The researcher from Liège first wanted to know which chemical elements were important for the development of the chemosynthetic bacteria. The experiment was seemingly simple: five recipients containing five different environments (control sea water, sea water enriched either with sulphur, methane iron or hydrogen), plus a supply of dissolved organic or inorganic carbonaceous molecules were placed in pressurised aquarium. A shrimp was immersed in each of the test tubes for several hours. A biological or isotopic marker then allowed the researchers to measure the proportion of carbon incorporated by the bacteria, i.e. to check in which of the five environments the microbe produced the most new organic matter. “After being immersed for several hours”, Julie Ponsard states, “we didn’t see any great difference between the environments. Every chemical element seems to be as important and is potentially used by the bacteria.”

The second part of the study aimed to measure a possible transfer of dissolved organic matter between the bacteria and the shrimp. For this demonstration of the existence of symbiosis, the researcher from Liège dissected her shrimps into several pieces, in order to check in which part of the animal the transfers of biological and radioactive markers could be observed. Result: firstly, the greatest number of tracers were found in the shell, as well as in the other tissues where the bacteria are established; secondly, in the gills, which are also tegumentary tissues and highly vascularised, and the uropods (shrimp’s tail); then the muscles, the digestive system, etc. “Our observations (published in the journal International Society for Microbial Ecology [ISME]) (1) confirm the symbiotic hypothesis”, Julie Ponsard explains. “A large amount of the small dissolved organic molecules are indeed transferred between the bacteria and the shrimp through the shell, which is in fact permeable. The digestive system is secondary. Which doesn’t mean that it's useless because, in situ, the shrimp could continue to consume non-symbiotic bacteria, and even other prey. The abyssal conditions have perhaps obliged the creature to develop two complementary feeding systems.”

And its cousin in the North Sea?

To plunge further into her research, Julie Ponsard also turned her attention to a very distant cousin of the abyssal shrimp: the North Sea brown shrimp. We could indeed hypothesise that in this shrimp’s environment dominated by photosynthesis, it has developed a very different diet. Nevertheless, dissolved organic substances abound in the marine environment and could constitute a significant food source. Crangon crangon, its scientific name, lives in the shallow waters of the continental shelf. At low tide, you just need to drag a net across the sand to collect large amounts. For her experiment, Julie Ponsard went to Bray Dunes (France) in July 2010 and Wimereux in September 2011. Morphologically, the brown shrimp is quite different from the abyssal shrimp. The branchial space between the shrimp's body and its shell is far less voluminous and doesn’t house any symbiotic bacteria, except accidentally, in small quantities. “But we wanted to show that its shell was permeable to the small dissolved molecules”, Julie Ponsard explains, “thus supporting the hypothesis of the symbiotic diet of its abyssal cousin.”