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Reflexions, le site de vulgarisation de l'Université de Liège
The dance of the invading jelly-fish
11/17/14
Almost everywhere on Earth, entire sections of coastlines are occasionally invaded by jelly-fish. While
the periodicity of such phenomena has been relatively well-known (sometimes several centuries!), the
mechanisms involved are relatively unknown. It is to be noted that the physiology of these "Cnidaria", which
are sometimes smaller than is generally thought, remains an important area for scientific investigation. In her
doctoral thesis, Amandine Collignon describes (for the first time) the entire development cycle of a key species,
Chelophyes appendiculata, showing reproduction that is both sexual and asexual! Even more worryingly,
she has also shown that jelly-fish, like other local marine species, are largely contaminated by microplastic
residues. The work was carried out at the Stareso oceanographic station (University of Liege) in Calvi bay
(Corsica).
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Stories about jellyfish are not often told to children on
long winter nights... In most adults they evoke a kind of repulsion tinged with dread. Their tentacles which carry
cells that can inflict stings (only a minority of which are actually urticating), do nothing to allay the antipathy
aroused by these flaccid viscous masses that invade beaches at certain times.
These same jellyfish, which are part of a phylum or branch called cnidaria, can elicit gasps of wonder from the
same public that yesterday hated them so intensely whenever they are displayed in an aquarium. The ethereal
way in which these invertebrates with neither a shell nor skeletal structure drift through the water, is a delight
to watch, just like the extreme diversity of their umbrella-like shapes: these shapes can be round or angular,
flat, or dome-shaped, transparent or colored etc. These strange animals, which belong to the same phylum
as coral and sea-anemones can therefore also become objects of curiosity or even fascination. Comprised
of 95 % water, these animals are without a heart and nervous system as we define them. They are capable
of "melting" whenever there is a lack of food only to regenerate themselves and reform when a food supply
becomes available again. Some species (almost a thousand of them and forming a grouped into 300 different
kinds) have up to 800 tentacles per individual.
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Existing in micro, macro and meso forms, there are jellyfish to suit all tastes
What the wider public does not know is that apart from the "typical" jellyfish that exist, there is a multitude of
others that are infinitely smaller, some only measure a few centimeters in length, and others are practically
invisible to the naked eye. These less spectacular species are interesting to study from the point of view of
achieving a better classification of a phylum that is still less well-known and subject to frequent reorganizations,
but also from the point of view of better understanding the proliferation phenomena concerning these species
that have been recorded almost everywhere throughout the globe over the last three decades or so.
The tourist industry will still recall the episode in 2006, which saw veritable clouds of pelagia invade the bathing
waters of sea resorts. Pelagia is a stinging species with a pink umbrella of around 10 to 12 centimeters in
diameter and which is also found along the North-sea coastline. Over recent years, other proliferations of
jellyfish have been recorded from the Mediterranean as far as Ireland, the Gulf of Mexico, the Black Sea, the
Caspian Sea, the United States and Japan.
Episodes like this make the front headlines of newspapers due to the fact that they make life difficult for
holidaymakers. They can also cause problems for fishermen. Due to the fact that there are millions of them
and they can contain up to 20% of zooplankton in volume which can clog up fishermen's nets. They are also
predators - jellyfish are carnivores - they eat the eggs of fish both in the natural environment and on fish
farms. In other regions, particularly in Scandinavia, large volumes of jellyfish referred to as "swarms" have
been known to clog up the cooling pipes of electric power stations. This shows the impact such proliferations
can have on the economy!
Proliferations that cause major headaches
"These episodes are dreaded throughout the world today", comments Amandine Collignon, the author of a
doctoral thesis (1) which she defended last spring at the Oceanology Laboratory (Department of Biology,
Ecology and Evolution) of the University of Liege under the supervision of professors Jean-Henri Hecq and
Anne Goffart. "But we still don't understand why these proliferations occur and we also need to distinguish
between purely subjective theories and real scientific data. If, for example, the fishing industry decides to
frequent a new fishing zone, the fishermen might be tempted to interpret an abundance of jellyfish as an
abnormal phenomenon whereas these animals might have been present in this area for a long time".
A prudent approach also needs to be applied to phenomena occurring in tourist resorts: "the recorded jellyfish
'invasions' of beaches do not necessarily mean that they are abnormally numerous in the sea. Sometimes a
conjunction of relatively harmless events can create a false impression. This is the case, for example, when
the vertical migration of jellyfish (a phenomenon that is frequent at the end of the day in temperate waters) is
followed by an encounter with a shoal of predatory fish. This swarm of jellyfish that were initially harmless and
healthy can then become transformed, under the effect of other factors such as the turbulence of currents,
into a 'passive' swarm made up of exhausted and wounded individuals. They drift in strong currents and are
washed up on beaches". These somewhat benign 'invasions' are interpreted as aggressive by tourists who
are disturbed by the presence of the creatures…
Corsica: an ideal study location
In order to study such phenomena, and particularly to study the development of jellyfish populations, the
University of Liege has access to an invaluable tool: the Stareso oceanography station. Built at the beginning
of the 1970s and having the particularity of being located far from densely populated areas, the station is
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literally 'immersed' in the terrestrial and maritime environment of Calvi Bay in the North-West of Corsica and
is thus protected from human influences such as pollution. In addition, the station has amassed an impressive
volume of temporal data (meteorological, physical, chemical and biological) relative to the marine ecosystem.
Once a week, a marine expert combs a transect (an established route that is regularly followed) of the bay
in a boat equipped with a 200 micrometer steel mesh fishing net for a period of twenty minutes. The expert
also gathers precious zooplankton, a fundamental research 'material' for the investigations carried out by the
oceanographers from Liege: fish larvae, prawns, eggs, sea-urchin eggs, appendicularians, gastropods, etc.
A group of jellyfish that are particularly represented in Calvi Bay also feature in this daily fishing expedition:
Siphonophores and Calycophores, are of particular interest to Amandine Collignon "I chose this particular
group of medium-sized jellyfish for my thesis because it is an abundant lesser-known species in the
Mediterranean and also because our samples have revealed that these organisms, which are made up of
several individual categories with special functions, are present all year round in the marine ecosystem,
particularly the polymorphous species Chelophyes appendiculata. In fact, I could not run the risk of studying
only jellyfish whose interannual cycles are known to be spread over six to ten years for my four-year doctoral
thesis. This is why I concentrated on the Calycophores while also studying one of the best-known scyphozoan
jellyfish (Editor's note: the 'real' large jellyfish) in Europe: Pelagia (Pelagia noctiluca). My objective was to
determine the variability of these jellyfish and the influence that biotic factors (presence of phytoplankton and
zooplankton) and abiotic factors (wind, temperature, ocean currents, pollution, etc.) have on these".
Reproduction: with and without sex
It is impossible to understand the annual or interannual variations of jellyfish populations - the infamous
proliferations - if we do not first understand their morphology and the different stages of their reproduction. "It
is not always easy to distinguish an individual from a mature colony (or super organism) given that the size
of the colony itself rarely exceeds two centimeters in length", explains the young researcher. She describes
the development cycle of the key species Chelophyes appendiculata, for the first time. "Broadly speaking, it
involves a first phase during which the colony develops from the larvae giving rise to a burgeoning population
of young individuals each of which has a gastrozooid (eating apparatus), a (protective) bract and a roughlyshaped (reproductive) gonophorous. At the end of this first asexual polygastric phase, the newly-budded young
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individuals which have matured are released from the initial colony in the form of independent individuals called
eudoxia. The latter only have one gastrozooid and the mature gonophore produces male and female gametes
which give rise to the larvae. This second phase of reproduction is called "monogastric sexual reproduction".
The following phase of the work consisted in establishing the annual and interannual fluctuations of the
populations based on samples fished every week in Calvi Bay between 2003 and 2013. "To process such
differences in values for biological variables, I used the statistical method of standard deviation and curves. For
the sake of brevity, this statistical method consisted in establishing averages and then averages of averages
from the 600 species of zooplankton gathered". By comparing the results obtained with the data relating to
the different biotic and abiotic parameters examined (including temperature, wind-speed, the abundance of
zooplankton, the concentration of chlorophyll a in the water column) the researcher from Liege firstly noticed
that sea temperature has a direct positive effect on asexual reproduction: the more the temperature increases,
the more numerous the gonophores are which favors reproduction in Chelophyes appendiculata. She then
observed that the temperature of the water has a directly negative effect on asexual reproduction via the trophic
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cascade. This means that the more the temperature rises, the weaker the reproduction of phytoplankton will
be. Logically, the zooplankton diminishes in turn, resulting in a decrease in the amount of gonophores.
A first in the Mediterranean
This direct correlation between the abundance of phytoplankton and zooplankton and jellyfish in turn seems
logical. It still remained to explain this 'trophic cascade' - which had never before been done for this species
in the Mediterranean - but also to situate this trophic cascade in time. A time lapse of one month between
the maximum amount of chlorophyll (in March) and that of zooplankton (in April) was observed. Then another
time lapse of one month between the maximum abundance of zooplankton and Chelophyes (in May) was
observed. It was still necessary to precisely describe the mechanism that triggers this chain reaction from the
phytoplankton at the end of winter occurring from February to the beginning of March and up to the jellyfish
stage. In winter, or more precisely from December to February, the winds are stronger and therefore they stir
up the water column and suspend the nutrients that are indispensable to the growth of phytoplankton. "We can
conclude that the wind also exerts a directly positive influence on asexual reproduction in the Calycophores
and more broadly speaking, that their development cycle in Calvi Bay is part of the seasonal sequence of
events in the ecosystem of plankton", explains Amandine Collignon.
Another important and somewhat worrying fact is that Amandine Collignon's thesis also included an analysis of
zooplankton samples captured in the surface water of Calvi Bay and elsewhere in the Western Mediterranean
and these revealed that jellyfish, like other local marine animals, are extremely contaminated by microplastic
residues. A staggering 40 % of stations sampled from 2010 to 2014 contained synthetic particles varying in size
from 0.3 to 5 milimeters: polystyrene, filaments, residue from plastic Clingfilm, etc. The researcher dolefully
explains, "Jellyfish are fragile animals. They ingest these fragments. Researchers are only beginning to study
the mechanics of this problem, and we are still unaware of the overall physiological impact this can have on
the reproductive cycle of Cnidaria, which is not quite the case for other animals. The other worrying aspect is
that Calvi Bay, which up to now was reputed for the quality of its ecosystem, is not immune to accumulations
of plastic. Fragments tend to accumulate in the gyres (whirlpools of water) at the bottom of bays and then
cannot ever be dispersed unless algae attach themselves to the surface of these fragments causing them to
sink to great depths. But what are the effects of this on the ecosystem? It remains a mystery for now".
Predicting the 'peaks': a delicate exercise
All these discoveries still tell us nothing about the reasons for the cyclical proliferations of one of the most wellknown jellyfish on the beaches of France: Pelagia (Pelagia noctiluca). The same applies to Aurelia (Aurelia
aurita) which, in Belgium too, from Panne to Knokke-le-Zoute, sometimes spoils the summer for bathers.
Amandine Collignon reminds us that this phenomenon of cyclical proliferation has been reported for over two
hundred years! Thus it is not only a recent problem. "Due to a lack of precise data, it would certainly be
premature to put it down to global warming. My research only focused on Calvi Bay and I cannot offer an exact
explanation for the cyclical proliferation of Pelagia (around every ten years). On the other hand, we can confirm
the great annual and interannual variability of the species. In Calvi Bay, for example, the years 2004 and
2005 were marked by a gradual increase in populations followed by a net increase in 2006. Then, during the
summers of 2007 and 2008, we observed an important decrease in populations. The summer of 2014 seems
to have seen a new 'peak', but we do not yet have all the data to confirm these empirical observations. What
is clear, moreover, is that the active swarms of Pelagia generally occur at the end of the spring just below the
surface of the water. This phenomenon is broadly spread over a few hours. It happens around terrestrial points
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where the surface of the water is protected from wind, where there is less turbulence and where zooplankton
accumulates.
These swarms do not remain for long,
barely a few days. They migrate vertically at close of day but they are also influenced by wind fluctuations. In
the end, only dying individuals that join the passive swarms carried by the North or North-East winds remain
on the surface near the coasts".
Will we one day be able to predict the appearance of masses of jellyfish on coastlines, both in the Mediterranean
and the North Sea? "At the present time, it is an extremely hazardous exercise", comments the oceanographer.
"We cannot affirm that only one factor - temperature, the presence of plankton, etc. - plays a role, but rather a
conjunction of factors is involved. In addition, local conditions vary greatly according to whether you are in an
oligotrophic environment like the Mediterranean Sea (where there are not many nutrients available for plant
and animal plankton), or eutrophic like the North Sea (rich in nutrients). Public authorities often ask scientists
for this kind of prediction but they must understand that our studies are necessarily long-term commitments.
Providing funding just because there is some kind of public outcry following a proliferation of jellyfish is illadvised. We need continuity, especially when nothing spectacular is seems to be occurring".
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(1) Abondance et variabilité des méduses en baie de Calvi (Abundance and variability of jellyfish in Calvi
Bay), doctoral thesis, Amandine Collignon, University of Liege, 2014.
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