Copepods, the most abundant planktonic metazoans, constitute an intermediate trophic position between phytoplankton and higher trophic-level animals such as fish and jellyfish. Fish and jellyfish are ...adversaries because they often compete for prey copepods and also can be prey of each other. The classical food chain represented by phytoplankton–copepod–fish is the main process leading to efficient and sustainable production of fish as human food. At present, more than 75% of world fish stocks are fully or over exploited. On the other hand, jellyfish populations have increased world-wide, particularly in waters under significant human influences. Two such cases are seen in East Asian waters, where massive blooms of moon jellyfish (
Aurelia aurita
s.l.) and giant jellyfish (
Nemopilema nomurai
) have repeatedly occurred in recent decades, causing severe damage to local fisheries. In this article, I will review the pivotal role of copepods in marine ecosystems, particularly in the Inland Sea of Japan, where the annual fish catch per unit area is among the world’s highest. Then, I will describe an ongoing ecosystem shift from dominance by fish to dominance by jellyfish as a consequence of human forcing. Finally, I will propose to create “sato-umi”, a coastal sea with high productivity and biodiversity with wise human interaction, where copepod production would most efficiently transforms into food for humans.
Although recent articles state that jellyfish populations are increasing, most available evidence shows that jellyfish abundances fluctuate with climatic cycles. Reports of increasing problems with ...jellyfish, especially in East Asia, are too recent to exclude decadal climate cycles. Jellyfish are infamous for their direct negative effects on human enterprise; specifically, they interfere with tourism by stinging swimmers, fishing by clogging nets, aquaculture by killing fish in net-pens and power plants by clogging cooling-water intake screens. They also have indirect effects on fisheries by feeding on zooplankton and ichthyoplankton, and, therefore, are predators and potential competitors of fish. Ironically, many human activities may contribute to increases in jellyfish populations in coastal waters. Increased jellyfish and ctenophore populations often are associated with warming caused by climate changes and possibly power plant thermal effluents. Jellyfish may benefit from eutrophication, which can increase small-zooplankton abundance, turbidity and hypoxia, all conditions that may favor jellyfish over fish. Fishing activities can remove predators of jellyfish and zooplanktivorous fish competitors as well as cause large-scale ecosystem changes that improve conditions for jellyfish. Aquaculture releases millions of jellyfish into Asian coastal waters yearly to enhance the jellyfish fishery. Aquaculture and other marine structures provide favorable habitat for the benthic stages of jellyfish. Changes in the hydrological regime due to dams and other construction can change the salinity to favor jellyfish. Accidental introductions of non-native gelatinous species into disturbed ecosystems have led to blooms with serious consequences. In many coastal areas, most of these environmental changes occur simultaneously. We summarize cases of problem jellyfish blooms and the evidence for anthropogenic habitat disruptions that may have caused them. Rapid development in East Asia makes that region especially vulnerable to escalating problems. We conclude that human effects on coastal environments are certain to increase, and jellyfish blooms may increase as a consequence.
Once-to-twice weekly sampling at nocturnal high tide for a year enabled us to detail the seasonal population dynamics, production, and feeding of the chaetognath Aidanosagitta crassa in a eutrophic ...inlet of the Inland Sea of Japan. The population density fluctuated from 1.0 ind (or 0.0029 mg C) m−3 to 3,370 ind (or 14.69 mg C) m−3, with a small, protracted peak in winter and a large, sharp peak in summer. The population consisted of 3 subpopulations, each of which produced 6–7 generations, yielding a total of 19 generations. In most generations, the mortality was notably higher in the egg to small juvenile (body length: ca. 2 mm) stage and in the adult stage than in the large juvenile stage. Basic population parameters, e.g., time to first maturity (range: 32–71 d), minimum size for reproduction (4.0–8.9 mm), mean body length of mature adults (5.5–10.4 mm) and life span (34–96 d), were negatively correlated with temperature. The specific growth rates (0.07–0.22 d−1) were a positive function of temperature, yielding an annual population production rate of 91.49 mg C m−3 y−1. Only one third of A. crassa contained prey in the gut, and their feeding habit was completely carnivorous, with preference for progressively larger prey with their growth. Estimated carbon weight-specific feeding rates (0.01–0.81 d−1) sustained net growth with a mean gross growth efficiency of 45% in warm seasons (≥ca. 15°C). This highly intensive survey of the A. crassa population serves as a benchmark for intra- and interspecific comparisons of ecophysiological and population parameters.
The rhizostome jellyfish Nemopilema nomurai, which is endemic to the East Asian Marginal Seas (i.e. the Bohai, Yellow, East China and Japan Seas), is unique both by its enormous body size (ca. 2 m ...maximum bell diameter and 200 kg wet weight) and propensity for occasional population explosions. Massive blooms of this species have historically been reported only once per ca. 40 years (i.e. in 1920, 1958 and 1995), but have become increasingly frequent recently (i.e. in 2002, 2003, 2005 and 2006). Both accumulated knowledge on the spatiotemporal distributions and physical modeling of the water circulation show that the medusae are released from benthic polyps during April-June in the Yellow Sea and East China Sea, and transported by the Tsushima Current to the Japan Sea. The bloom in 2005 might be the largest ever in history; as many as 3-5 x l0E8 medusae passed through the Tsushima Strait daily during the summer and there were more than 100,000 complaints from commercial fishermen. The recent blooms of N. nomurai may have been caused by environmental changes, such as increased water temperature, eutrophication, coastal modification, and over-fishing in Chinese coastal waters. Frequent jellyfish blooms can apparently be a threat to the fisheries sustainability of the East Asian Marginal Seas, one of the world's most productive fisheries grounds.
Jellyfish (Cnidaria, Scyphozoa) blooms appear to be increasing in both intensity and frequency in many coastal areas worldwide, due to multiple hypothesized anthropogenic stressors. Here, we propose ...that the proliferation of artificial structures - associated with (1) the exponential growth in shipping, aquaculture, and other coastal industries, and (2) coastal protection (collectively, "ocean sprawl") - provides habitat for jellyfish polyps and may be an important driver of the global increase in jellyfish blooms. However, the habitat of the benthic polyps that commonly result in coastal jellyfish blooms has remained elusive, limiting our understanding of the drivers of these blooms. Support for the hypothesized role of ocean sprawl in promoting jellyfish blooms is provided by observations and experimental evidence demonstrating that jellyfish larvae settle in large numbers on artificial structures in coastal waters and develop into dense concentrations of jellyfish-producing polyps.
Presettlement processes likely influence the geographical distribution and abundance of scyphozoan jellyfish medusae. In East Asian coastal waters, Aurelia aurita s.l. spawning coincides with the ...summer monsoon season, and extreme rainfall events subject planulae to hyposaline conditions, presumably jeopardizing their survival and settlement. In this study, laboratory experiments were conducted to determine how prolonged exposure to hyposalinity (25, 20, and 15, in addition to control salinity 32) affects swimming speed, survivorship and settlement of planula larvae, and subsequent development of metamorphosed polyps. Nearly all planulae (≥95%) in 32, 25, and 20 were geonegative during the first 4h of exposure to respective salinities, but the majority of planulae (70%) in 15 were geopositive. Although no mortality was induced, hyposaline conditions affected larval behavior and the sequence of post-metamorphosis events. Salinity of 15 significantly increased planktonic larval duration and settled polyps had morphological deformities. At a salinity of 20, settled polyps had delayed tentacle development compared to control and 25 treatments. These anomalous results demonstrate that the response of planulae to environmental stress is more complex than previously assumed and may depend on the ability of the settled polyps to develop viable feeding tentacles. Salinity is proposed to be a principal factor influencing planulae dispersion and distribution in temperate monsoon regions.
•Tested salinities (15–32) reflective of temperate monsoon on planulae ecophysiology•A. aurita s.1 planulae survived prolonged exposure (≥6d) to salinity of 15.•Salinities <20 caused geopositive response of planulae.•Larval duration prolonged by severe hyposalinity, reduced by moderate hyposalinity.•Salinities ≤20 caused deviations from normal sequence of post-metamorphosis events.
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.
•We reviewed environmental and ecosystem changes in two LTER sites in the N hemisphere.•We examined the effects of anthropogenic pressures on moon jellyfish populations.•A synchronous shift in ...climate was detected in the 1980s in both systems.•Providing additional space for polyps, marine urbanisation-enhanced jellyfish population.•Fish-dominated ecosystem switched to “jellyfish-permeated” environment.
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A prominent increase in the moon jellyfish (genus Aurelia) populations has been observed since 1980 in two semi-enclosed temperate seas: the northern Adriatic Sea and the Inland Sea of Japan. Therefore, we reviewed long-term environmental and biotic data from the two Long-Term Ecological Research (LTER) sites, along with the increase in the moon jellyfish occurrence to elucidate how these coastal seas shifted to the jellyfish-dominated ecosystems. The principal component analysis of atmospheric data revealed a simultaneous occurrence of similar climatic changes in the early 1980s; thereafter, air temperature increased steadily and precipitation decreased but became more extreme. Accordingly, the average seawater temperature from March to October, a period of polyps' asexual reproduction i.e. budding, increased, potentially leading to an increase in the reproductive rates of local polyp populations. Conspicuous eutrophication occurred due to the rise of anthropogenic activities in both areas from the 1960s onwards. This coincided with an increase of the stock size of forage fishes, such as anchovy and sardine, but not the population size of the jellyfish. However, by the end of the 1980s, when the eutrophication lessened due to the regulations of nutrients loads from the land, the productive fishing grounds of both systems turned into a state that may be described as ‘jellyfish-permeated,’ as manifested by a drastic decrease in fish landings and a prominent increase in the intensity and frequency of medusa blooms. A steady increase in artificial marine structures that provide substrate for newly settled polyps might further contribute to the enhancement of jellyfish population size. Elevated fishing pressure and/or predation by jellyfish on ichthyoplankton and zooplankton might jeopardize the recruitment of anchovy, so that the anchovy catch has never recovered fully. These semi-enclosed seas may represent many temperate coastal waters with increased anthropogenic stressors, which have degraded the ecosystem from fish-dominated to jellyfish-dominated.
Cyanea nozakii is a scyphozoan jellyfish that causes problematic blooms in Chinese coastal waters, whilst its polyps have never been found in the field. This laboratory study examined the effects of ...salinity, light intensity and biofouling on planula settlement and subsequent development to polyps for narrowing the potential polyp habitat with regard to these factors. Planulae of C. nozakii could complete settlement, metamorphosis through planulocysts and development into 4-tentacle polyps within 5 days over a broad salinity range from 13 to 32. The planulae were photonegative with significantly lower settlement success at 1700 lx compared to 80 lx and darkness. The settlement was severely inhibited by the exacerbation of biofoulings mainly consisted of ascidians and bryozoans; it was blocked completely on substrates with heavy biofouling (covered area: ≥95%, mean wet weight: 1400 mg·cm−2) and substantially on those with light biofouling (50%, 16.5 mg·cm−2). The settled polyps were outcompeted by the biofouling proliferation and finally eliminated. These imply that neither salinity nor light intensity is a limiting factor but the availability of proper hard substrates with no or scarce biofouling is a bottleneck for C. nozakii polyps to establish the population in Chinese coastal waters
•C. nozakii planulae could complete settlement and subsequent metamorphosis over a broad salinity range from 13 to 32.•The planulae were photonegative with significantly lower settlement success at 1700 lx than 80 lx and darkness.•The settlement of planulae was severely inhibited by the exacerbation of biofoulings.
Common moon jellyfish, Aurelia spp. bloom seasonally in eutrophic or polluted coastal waters around the world. We hypothesized that podocysts, a part of asexual reproduction of the benthic polyps, ...were important in perpetuating populations of Aurelia aurita s.l. in anthropogenically perturbed waters. We examined the effects of temperature, salinity, dissolved oxygen concentrations, and food on the encystment and excystment of A. aurita podocysts. Podocysts were formed only by unfed or poorly-fed polyps (≤4.8 μg C polyp−1 day−1), indicating that starvation was the primary cause of encystment, while increased temperatures accelerated podocyst production rate. Encystment was never induced by changed salinity (15–32) or dissolved oxygen concentration (1–5 mg O2 l−1). Excystment occurred only when podocysts were returned to 19°C from 28°C and to oxic waters from hypoxic (0.2–1.0 mg O2 l−1). The podocysts were capable of surviving for up to 3.2 year. Histology revealed that newly-formed podocysts contained rich organic reserves (e.g., carbohydrate, protein, and lipid) that were gradually consumed while encysted. Podocysts may contribute minimally to increasing A. aurita polyp abundance, but they can insure maintenance of the population in adverse environmental conditions and in predator attacks. Podocysts may also enable the population to extend to areas where polyp survival is marginal.
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