Biology and ecology

Giant Pearl Mussels live mainly on stable surface soils, wedged in sediment rocks of shoals and pebbles. The specie can be found in the areas without much erosion and which level of depth is between 0.2 and 2.5 meters. In the Charente river however, some species can be observed until 8 meters water depth. In Spain, it can even be found in artificial channels. (Imperial Canal of the Ebro).

The Giant Pearl Mussel lives in vertical position, partly buried in the sediments, in areas with strong current (up to 1m.s-1) and of water enriched in calcium, said “hard water”: the Charente pH is therefore of 9 in the Taillebourg area according to the Water Agency.

The different studies carried out on Giant Pearl Mussel populations in Spain, on the Vienna and in La Creuse reveal that this specie lives in groups. However, we can note that the studied rivers are affected by human activities (dam, contamination…) and that this distribution can also be explained by the fragmentation of the natural habitat of Margaritifera auricularia

The Giant Pearl Mussel is a filter feeder but its exact diet is not known. Like all the Naiads, it feeds on organic particles transported by the water stream. Water passes into and out of the mantle cavity through the frilled siphons (kind of gills). The food particles in the water gathers on the siphons full of mucus and are driven to the mouth with microscopic silks. The particles too large are directly rejected by the exhaling orifice.

The Giant Pearl Mussel usually measures between 15 and 17 centimeters in the adult stage and can be up to 20 centimeters. According to measures led by Rafael Araujo, Giant Pearl Mussel populations spread in the Ebro in Spain are of 15 to 17 centimeters, and almost no specimen smaller than 12 centimeters has been identified. The population is indeed ageing. Similarly, most of the specimens identified in Vienna measure more than 13 centimeters and are adult populations. The identifications and measures carried out in most of the rivers where Giant Pearl Mussel live are characterized by the absence of mussels at the juvenile stage.
It is indeed admitted that a specimen is sexually mature from a size of 16 centimeters, which would correspond to about 50 years. The older of the specimens known in the Ebro would be 159 years, for a size of 20 cm. These size classes show that all these known populations are ageing.

As all the big freshwater mussels (named Naiads or Unionoidea), the Giant Pearl Mussel has a characteristic reproduction: Starting usually in March, the male spread its seed in the river and the female is fertilized from a distance. However, if the female is not situated downstream of a male, it can fertilize itself, being hermaphrodite.

Then, the mussels develop thousands of larvas called glochidias that they spread in the river at the end of March, when the number and the size of the larvas starts preventing the mussels from breathing. Madam Giant Pearl Mussel waits for the passing of host fishes and frees its glochidias in this moment. The glochidias, small larvas with hooks encyst on the gills of the host fish with a cocoon so they can develop thanks to the body fluids of their host. The most known host fish is the European Sturgeon (Acipenser sturio).

About a month after, developed enough, the juvenile mussels drop from the host fish gills and continue growing in the soil where they fell, usually in gravely pebbled soils.