Distribution of moon jellyfish Aurelia aurita in relation to summer

Estuarine, Coastal and Shelf Science 86 (2010) 485–490
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Estuarine, Coastal and Shelf Science
journal homepage: www.elsevier.com/locate/ecss
Distribution of moon jellyfish Aurelia aurita in relation to summer hypoxia in
Hiroshima Bay, Seto Inland Sea
Jun Shoji a, *, Takaya Kudoh b, Hideyuki Takatsuji b, Osamu Kawaguchi b, Akihide Kasai c
a
Takehara Fisheries Research Station, Hiroshima University, Takehara, Hiroshima 725-0024, Japan
Fisheries and Marine Technology Center, Hiroshima Prefectural Technology Research Institute, Ondo, Kure, Hiroshima 737-1207, Japan
c
Laboratory of Fisheries and Environmental Oceanography, Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto 606-0068, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 January 2009
Accepted 1 March 2009
Available online 10 March 2009
Biological and physical surveys were conducted in order to investigate the relationship between environmental conditions and the distribution of moon jellyfish Aurelia aurita in Hiroshima Bay, western Seto
Inland Sea, Japan. Moon jellyfish and ichthyoplankton were collected at 13 stations in Hiroshima Bay
during monthly surveys from July to September in 2006 and 2007. Surface temperature in 2006 was
significantly lower during the August and September cruises and surface salinity was lower during all
cruises than in 2007. Moon jellyfish was the most dominant gelatinous plankton collected, accounting for
89.7% in wet weight. Mean moon jellyfish abundance in 2006 was higher than that in 2007 from July
through September, with significant inter-year differences for July and September. Variability in
precipitation and nutritional input from the Ohta River, northernmost part of Hiroshima Bay, were
suggested as possible factors affecting the inter-annual variability in moon jellyfish abundance in the
coastal areas of northern Hiroshima Bay. Moon jellyfish were more abundant in the coastal areas of
northern Hiroshima Bay, where the dissolved oxygen (DO) concentration was lower, while low in the
central part of the bay. Japanese anchovy Engraulis japonicus eggs were most dominant (58.1% in number)
among the ichthyoplankton and were abundant in the central area of Hiroshima Bay. Explanatory
analysis was conducted to detect possible effects of environmental conditions on the abundance of moon
jellyfish and Japanese anchovy eggs during the summer months in Hiroshima Bay. Of the environmental
conditions tested (temperature, salinity and DO of surface and bottom layers at each sampling station),
bottom DO had the most significant effect on the moon jellyfish abundance: there was a negative
correlation between the bottom DO and the moon jellyfish abundance in Hiroshima Bay during summer.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
moon jellyfish
Japanese anchovy
dissolved oxygen
Hiroshima Bay
Seto Inland Sea
1. Introduction
The moon jellyfish Aurelia aurita is widely distributed
throughout the coastal waters of the world and has been considered as an important predator of zooplankton because of its high
consumption rates (Möller, 1984; Costello and Colin, 1994). Increase
in abundance of the moon jellyfish, the same as other large gelatinous zooplankton such as cnidarians and ctenophores, has
occurred in estuarine and ocean ecosystems all over the world
(Purcell and Arai, 2001; Brodeur et al., 2002), potentially causing
a significant impact on smaller zooplankton and their predators in
pelagic ecosystems (Uye and Ueta, 2004; Haslob et al., 2007). In
Japan, the moon jellyfish is the most common jellyfish species in
coastal waters and its biomass has increased over recent decades
* Corresponding author.
E-mail address: [email protected] (J. Shoji).
0272-7714/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ecss.2009.03.001
in Tokyo Bay (Toyokawa et al., 2000; Ishii, 2001) and the Seto Inland
Sea (Uye and Ueta, 2004). Uye et al. (2003) observed moon jellyfish
aggregations of up to 250 individuals m2, which were estimated to
consume nearly 100% of the mesozooplankton biomass during
summer months in the coastal waters of the western part of the
Seto Inland Sea. Clarification of the mechanism of moon jellyfish
blooms and effects of environmental conditions on moon jellyfish
abundance are indispensable for forecasting and regulation of
moon jellyfish blooms. However, much of the information available
in the literature is of qualitative evaluations of moon jellyfish
distribution and its dynamics in nature.
The Seto Inland Sea is a semi-enclosed basin, which is surrounded by heavily urbanized areas with extensive industrial and
agricultural development, and receives significant freshwater from
the surrounding watersheds (Ochi et al., 1978; Okaichi et al., 1996).
Eutrophication of coastal waters has been common in Hiroshima
Bay, the innermost part of the Seto Inland Sea (Okaichi et al., 1996).
Nutritional input through the Ohta River, the northernmost part of
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J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485–490
Hiroshima Bay, and estuarine circulation enhance both primary and
secondary production in Hiroshima Bay (Hashimoto et al., 2006).
Chlorophyll-a levels and therefore primary production of Hiroshima Bay are among the highest of the nine bay areas of the Seto
Inland Sea (from east to west: Kii Channel, Osaka Bay, Sea of Harima, Bisan Archipelago, Sea of Hiuchi, Bingo-Geiyo Archipelago, Sea
of Aki including Hiroshima Bay, Sea of Iyo, Sea of Suo: Okaichi et al.,
1996). During summer hypoxia prevails in the northern coastal
areas of Hiroshima Bay, with dissolved oxygen (DO) concentrations
of <2 mg L1 (Okaichi et al., 1996, present study). Moderate
decreases in DO to near hypoxic conditions (<3 mg L1), although
not lethal during short-term exposure, can reduce the ability of
larval fish to escape and avoid predation and to capture prey
(Breitburg et al., 1994). Recent laboratory experiments reported
that bell contraction rate and predation rate on fish larvae by moon
jellyfish under oxygen concentrations < 2 mg L1 were similar to
those under higher oxygen concentrations (4–6 mg L1), indicating
that moon jellyfish are highly tolerant to low oxygen concentrations, as are several other jellyfish species (Breitburg et al., 1994;
Keister et al., 2000; Shoji et al., 2005; Thuesen et al., 2005). It has
been suggested that a shift from size-selective to non-size-selective
predation prevails in the predator–prey interaction and that relative importance of trophic flow from ichthyoplankton to moon
jellyfish increases during summer hypoxia in coastal waters (Shoji,
2008). Investigation of the abundance and distribution of moon
jellyfish in relation to environmental conditions is therefore
important for understanding the effects of hypoxia on trophic flow
in plankton communities.
In the present study, biological and physical surveys were conducted in the Hiroshima Bay in order to examine the effects of
environmental conditions (temperature, salinity, and DO) on moon
jellyfish distribution. Pattern of the horizontal distribution and
effects of the environmental conditions on moon jellyfish were
compared with the most dominant ichthyoplankton species, Japanese anchovy Engraulis japonicus in summer 2006 and 2007.
2. Materials and methods
2.1. Field survey
Biological and physical surveys were conducted during monthly
research cruises from July to September in 2006 and 2007 on the RV
Aki, Fisheries and Marine Technology Center, Hiroshima Prefectural
Technology Research Institute (HPTRI), in Hiroshima Bay (Fig. 1).
Moon jellyfish were collected with a plankton net (0.45 m in
diameter, 0.33 mm in mesh aperture) with a flow-meter at 13
stations in Hiroshima Bay. A vertical haul with the plankton net
from 1 m above the sea bottom to the surface was made at each
station. Moon jellyfish were sorted from the sample, identified and
measured for bell diameter (mm) and wet weight (g) on the boat.
Ichthyoplankton samples were preserved in 10% formalin. Vertical
profiles of temperature ( C), salinity and dissolved oxygen (DO)
concentration (mg L1) were measured during each sampling
event.
2.2. Laboratory procedure
Jellyfish and ichthyoplankton were enumerated as kg m2 and
number m2 according to the flow-meter counts. Of the total
jellyfish catch for the six cruises, the moon jellyfish Aurelia aurita
was most dominant, accounting for 89.7% of the total wet weight
(see the Results). Other jellyfish species was excluded from the
further analysis due to the small sample size. Ichthyoplankton were
sorted and identified to the lowest taxa possible. Eggs of Japanese
anchovy Engraulis japonicus, the most dominant ichthyoplankton
Fig. 1. Map showing the 13 sampling stations in Hiroshima Bay, Seto Inland Sea,
southwestern Japan. Monthly physical and biological surveys were conducted from
July to September in 2006 and 2007.
collected during the six cruises (58.1% in number of the total ichthyoplankton, see the Results) were processed for analysis of
seasonal change in abundance and horizontal distribution in Hiroshima Bay. Other ichthyoplankton were excluded from the further
analysis due to the small sample size.
Exploratory analysis was conducted in order to detect possible
effects of the environmental factors on variability in the moon
jellyfish and Japanese anchovy egg abundance. Temperature,
salinity and DO of surface and bottom layers at each sampling
station were used as explanatory variables and abundance of moon
jellyfish and Japanese anchovy eggs as dependent variables. The
environmental conditions (temperature, salinity and DO) and
moon jellyfish and Japanese anchovy egg abundance were
compared between the two years by the use of Wilcoxon test.
3. Results
3.1. Environmental conditions
Mean sea surface temperature ranged between 24.0 C (July)
and 26.8 C (August) in 2006 and between 23.0 C (July) and 28.9 C
(August) in 2007 (Fig. 2). There was a significant inter-annual
difference in the surface temperature in all months (Wilcoxon test,
d.f. ¼ 1, July: P ¼ 0.004; August: P ¼ 0.0006; September:
P < 0.0001). The surface temperature was lower in northern Hiroshima Bay during all cruises (Figs. 3 and 4).
Mean surface salinity increased from 20.8 in July to 28.1 in
September in 2006 and from 27.8 in July to 31.8 in September in
2007 (Fig. 2). In all months, surface salinity in 2007 was significantly higher than that in 2006 (Wilcoxon test, d.f. ¼ 1, July and
September: P < 0.0001; August: P ¼ 0.005). The surface salinity was
lower in the northern part of Hiroshima Bay.
The mean bottom DO for all 13 stations ranged between
3.3 mg L1 (September) and 5.0 mg L1 (August) in 2006 and
3.3 mg L1 (August) and 4.7 mg L1 (July) in 2007 (Fig. 2). In August,
J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485–490
487
there was a significant inter-annual difference in the DO (Wilcoxon
test, d.f. ¼ 1, P ¼ 0.003). The bottom DO was lower in the northern
and/or eastern parts of Hiroshima Bay throughout the cruises. In
September 2006 and August and September 2007, some stations
indicated a bottom DO of <2.0 mg L1 (Figs. 3 and 4).
3.2. Occurrence and distribution of moon jellyfish
Of the total jellyfish collected throughout the cruises, moon
jellyfish was most abundant, accounting for 89.7% in wet weight.
Bolinopsis mikado and unidentified jellyfish species composed only
10.3%. The mean moon jellyfish abundance was highest in July in
both 2006 (4.68 kg m2) and 2007 (1.24 kg m2: Fig. 2). The interyear difference in the moon jellyfish abundance was significant in
July (P ¼ 0.014) and September (P ¼ 0.013). There was no significant
difference in the bell diameter of moon jellyfish between the two
years (2006: 177.6 32.2 mm; 2007: 174.0 19.6 mm).
Moon jellyfish were more abundant in the northern part of
Hiroshima Bay (Figs. 3 and 4). The areas of high abundance corresponded with those of low bottom DO. In July 2006 and 2007, the
highest moon jellyfish abundance was observed in the eastern part
of the bay, where bottom DO was lower than 2.0 mg L1. Moon
jellyfish were sparsely distributed in the central area of the bay
throughout the cruises.
3.3. Occurrence and distribution of Japanese anchovy egg
A total of 654 fish eggs and 88 larvae were collected during the
six cruises. Japanese anchovy eggs were the most dominant,
accounting for 65.9% in number of the total fish eggs. Japanese
anchovy egg abundance was highest in August in 2006 (276.3 m2)
and in July in 2007 (39.4 m2: Fig. 2). There was a significant
difference in the Japanese anchovy egg abundance in August
(P ¼ 0.022) between the two years.
Japanese anchovy eggs were more abundant in the central or
southern parts of Hiroshima Bay from July through September in 2006
and in July in 2007 while sparsely distributed in the coastal areas of
the northern part of the bay throughout the six cruises (Figs. 3 and 4).
3.4. Effect of environmental conditions on moon jellyfish
distribution
Of the environmental conditions tested, only bottom DO had
a significant effect on the moon jellyfish abundance (F ¼ 4.4758,
P ¼ 0.0379: all data combined). Moon jellyfish were more abundant
in the areas with lower levels of bottom DO (<3.0 mg L1). An
exponential model was fitted to the plots of moon jellyfish abundance to bottom DO in July and September 2006 (Fig. 5).
4. Discussion
4.1. Effect of dissolved oxygen concentration on moon jellyfish
Fig. 2. Seasonal changes in physical and biological conditions in Hiroshima Bay from
July to September in 2006 (open circles) and 2007 (closed circles). Mean sea surface
temperature (WT), salinity (SA), bottom dissolved oxygen concentration (DO), abundance of moon jellyfish (MJ) and Japanese anchovy egg (AE) are shown with vertical
bars as standard deviation. Asterisks indicate significant difference between the years
(Wilcoxon test, *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001).
Correspondence of high abundance of moon jellyfish with the
areas of moderate bottom-layer hypoxia (<3 mg L1) was
observed in Hiroshima Bay during summer 2006 and 2007.
Previous laboratory experiments showed a high tolerance to low
DO conditions (<2.0 mg L1) in moon jellyfish: bell contract rate
(an index of feeding activity) of the moon jellyfish was constant
over the DO levels tested (1.0–5.8 mg L1: Shoji et al., 2005).
Strong tolerance to low DO levels has also been observed in
Aurelia labiata and sea nettle scyphomedusa Chrysaora quinquecirrha under laboratory conditions (Breitburg et al., 1994;
Thuesen et al., 2005) and high densities of the ctenophore
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Fig. 3. Horizontal distribution of sea surface temperature (WT), bottom dissolved oxygen concentration (DO), and abundance of moon jellyfish (MJ) and Japanese anchovy eggs (AE)
from July to September in 2006 in Hiroshima Bay.
Fig. 4. Horizontal distribution of sea surface temperature (WT), bottom dissolved oxygen concentration (DO), and abundance of moon jellyfish (MJ) and Japanese anchovy eggs (AE)
from July to September in 2007 in Hiroshima Bay.
J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485–490
489
Fig. 5. Plots of moon jellyfish abundance to bottom dissolved oxygen concentration (DO) in each month. The effect of DO on moon jellyfish abundance was significant in July and
September in 2006.
Mnemiopsis leidyi in low DO concentrations in Chesapeake Bay
(Keister et al., 2000).
Contrastingly, ichthyoplankton were less abundant in the
northern part of Hiroshima Bay during the summer months.
Swimming and/or feeding performance of major fish predators
have been observed to significantly decrease at DO levels < 2.0 mg L1 in coastal waters (juvenile striped bass Morone
saxatilis, adult naked goby Gobiosoma bosc and juvenile Spanish
mackerel Scomberomorus niphonius: Breitburg et al., 1994; Shoji
et al., 2005). Therefore, competition for prey and space between
moon jellyfish and fish weakens as DO decreases in coastal
habitats. As a result, the relative importance of trophic flow from
plankton to jellyfish is considered to increase in coastal waters
where summer hypoxia prevails (Breitburg et al., 1994; Shoji et al.,
2005). We suggest that low DO conditions in the coastal waters of
Hiroshima Bay during summer were favorable for moon jellyfish
feeding, growth and survival and resulted in the high abundance
in the northern area.
4.2. Distribution of moon jellyfish in Hiroshima Bay
In addition to the strong tolerance to low DO level, supply of
young stages of moon jellyfish and retention of them within the
coastal areas of northern Hiroshima Bay should be included in
the possible factors that explain the high moon jellyfish
abundance in this area. Hiroshima Bay is located at the most
inner part and is the most enclosed area in the Seto Inland Sea
(Okaichi et al., 1996). Moon jellyfish polyps use artificial
substrate such as concrete blocks, plastic floats and oyster beds
(Yasuda, 2003). In Hiroshima Bay, the natural shoreline has
decreased to less than 40% of its original existence (Okaichi
et al., 1996), with much of it replaced by concrete seawalls
which are favorable for jellyfish polyps. Intensive oyster culture
in Hiroshima Bay (ca. 50% of the total landings of oyster
cultured in Japan) can provide suitable substrate for the moon
jellyfish polyps. We conclude that a combination of environmental factors such as available substrate for the polyps and
high retention within the northern bay, in addition to their
biological features (strong tolerance to low DO conditions),
enhances the moon jellyfish population in the northern part of
Hiroshima Bay.
During most of the cruises, on the contrary, highest abundances of Japanese anchovy eggs were observed in the central
part of Hiroshima Bay, where bottom DO was higher. The spatial
difference in the Japanese anchovy abundance between the
northern and central parts of the bay indicates that: (1) Japanese
anchovy spawning was less intensive in and around the northern
part of Hiroshima Bay, and/or (2) egg mortality was higher in
northern part of Hiroshima Bay. It is likely that Japanese anchovy
eggs were more vulnerable to predation by moon jellyfish in the
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J. Shoji et al. / Estuarine, Coastal and Shelf Science 86 (2010) 485–490
northern part of Hiroshima Bay where moon jellyfish were
abundant during summer 2006 and 2007. Survival rates of bay
anchovy Anchoa mitchilli eggs have been reported to markedly
decrease at DO < 3.0 mg L1 (Chesney and Houde, 1989) although
there is no information on how low DO affects survival of Japanese anchovy eggs. In Chesapeake Bay, distribution of bay
anchovy spawners was confined to the southern (near bay mouth)
region when summer hypoxia prevailed in the northern (inner
bay) region (Jung and Houde, 2004). We conclude that the
northern part of Hiroshima Bay was less favorable for Japanese
anchovy spawning and/or egg survival due to low DO during
summer 2006 and 2007.
4.3. Inter-annual difference in moon jellyfish abundance
Global warming and increase in temperature of shallow
waters have been suggested to potentially enhance moon jellyfish blooms in coastal waters of Japan (Uye and Ueta, 2004). In
the present study, however, moon jellyfish abundance was
higher in 2006, when the temperature was lower during late
summer in Hiroshima Bay. Therefore, the inter-annual difference
in moon jellyfish abundance cannot be explained by temperature condition alone, and other composite factors may better
explain the higher moon jellyfish abundance in 2006 in Hiroshima Bay.
Among the environmental factors tested, difference in salinity
was most significant between the two years from July through
September. The lower salinity in 2006 reflects higher precipitation during summer, which might be favorable for growth and
survival of moon jellyfish. The total precipitation in Hiroshima
City from January through July (1120.0 mm) and from July
through September (689.0 mm) in 2006 was double the values in
2007 (January through July: 550.0 mm; July through September:
345.5 mm, Japan Meteorological Agency: http://www.jma.go.jp/
jma/). Higher freshwater flow through the Ohta River, the
northernmost part of Hiroshima Bay, increases primary and
secondary production within the bay by enhancing the nutritional input and estuarine circulation (Hashimoto et al., 2006). In
2006, chlorophyll-a levels in the northern part of Hiroshima Bay
were significantly higher than in 2007 (Kawaguchi et al.,
unpublished data). Previous stomach contents analysis (Uye
et al., 2003) and recent stable isotope analysis (Shoji et al.,
submitted for publication) showed copepods are the major prey
source of moon jellyfish in the Seto Inland Sea. Increase in
freshwater flow through the Ohta River should enhance growth
and survival of moon jellyfish in the northern part of Hiroshima
Bay by increasing copepod production. We still lack ecological
information on younger pelagic (ephyra) and benthic stages
(polyp and strobila) of moon jellyfish in nature. Further investigation on the effect of variability in freshwater flow through
the Ohta River on primary and secondary production in Hiroshima Bay in relation with growth and survival of moon jellyfish ephyra, polyp and strobila stages would help to clarify the
mechanism of moon jellyfish bloom, distribution and inter-year
fluctuation in Hiroshima Bay.
Acknowledgements
We are grateful for the crews of RV Aki, HPTRI, for their help
during the field sampling.
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