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 roughly-shaped (reproductive) gonophorous. At the end of this first asexual polygastric phase, the newly-budded young 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 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”.