Journal of Oceanography, Vol. 58, pp. 451 to 459, 2002
On the Polyps of the Common Jellyfish Aurelia aurita in
Kagoshima Bay
HIROSHI MIYAKE1*, MAKOTO T ERAZAKI1 and YOSHIKO KAKINUMA 2
1
Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku,
Tokyo 164-8639, Japan
2
Faculty of Science, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan
(Received 22 November 2000; in revised form 6 October 2001; accepted 10 October 2001)
There is the natural habitat of polyps of the common jellyfish Aurelia aurita in the
Taniyama area, Kagoshima Bay. We examined the attachment substrata, density,
colony structure and strobilation of the polyps. The polyps were observed only on the
horizontal undersurface of floating piers. They attached specifically to Mytilus shells,
solitary ascidians, calcareous polychaete tubes, muddy amphipod tubes and the gap
space that fouling animals peeled off the substrata. The polyp colonies were distributed in patches. Spatial distribution patterns of the polyps within their colonies were
uniform. Strobilation occurred during late December to March, when water temperatures were 16–17°C, and a large number of ephyrae were released. An increase in
man-made structures such as floating piers in coastal areas may lead to bloomings of
Aurelia aurita medusae.
Keywords:
⋅ Polyp,
⋅ Aurelia aurita,
⋅ artificial substrata,
⋅ colony,
⋅ distribution
pattern,
⋅ Iδ-Index,
⋅ strobila,
⋅ floating pier,
⋅ undersurface.
and the interaction between polyps and their environment.
We recently discovered polyps of Aurelia aurita in
Kagoshima Bay (Miyake et al., 1997). The purpose of
this paper is to report the distribution pattern and colony
forms of the polyps with special regard to intra- and
interspecific interactions and the occurrence of strobilation.
1. Introduction
From matured Aurelia aurita medusae, planulae are
released and attach to some suitable substrata and develop
into polyps. Each polyp reproduces asexually by budding,
fission, or pedal disk (cyst) (Kakinuma, 1975) and forms
a colony.
There are few reports about the polyps of Aurelia
aurita in the field. Because of the lack of information
about the habitat of polyps, it has been difficult to find
polyps in the field. Therefore, their natural history has
been inferred mainly from studies on polyps attached to
artificial plates (Hernroth and Gröndahl, 1983, 1985a,
1985b; Yasuda, 1988; Gröndahl, 1989) and those kept in
aquaria (see Arai, 1997). Detailed ecological studies on
polyps in situ have been published from the Mariager
Fjord in Denmark (Ussing, 1927), the Kiel Fjord in Germany (Thiel, 1962), and the Gullmar Fjord in Sweden
(Hernroth and Gröndahl, 1985a; Gröndahl, 1988a, b).
These studies describe the occurrence of strobilae, polyps, and the physical environment in which they were
found, i.e., temperature and salinity regimes. However,
little is known about the distribution pattern of polyps
2. Materials and Methods
The survey site was located in the Taniyama area,
Kagoshima Bay, southern end of Kyushu, Japan (Fig. 1).
This site is in a man-made canal that is 1300 m long and
30 m wide, 3 m deep at ebb tide and 6 m deep at high
tide. The wall of the canal was a metal plate extending to
4 m from the bottom and the upper part was concrete.
Many vessels such as leisure boats and many floating piers
are anchored in the canal.
Investigations were conducted by SCUBA at least
once per month from April 1996 to March 1997 in order
to clarify the period of strobilation. Temperature, salinity
(ALEC ELECTRONICS, MODEL ACT20-1), and dissolved oxygen (YSI, Model 55/25 FT) were measured in
the habitat of the polyps.
We observed the substrata of polyps in situ and collected six to twelve colonies of polyps with the substrata
during every investigation. We examined polyp colonies
and their substrata under a stereoscopic microscope to
determine the micro-distribution of polyps, colony form,
* Corresponding author. E-mail: [email protected]
Present address: Japan Marine Science and Technology Center, 2-15
Natsushima-cho, Yokosuka 237-0061, Japan.
Copyright © The Oceanographic Society of Japan.
451
Fig. 1. Map of the survey area, Taniyama, Kagoshima Bay, Japan.
Fig. 2. Seasonal changes in temperature, salinity and dissolved oxygen at the survey site.
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H. Miyake et al.
Fig. 3. Colony of Aurelia aurita polyps (P) on Mytilus sp. There were many muddy amphipod tubes (AT) on the shell.
and presence or absence of strobilae. Strobilation ratios
were based on the strobilae components of all polyps collected on the same day.
The density of polyps on the collected substrata was
also measured. For the colonies on the almost flat substrata, spatial distribution patterns were determined by
the Iδ-Index (Morisita, 1959). Distribution patterns were
classified into six patterns from the shape of the Iδ-quadrat
size relationship: 1. Random distribution; 2. Uniform distribution; 3. Aggregated distribution with small clumps
and random intra-clump distribution; 4. Aggregated distribution with small clumps and uniform intra-clump distribution; 5. Aggregated distribution with large clumps
and random intra-clump distribution; 6. Aggregated distribution with large clumps and uniform intra-clump distribution (Morisita, 1959). The size of peaks in the
Iδ(s)/Iδ(2s) curve indicates the size of a clump (unit colony
size) or the size of aggregation of clumps (Morisita, 1959).
3. Results
3.1 Environmental parameters
Temperature ranged from 14.2 to 30.1°C at the surface and from 14.2 to 29.5°C at the bottom (3–6 m depth)
(Fig. 2). Salinity varied from 25.6 to 34.0 at the surface
and from 30.0 to 34.1 at the bottom (Fig. 2). Salinity at
the surface was low in summer, the lowest value being
25.6, but salinity at the bottom was normally 30.8. The
dissolved oxygen concentration was lower from August
to January (Fig. 2). The maximum value of dissolved
oxygen was 8.37 mg/l in July and the minimum was 5.57
mg/l in November.
3.2 Substrata
There were many shells and remains of fouling organisms on the bottom and muddy sediment had accumulated. Polyps were found on the undersurface of floating piers and buoys. Floating piers were rectilinear, 15 m
long, 3 m wide and 1.5 m thick. These piers were made
of a polystyrene float with a concrete frame and deck.
The lower one meter of the pier was submerged.
No polyps were present on the sides of the floating
piers, the wall of the channel or on the bottom of ships.
They were discovered only on the undersurface of the
floating piers and of the buoys (Fig. 3). The polyps attached to these substrata in patches. Most of them were
attached to solitary ascidians, barnacles, Mytilus,
polychaete tubes and amphipod tubes (Table 1). They also
attached densely to the gap spaces that fouling animals
had peeled off the substrata. In particular Mytilus, solitary ascidians and the gap spaces were the major substrata for polyp attachment. The Mytilus shells that had
attached polyps were larger than 10 mm long, and in particular polyps formed large colonies on shells longer than
50 mm. However, no polyps were observed on the shells
of dead Mytilus. Polyps were never found on colonial
On the Polyps of Aurelia aurita in Kagoshima Bay
453
Table 1. Substratum for Aurelia aurita polyp colonies.
: occurred commonly; 䊊: occurred; ×: did not occur.
ascidians nor soft solitary ascidians like Ciona, but only
on hard ascidians like Styela. The polyps were few on the
shells of living barnacles, but were numerous on dead
barnacles. Polyps also formed extremely large, dense
colonies on freshly vacated polystyrene substratum, where
large fouling organisms had fallen off or were removed.
In addition, a fallen leaf and the cellophane cover of a
cigarette package also provided substrata for polyps.
3.3 Distribution pattern
The densest part in any colony reached 88 inds./cm2.
The average densities of polyps were 7.3–17.6 inds./cm2
on Mytilus and 3.1–18.5 inds./cm 2 on polystyrene. These
variations in the densities were derived from conditions
of colonies.
Every colony had an aggregated distribution with
small clumps and intra-clump distribution was uniform
or random independently of the season (Fig. 4). Typical
distribution patterns of polyps on major substrata (Mytilus
and polystyrene substrata) are outlined below.
One of the typical distribution patterns of polyps on
Mytilus (Fig. 5), the density of polyps was 7.4 inds./cm 2,
and the Iδ-value was about 3 at a quadrat size of 9.4 mm2
(256 quadrats). According to the shape of the Iδ-quadratsize graph, colonies were in an aggregated distribution
pattern with small colonies, and polyps were evenly dis-
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H. Miyake et al.
Fig. 4. I δ-quadrat size relationships of polyp colonies on some
substrata.
tributed within the colony. The unit colony size on Mytilus
was 9.4–18.8 mm 2, and larger colonies (150–300 mm2)
were formed by aggregations of the unit colonies (Fig.
7a).
One of the typical distribution patterns of polyps on
the polystyrene substratum to which amphipod tubes were
attached (Fig. 6a), the density of polyps was 6.1
Fig. 5. Polyp distribution on both sides of a Mytilus shell attached to the undersurface of a floating pier.
Fig. 6. Polyp distribution on a polystyrene substratum on the undersurface of a floating pier. (a) A substratum where few species
existed. (b) A substratum where many other species existed.
On the Polyps of Aurelia aurita in Kagoshima Bay
455
middle of January to early February when water temperature was 14–15°C, and continued through late February
(Fig. 8). Not all polyps in a colony developed into strobilae. The strobilation ratio was 3.2% of the polyps collected in late December, 10.1% in January (the peak
month) and 6.1% in late February (Fig. 9). Strobila had
7–9 segments, though the maximum segment number was
14.
Fig. 7. Iδ -quadrat size relationships and I δ(s)/I δ(2s)-quadrat size
relationships of polyp colonies. (a) On Mytilus sp. in Fig.
5. (b) On a polystyrene substratum in Fig. 6a. (c) On a polystyrene substratum in Fig. 6b.
inds./cm 2. The Iδ value was about 1.4 at a quadrat size of
31.3 mm2 (64 quadrats). Colonies had an aggregated distribution with large colony sizes, and polyps were evenly
distributed within the colony. The unit colony size was
62.5–125 mm2 (Fig. 7b).
One of the typical distribution patterns of polyps on
the polystyrene substratum to which many fouling species were attached (Fig. 6b), the density of polyps was
3.1 inds./cm2. The Iδ value was about 2.0 at a quadrat size
of 7.8 mm 2 (256 quadrats). Colonies had an aggregated
distribution with small colony sizes, and polyps were distributed evenly within the colony. The unit colony size
was 7.8–15.6 mm 2, and larger colony sizes of 125–250
mm2 were formed by aggregations of the unit colonies
(Fig. 7c).
3.4 Strobilation
Strobilation began from late December when water
temperature fell to 16–17°C. It reached its peak from the
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H. Miyake et al.
4. Discussion
The in situ habitat of Aurelia polyps was found at
Taniyama. This area was one of developmental areas of
ephyrae in Kagoshima Bay. Polyps were attached only to
the horizontal undersurface of artificial structures like
floating piers or buoys to 1 m depth. There were many
sessile organisms on the undersurface of the floating piers
or buoys, however polyps were observed on Mytilus sp.,
barnacles, dead oyster shells, solitary ascidians,
polychaete tubes, amphipod tubes and gap spaces. We also
observed polyps only on the undersurface of floating piers
at the Murotsu fishing port in Shimonoseki, Yamaguchi,
Japan. They were attached only to Mytilus, hard solitary
ascidians and floats, though there were many natural substrata that had overhang structures such as bare rocks or
stone wall embankments and many sessile organisms suitable for polyp attachment around there. There may be
substratum selection in the polyp of Aurelia. Previously
reported substrata for the polyps of Aurelia include
Mytilus, algae, the shells of living or dead barnacles, tunics of ascidians, hydrozoans and bare rocks (Ussing,
1927; Sjorgern, 1962; Thiel, 1962; Rasmussen, 1973;
Hernroth and Gröndahl, 1985a) (Table 1). Green algae
and brown algae were also used as polyp substrata in
Kagoshima Bay. However, these substrata are not permanent, since they disappear after late spring.
One of the striking features in this study is that polyps have a distinct preference for the undersurface of manmade structures like floating piers and buoys. Even if
suitable substrata exist on the upper or vertical sides of
man-made structures, no polyps were attached to those
sides. Brewer (1978) explained this phenomenon using a
laboratory experiment on planula settlement. Nearly all
planulae attached to the undersides of structures. However, a small number of planulae also attached to the
upperside. In our laboratory tank, polyps also formed
colonies on the bottom and the wall of aquaria. It was
observed that the polyps on the bottom of aquaria perished from being buried under sediments and that polyps
which formed colonies on the wall of aquaria had possible attachment substrata reduced by the encroachment of
algae and then asexual reproduction was repressed. Finally some polyps formed cysts on the wall and others
died, consequently colonies disappeared from the wall and
bottom of the aquaria (Miyake, unpublished). The reason
Fig. 8. Aurelia aurita strobilae: A: Young white strobila with tentacles, that had 9 segments. B: Maturing strobila. C: Matured
strobila that had 6 segments.
Fig. 9. Strobilation ratio for the entire population that was observed.
as to why no polyps were found on the upper side or vertical sides in this study may be a difference in the survival rates of polyps between the undersurface and other
surfaces. Polyps that develop from planulae attached to
the upper sides or vertical sides of a structure may survive for a while, but they are either buried in sediment or
eliminated by interspecific competition mainly with algae. As a result, it may be that colonies of polyps were
observed only on the undersurface of substrata.
Aurelia aurita exhibits internal and external water
currents produced by ciliary action (Southward, 1955).
Considering that polyps hang down from the undersurface
of substrata, it is very convenient for polyps to remove
sediments or suspension materials covering the body with
ciliary currents, thus maintaining a suitable environment
for the polyp.
Strobilation took place from late December to March
when water temperature dropped below 16° C in
Kagoshima Bay. The induction of strobilation was accelerated by varying temperature and light intensity
(Kakinuma, 1962, 1975; Custance, 1964, 1966), the addition of iodine to rearing water (Spangenberg, 1964,
1967, 1971), high density of polyps in a colony (Chiba,
1969) or low food density conditions (Thiel, 1962). After
late autumn when food concentrations and temperature
decrease, polyps may undertake strobilation and ephyrae
can be observed in early spring. The strobilation ratio
peaked at 10% in all populations collected in January. It
was very low compared with that reported for other localities, much less than 40% reported at Urazoko Bay
(Yasuda, 1988) or 80% in Kiel Fjord (Thiel, 1962). Strobilation did not occur in young polyps of age less than
one month and was faster in old age polyps of about two
to three months (Miyake, unpublished). Kakinuma (1975)
observed that more than one-month-old polyps strobilated
simultaneously. Considering the above mentioned strobilation, food conditions and temperature conditions for
individual polyps do not change very much in colonies
which live in the same area, so the main cause of the difference in the strobilation ratio in different localities may
be the age composition of the population in a colony. In
On the Polyps of Aurelia aurita in Kagoshima Bay
457
short, colonies including many old age polyps may exhibit a high ratio of strobilation.
Colony form showed an aggregated distribution.
Polyps settled regularly or randomly within the colony.
This suggests that there is an intraspecific spatial relationship in polyps. The Iδ value was higher on substrata
where many other species existed than on substrata where
few species existed.
The densest part of a colony in this study, up to 88
inds./cm 2 , was very high compared with the 6–40
inds./cm 2 reported in Gullmar Fjord, Sweden (Gröndahl,
1988a, b). These observations, including ours, suggest that
polyps form large colonies using a variety of asexual reproductive modes in competition with other fouling species. There were some variations in density on every substratum in every investigation throughout the year. It may
be that those variations did not derive from seasonal variation, but from the structure of the colony such as the age
composition, the number of planulae recruited on the substrata and the existence of other species. It was very difficult to set up the quadrat on the undersurface of a floating pier. Moreover, it was impossible to replace the
colony, which was observed under a stereoscopic microscope in order to observe the colony structure, to the same
place to which it attached in nature. Accordingly we were
unable to follow the development of the same colony at
the same place continually. It was also very difficult to
obtain quantitative data on colony density, because the
polyp colonies were distributed in patches and the density value would change according to the method used to
decide the total area of the colony substratum. To clarify
the population dynamics of colonies, an experiment using a flat float made of polystyrene in the same area and
also an experiment on colony formation in the laboratory
must be conducted.
In this study, it was clear that favorable conditions
for the settlement of Aurelia polyps were the presence of
gap spaces and sessile organisms with hard bodies, on
the undersurface of structures. The undersurfaces of natural structures are not common in the field. Almost all of
the structures that have a flat undersurface are artificial
structures such as floating piers, buoys, quay walls and
embankments. Artificial structures are common in the
coastal area of Japan. Recently, because of the great influence of Aurelia aurita on the coastal marine ecosystem and on our coastal economic activities like fishing
activities and electric power plant cooling (Matsueda,
1969), field studies of the Aurelia medusa stage have been
increasing. The increase in underwater structure of anthropogenic origin could be related to recent increases in
jellyfish biomass in pelagic ecosystems. Thus, we need
to research not only the medusa stage, but also the polyp
stage to understand the population dynamics of Aurelia
aurita.
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H. Miyake et al.
Acknowledgements
We would like to thank Dr. Mary N. Arai (Pacific
Biological Station, Nanaimo, B.C., Canada), Dr. Richard
D. Brodeur (Northwest Fisheries Science Center, Hatfield
Marine Science Center, NOAA, USA), Dr. Dhugal J.
Lindsay and Dr. James C. Hunt (JAMSTEC) for critical
reading and improvement of the manuscript. We thank
also Drs. Toshihiro Ichikawa and Masanori Sato at
Kagoshima University for providing everything for our
study. We thank students from the Department of Biology, Faculty of Science, Kagoshima University for helping with the field investigations. Heartfelt thanks go to
Dr. Ueno at the National Fisheries University who gave
us the chance to SCUBA dive at the Murotsu fishing port.
Finally, we appreciate useful comments and criticism of
the manuscript from the anonymous reviewers.
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