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Life History and Environment of Aurelia aurita
Hiroshi MIYAKE 1), Kenji IWAO 1) and Yoshiko KAKINUMA 1)

Abstract

We investigated the seasonal occurrence, growth, age, and life history of Aurelia aurita in the Taniyama areaof Kagoshima Bay, also some of the environmental factors such as the specific gravity, pH and the temperature,
influence the appearance of Aurelia aurita in the surface layer. The size of an individual depends on environmental
factors. In contrast the number of branching points of radial water vascular canals increasing with time, independently of environmental factors, and therefore could be used as an age index.
By using age index, the population of Aurelia aurita sampled may consist a mixture of two cohorts. The
assumption may be that the life span of Aurelia aurita is up to two years.
By examining all stages of the life history of Aurelia aurita in Kagoshima Bay and comparing it with that ofother locatives, it is found that the life form of Aurelia varies with their environmental circumstances. We suggestthat how the habitat influence the life history in Aurelia should now be reviewed.
Key words: Aurelia aurita, Environment, Growth, Age, Water vascular system, Populations, Life cycle.

Introduction
The Scyphozoan Aurelia aurita is seen commonly in cosmopolitan waters. It has alternation of generations, between a sexual planktonic generation and an asexual benthic generation.
There are many ecological reports of the medusa stage in north Europe, north America
and Japan. It was reported by MOLLER (1980), YASUDA (1971), HAMNER and JENSSEN(1974), OLESEN (1994) and OMORI et. al. (1995) that Aurelia medusae grew in size andmatured from early spring to summer and then the medusa size regressed. They suggested
two reasons for that phenomenon; one is senescence of medusae after spawning and theother is the invasion of a new generation of young medusae. However, morphologicalcharacters, aging and mortality of medusae were not referred to and the reasons for size
regression phenomena have never been proved.
The aim of this study is to clarify the life history of Aurelia aurita in Kagoshima Bay.
We investigated seasonal occurrence, growth, maturation, aging and their environmental
factors. In the laboratory, we studied the organ differentiation of canal system of medusa incultures.
1)

Department of Biology, Faculty of Science, Kagoshima University, Kagoshima 890, Japan

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Materials and Methods
I. The morphogenesis of medusae cultured in the laboratory.
Ephyrae used in this experiment were liberated from polyps in the laboratory by
Kakinuma’s method (KAKINUMA, 1975).
Experiments were carried out between mid-June and mid-September 1993. The range ofroom temperature was from 20 to 27℃. Ephyrae and metephyrae were cultured in tall-glass
petri-dishes, 9 cm diameter, 12 cm high, filled with 500 ml filtered sea water. In order tosupply oxygen and keep organisms from sinking to the bottom of the vessel, we madecircular water flow by air lift using a piece of a acrylic tube 1 mm in diameter and 20 cm
in length. Cultures were fed with excess of Artemia sp. larvae and old culture sea water was
replaced with fresh sea water once a day. Young medusae were transferred to a glass medusa rearing tank devised by Michihiro Tabata, 25 cm diameter and 20 cm high, filled with
7 filtered sea water. They were also fed with excess Artemia sp. larvae once in a day andold sea water was replaced with fresh sea water every three days. Larger medusae were thencultured in a 60 cm wide tank with 40 filtered sea water.
For these jelly fish measurements of size (mm, diameter), wet weight (g) and the number of water vascular canal radial branches were taken every 7 to 10 days. The water vascular system branches and elongates from the margin of umbrella, so we counted the maximum number of branching points of radial canals (Fig.1 KAKINUMA et al., 1993).

Fig. 1 The methods of measurement of medusa diameter and counting the number ofbranching points.

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II. Ecological study of medusae at the Taniyama area in Kagoshima Bay
Between December 1993 and December 1994, three stations around the Taniyama areain Kagoshima Bay at the southern end of Kyushu Island, Japan were surveyed every 10
days (Fig. 2). Sampling was done using a 45 cm diameter plankton net with a mesh sizeof 2 mm. Within 2hr. after sampling, live medusae caught in the field were observed andmeasured. Data includes medusa diameter (mm), wet weight (g), the number of water
vascular branches, maturation of gonad and the presence of planulae in the brood sacs on
the oral arm. Environmental factors taken at each sampling points were as follows: weather
condition, water transparency, temperature, pH, specific gravity and salinity measurement
in day time, and the settled volume of plankton caught by oblique haul towed 6 to 10 m

Fig. 2 The location of investigated station at the southern end of Kyushu
island, Japan276

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using a 30 cm diameter plankton net with a mesh size of 50 m.

Result and Discussion
I. The morphogenesis of medusae cultured in the laboratory
The relationship between the number of branches and the number of days show logarithmic growth (Fig. 3 B ＝ 3.590 log D 2.462 r ＝ 0.969; B ＝ the number of branching
points, D ＝ days after ephyrae liberated from strobilae). At 10 days the first branching
point appeared. Then at 18 days the second appeared. At 34 days the third branching points
appeared, and after 63 days the forth ones appeared (Fig. 3). The most striking part in
Figure 3 was that the medusa size decreased by their environmental factor or their physiological conditions (Fig. 3 arrows), however the number of branches continued to increase.
So it is the best way to take the number of branches to indicate their aging.
II. Ecological study of medusae at the Taniyama area in Kagoshima Bay
We found the habitat of polyps at station II. Station II is located in a channel where mostof the vessels and floating piers were anchored. No polyps were seen on the side of floatingpiers or the wharf. They are seen only on the undersurface of floating piers. These floatingpiers were made of polystyrene. The polyps were attached non randomly. Most of themwere seen attached to solitary ascidians, Balanus, Mytilus edulis, polychaetae tubes andamphipod tubes (Table 1). These polyps advanced to strobilae from December to March,
afterwards the life style changes from benthic polyp stage to planktonic medusa stage.

Table. 1 Substrates of Aurelia aurita polyps.
Substrates under the surface of floating pier

Polyps present

Solitary ascidians
Colonial ascidians
Barnacle (Balanus)
Bivalves (Mytils edulis)
Polychaeta tubes
Amphipod tubes
Bryozoans
Actiniarians
Hydroids
Sponges

＋＋
−
＋
＋＋＋
＋
＋＋＋
−
−
−
−

Phaeophyseae
Other algae＋＋−

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Fig. 3 The growth of Aurelia aurita medusa in the laboratory.
Open circle shows size (diameter). Closed squares shows the increasing number ofwater vascular branching points. The error bar shows S.D.
(B: the number of branching points, D: days, L: medusa diameter)

In Kagoshima Bay, ephyrae appeared from early January to middle March 1994,
Metephyrae from late February to early April, and medusae from late February. In
Fig. 4, the time of occurrence of this species in plankton form is very different in different
localities, because of difference in their environment. Fig. 5 shows the differences in salinity and temperature ranges of environmental factors for Aurelia aurita it reveals that Aurelia
aurita has the ability to adapt itself to various environmental factors and could change its
life style with its environment. The specific gravity, pH and water temperature influence thepresence of medusae in the surface layer as shown by Fig. 6. In the upper right graph (pHtemperature), the cross mark in the clear circle within the medusa occurrence range
corresponds to the same mark in the upper left graph (pH specific gravity), that the278South Pacific Study Vol. 17, No. 2, 1997＊

Fig. 4 The times of occurrence of Aurelia aurita in different localities.
: Planulae: Ephyrae: Medusae＊

Personal communication

Fig. 5 The differences in range of temperature and salinity as environmental factor for
Aurelia aurita in different localities.

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,

,

Fig. 6 The environmental range of medusa occurrence.
A closed circle (●) indicates medusa occurrence. A cross (×) indicates no occurrence. The stippled area is the range of environmental factors for medusa occurrence(Specific gravity 1.0210 〜 1.0260, Water temperature 14 〜 28 ℃, pH 8.1 〜 8.6).

specific gravity mesurement is very low as also observe in the lower left graph (temperature
specific gravity). These three environmental factors, i.e., temperature, specific gravity andpH have the greatest influence on the occurrence of medusae.
In Kagoshima Bay, the growth of Aurelia medusa starts in January and by July they reacha peak diameter of 23 cm (Fig. 7). In Kiel Bight, Germany (MOLLER, 1980), the species

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Fig. 7 The growth of Aurelia aurita medusae in different localities.

started growing in May and reached a peak diameter of 20 cm in August. In Tomales Bay,USA (HAMNER and JENSSEN, 1974), growth started in March and reached a peak of 12 cm
by July. In Kertinge Nor, Denmark (OLESEN, 1994), growth started in April and reacheda peak of about 4 cm in July (Fig. 8). These four curves, all show that the growth rates andmedusa size decrease after summer. The reason why the curves differ is that ranges ofenvironmental factors vary in different places where the jellyfish is present.
There is a scattering in the relationship between medusa diameter and the number ofbranching points of the radial canals (Fig. 8). For the given same medusa diameter, thenumber of water vascular system branching points varied from 4 to 8. For example, when
there are 5 branching points, the medusa diameter ranges between 90 mm to
320 mm. The reason suggested is that environmental factors as well as physiological factors
control the growth of medusae.
The monthly change of age composition of the Aurelia population was indicated by the

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Fig. 8 The relationship between medusa diameter and the number of branching points ofradial canal.
number of branching points and the maturity of medusae (Fig. 9). The results obtained
show that the number of branching points ranged from 0 to 8. Considering these data (Fig.
9) together with unpublished observations on the organ differentiation with growth inlaboratory cultures, it is possible that the Aurelia samples indicate a succession of two
populations. In our samples it would have originated in 1993 and 1994. From the monthly
changes in number of mature medusae in Fig. 9, it may be that the older the medusae is, themore accelerated the maturity of its gonad is. The number of older medusae (i.e. numerous
branching points of radial canals) increased up to July but from August they began todisappear. Possibly by this time the 1993 population of medusae is being succeeded by the
1994 population. By summer, old medusae, which have more than 7 radial canal branching
points, have no directional swimming ability and drifted by the wind and waves. During
1994, water temperature and the amount of planktonic food available increased from March,and reached a peak in June. At the same time, Aurelia medusae attached planulae increased
from march, reached a peak in June and increased again slightly in September (Fig. 10).
Considering the age of this population (i. e. the number of branching of radial canals, Fig.
3), it may be suggested that from March to April only the medusa population of 1993
released planulae, and after that, medusae reached more than 4 radial canal branching points
in 1994 population also began to release planulae. So it may be that in Kagoshima Bay,
Aurelia release planulae twice, both in the first summer and in the second summer. In thatcase mortality of medusae occurs over a period of 8 to 20 months. The life span is up to two
years.
The importance fact described above is that we must consider not only the life cycle of282South Pacific Study Vol. 17, No. 2, 1997

Fig. 9 The monthly changes of the age of the Aurelia aurita population and sexual maturating of medusa during 1994.
The aging was taken by the number of water vascular canal branching points.
Dotted area indicates immature medusae. Black shows mature medusae with planulae.

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Fig. 10 Above: Seasonal change of water temperature and volume of plankton in 1994.
Below: Seasonal change in the frequency of medusae with attached planulae in 1994.

Aurelia aurita, but also the life cycle including the environmental factors play an importantrole for the survival of Aurelia population.

Acknowledgment
We thank Dr. Elaine Robson, School of Animal and Microbial Science, the University ofReading, for reading and making critical comments on this manuscript.

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