ephyra, remains limited. In this study, we experimentally investigated the effects of seawater temperature on the growth, feeding, and survival of C. pacifica ephyrae under controlled laboratory conditions. Five temperature treatments (12, 16, 20, 24, and 28 °C) were selected based on the species’ natural occurrence period. The results showed that ephyrae exhibited stable growth and feeding at 20–24 °C, with a high survival rate of approximately 90%, indicating that this range represents the optimal thermal condition for the ephyra stage. At 28 °C, growth and feeding were highest among all treatments; however, survival declined sharply to 22.5%, suggesting that elevated temperature may impose physiological stress. In contrast, at 12 °C, both growth and feeding activity were markedly reduced, and survival decreased to 32.5%. These findings demonstrate that temperature is a key environmental factor influencing the physiological performance and survival of C. pacifica ephyrae. This study provides essential baseline data for understanding the early life-stage ecology of this species and contributes to improving predictions of jellyfish population dynamics and potential distribution shifts in East Asian marine ecosystems under future environmental change.

Salinity is hypothesized to influence the abundance of scyphozoan medusae in the East Asian Marginal Seas, as their spawning largely coincides with the summer monsoon season, and extreme rainfalls subject planulae to reduced salinity, presumably jeopardizing the recruitment of polyps. We examined the effects of different salinities (5, 10, 15, 20, 25, and 32) on body size, swimming speed, survivorship and settlement of planulae, and subsequent development of metamorphosed polyps of three bloom-forming scyphozoans: Chrysaora pacifica, Nemopilema nomurai, and Rhopilema esculentum. Their physiology and behaviors were affected by osmotic stress, but differed by species. At 5, planulae of all species died. At 10, C. pacifica could not settle but survived as plankton, whereas N. nomurai and R. esculentum could settle and develop into four-tentacle polyps, yet the former showing delayed development and tentacle abnormalities. The lowest salinities permitting normal recruitment of polyps were 10, 15, and 20 for R. esculentum, N. nomurai, and C. pacifica, respectively. Inter-annual variations in monsoon rainfall can alter hydrographic conditions in the polyp habitats, affecting medusa population sizes in the following year. The robustness to low salinity of R. esculentum and N. nomurai larvae may enable them to establish yet-to-be-identified polyp populations in brackish-water environments.

The complete mitochondrial genome sequences of giant jellyfish Chrysaora pacifica, a scyphozoan species inhabiting the Bohai Sea water in China, is firstly described and analyzed in this research. The mitogenome is a circular molecule 16,964 bp in length, including 13 protein-coding genes (Cox 1, Cox2, Atp 8, Atp 6, Cox 3, ND2, ND5, ND 6, ND3, ND4L,ND1,ND4, Cob), 2 tRNAs (trnW, trnM), 2 rRNA genes (small subunit RNA and large subunit RNA). The neighbor-joining (NJ) phylogenetic tree in the related species showed that C. pacifica is close to Chrysaora quinquecirrha.

In the Seto Inland Sea of western Japan, metacercariae of three species of trematodes, Lepotrema clavatum Ozaki, 1932, Cephalolepidapedon saba Yamaguti, 1970, and Opechona olssoni (Yamaguti, 1934), were found in the mesoglea of the jellyfish Aurelia aurita s.l., Chrysaora pacifica, and Cyanea nozakii. Moreover, these jellyfish frequently harbored juveniles of the fish species Psenopsis anomala, Thamnaconus modestus, and Trachurus japonicus. The former two fish species are well-known medusivores. We investigated seasonal changes in the prevalence and intensity of these metacercariae in their host jellyfish from March 2010 to September 2012 and presumed that infection by the trematodes of the definitive host fish occurs through these associations. The mean intensity of metacercariae in A. aurita s.l. clearly showed seasonality, being consistently high in June of each year. The intensity of metacercariae in C. nozakii was highest among all jellyfish hosts and appeared to be enhanced by medusivory of this second intermediate, and/or paratenic host. Trophic interactions between jellyfish and associated fish were verified using both gut content and stable isotope analyses. The detection of trematodes and nematocysts in the guts of P. anomala and T. modestus juveniles, in addition to stable isotope analysis, suggests that transmission of the parasites occurs via prey-predator relationships. In addition, the stable isotope analysis also suggested that P. anomala is more nutritionally dependent on jellyfish than Th. modestus and Tr. japonicus. Dans la mer intérieure de Seto à l’Ouest du Japon, les métacercaires de trois espèces de trématodes, Lepotrema clavatum Ozaki, 1932, Cephalolepidapedon saba Yamaguti, 1970 et Opechona olssoni (Yamaguti, 1934), ont été trouvées dans la mésoglée des méduses Aurelia aurita s.l., Chrysaora pacifica et Cyanea nozakii. De plus, ces méduses hébergeaient fréquemment des juvéniles des espèces de poissons Psenopsis anomala, Thamnaconus modestus et Trachurus japonicus. Les deux premières espèces de poissons sont connues comme médusivores. Nous avons étudié les variations saisonnières de prévalence et d’intensité de ces métacercaires dans leurs méduses hôtes de mars 2010 à septembre 2012 et présumé que l’infection des poissons hôtes définitifs par les trématodes se produit à travers ces associations. L’intensité moyenne des métacercaires chez A. aurita s.l. a clairement montré une saisonnalité, étant toujours élevée en juin de chaque année. L’intensité des métacercaires chez C. nozakii était la plus élevée parmi toutes les méduses hôtes et semblait être renforcée par médusivorie de ce second hôte intermédiaire, et/ou par un hôte paraténique. Les interactions trophiques entre les méduses et les poissons associés ont été vérifiées en utilisant à la fois le contenu de l’intestin et des analyses des isotopes stables. La détection de trématodes et de nématocystes dans les intestins de juvéniles de P. anomala et de Th. modestus, en plus de l’analyse des isotopes stables, suggère que la transmission des parasites se produit par l’intermédiaire de relations proies-prédateurs. En outre, l’analyse des isotopes stables a également suggéré que P. anomala est plus dépendant des méduses pour sa nutrition que Th. modestus et Tr. japonicus.

Jellyfish possess venomous cnidae on their tentacles to capture and consume marine zooplankton. Nevertheless, the planktonic larvae of the smooth fan lobster (Ibacus novemdentatus), known as phyllosoma, prey on jellyfish and successfully ingest both tentacle tissue as well as constituent cnidae, despite the presence of the venom-filled explosively penetrant cnidae or nematocysts. In the present study, we hypothesized that phyllosomas have mechanical and/or physiological resistance to internal envenomation by ingested nematocysts. To test this hypothesis, we examined the feces of phyllosomas (n=5) that were fed with Japanese sea nettle (Chrysaora pacifica) and found both undischarged as well as discharged cnidae surrounded by peritrophic membrane. We surmise that this membrane may mechanically insulate the lining of the midgut from stinging nematocysts to avoid injection of jellyfish venom into the phyllosomas' body by nematocyst tubule penetration. We then tested physiological sensitivity of the phyllosomas (n=10) to crude extract of tentacle cnidae injected into their bodies. For this experiment, we used a crude venom extract prepared from nematocysts isolated from tentacles of a rhizostome jellyfish (Nemopilema nomurai) after exposure to high salt which disrupted tentacle integrity, and phosphate-buffered saline as a control. Nine out of 10 animals died after the injection of crude venom extract, while none of the animals died in the control group. These results indicate that the defense of phyllosoma larvae against the toxin of jellyfish is a combination of mechanical inactivation of the ingested nematocysts and chemical digestion of the toxin in the midgut rather than physiological resistance against the toxin.

Polyps of Chrysaora pacifica were found on sediments sampled from the sea bottom in Sagami Bay near the mouth of the Sagami River on 26 June 2009; they were identified from released ephyrae in the laboratory. This is the first record of wild polyps of C. pacifica. Polyps and/or podocysts were found from five among the six stations. They were found on 25 shells (2.5–9.2 cm in width, 1.6–5.3 cm in height) and on 22 stones (1.5–8.0 cm in width, 1.3–5.0 cm in height). The shells with polyps were mostly from the dead clam Meretrix lamarckii. Polyps and podocysts were mostly found on the concave surface of bivalve shells, or in hollows of the stones. The number of polyps and podocysts per shell ranged between 0–52 (median = 9) and 0–328 (median = 28); and those per stone were 1–12 (median = 2) and 0–26 (median = 1.5). The number, especially of podocysts, was much greater on shells than on stones. On a convex substrate they can easily be removed by being hit with other substrates during dredging and washing, and such a process may also occur in natural conditions. They were induced to strobilate and release ephyrae by decreasing the temperature from 22–23°C to 5–10°C.