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Artificial mesocosms

In our laboratory, we have installed two identical mesocosms of 1500L each. They are made of a main tank of 500L to grow mother colonies, of two times two experimental aquaria of 300L each that can be disconnected from the main water circuit to study coral frags in different physico-chemical conditions, and of complete filters: mecanical, biological, skimmer and chemical (calcreactors). The systems are completed with "refugia" where macroalgae are cultured to regulate nitrogen and phosphorus concentrations in the water, down to submicromolar values.

The whole system is monitored and controlled by a computer that stabilizes physico-chemical parameters to targetted values: salinity of 35 (psu), temperature of 26°C, pH(sws) around 8, alkalinity of 2.5meq/kg, etc. Powerful, electronically varied pumps simulate waves and artificial light using T5 fluorescent lamps and HQI bulbs simulate the hydrodynamics and light field of natural coral reefs. The seawater is made by mixing commercial salts with osmosed and demineralized water. Water change is 20% per month.

These mesocosms are populated, of course, with various species of scleractinians that we study, but also by a more diversified flora and fauna (algivorous fishes, various mollusks, crustaceans and worms). Indeed, we create a small community that stabilizes the system from an ecological point of view. Hence, it is really a miniature ecosystem, artificial, but similar enough to real coral reefs for our corals. Some of these corals also reproduce sexually in our aquaria, like Pocillopora damicornis.

These mesocosms were designed by Philippe Grosjean and realized by Exotic 2000. hey were redesigned since the first project, mainly for the filtration, lighting, and the system to hold coral frags. Olivier Detournay and Antoine Batigny have contributed to this evolution of our installations .

A series of pictures

The mesocosms during their installation (March 2005, picture: Exotic 2000). The two tanks on the front are coral frags production units of 500L each. On the left, one of the 300L experimental aquaria, and the 100L refugia on the top.

The first test with water. The 300L tanks at left, one 500L tank at right. The filter, at the rear, in its first version, was modified later on (April 2005).

Final version of the mesocosms, equilibrated and fully populated. Overview of the two complete mesocosms (April 2009).

One 500L tank in operation where many coral species are growing (April 2009).

Production tank of one mesocosm (April 2009).

Production tank of the second mesocosm (April 2009).

Mother coral colonies freshly imported (December 2006). In the center, (in pink), the mother colony of Seriatopora hystrix, our favorite biological model: its growth is so fast in our mesocosms that it naturally becomes our ideal subjet to study coral skelettogenesis.

After a little bit more than two years (April 2009), original frags have increased their size in our mesocosms! Most of the space is occupied by large clone colonies of Seriatopora hystrix, all coming from the same frag (see hereabove). Now, we can work on those clones, which reduces inter-individual variability in our experiments. The slight color change is mainly due to variations in light in comparison with the light field the initial frag was exposed to on the reef.

Production is more than enough to provide coral frags for our experiments (April 2009). Frags are suspect with nylon lines and are tagged to measure their growth more easily. Note, once more, how color of S. hystrix can change according to the intensity and spectrum of light, as well as, nutrients availability.

We also study other species (March 2008). Here, frags of Acropora tumida (green), of Pocillopora damicornis (beige), of Stylophora pistillata (purple, at the center and bottom), and of Caulastrea furcata (at the first plane, one large polyp on the right).

Another picture of frags (October 2009), with in addition to previous species, Acropora formosa (brown branches with light tip).

One of the four 300L experimental tanks (October 2009). The rack where frags are fixed is clearly visible, as well as one of the electronically pulsed pumps (Tunze Stream) on the right. The yellow surgeon fish is a Zebrasoma flavescens, a rather efficient algivorous fish.

Another stream pump (top left) in one of the 500L tanks. A couple of other surgeon fishes, Zebrasoma xanthurum (blue with yellow tail), Zebrasoma desjardinii (striped) graze algae. One can see also the head of a Pomacanthus annularis hidden beneath the rack (October 2009).

Finally, here is our mascot (October 2009), one giant clam Tridacna derasa of a good size. This mollusk also lives in symbiosis with zooxanthellae, as do zooxanthellate corals. This clam is about 40cm long and weight already several kilos... but it should still grow!


Why should we study corals in artificial conditions?

Shouldn't we understand how these animals work in their natural environment? Artificial biotopes, like our mesocosms, aren't they too different from natural conditions? Hence, observations done in aquaria, aren't they, consequently, totally artificial? Indeed, we don't try to extrapolate what we see there to what happens in the field, but we consider one constant: the animal is (genotypically) the same. Consequently, his ecophysiological study in a perfectly controlled environment can provide insights on its adaptation potentials in conditions that may be impossible to reproduce in the field. This opens possibilities to complement observations done on natural reefs in order to understand how a coral holobiont works. In artificial mesocosms, we can induce a large palette of phenotypical variations. More specifically, we try to explain precisely why such an holobiont is so efficient to create geological-scale reefs where biological diversity is so high.


Where does this picture come from?
No, it wasn't taken in the field... this is the home aquarium of one member of the EcoNum staff (click on the picture to learn more)! A sympathic way to continue working at home, isn't it?

A borderless aquarium!
This aquarium was presented during the "printemps des sciences" 2008 (click on the picture for a larger view). Water flows over the edges which are made of transparent polycarbonate of the same density as seawater. That way, edges become practically invisible, sandwitched between two layers of water.