Host Separation Syndrome and Chemical Dependency of Ectosymbiotic Decapods Associated with Echinoderms

Symbiotic relationships, defined as close and long-lasting associations between at least two different species, are ubiquitous throughout the biosphere. One key factor that sustains symbioses across generations is the establishment of a host-specific chemical environment allowing, amongst others, the survival of the symbiont and its ability to select a suitable host. These chemical signals, true metabolites from the host, are also known as kairomones as they provide benefits for the symbiont. In echinoderms, the nature of the kairomones attracting symbionts is only known for associations involving sea cucumbers and sea urchins hosts. Moreover, when separated from their hosts, the environment (both chemical and nutritional) of the symbionts becomes unsuitable leading to a "host separation syndrome". This syndrome, only described in a sea urchin-shrimps association, is characterized by an alteration of the behavior and the state of health, potentially leading to the death of symbionts.
The general aim of this thesis is to contribute to the knowledge of these kairomones playing a crucial role in symbiotic associations, notably those associating decapods with echinoderms which are frequent symbioses in the marine environment. First, the chemical characterization of the kairomonal chemical signature allowing host recognition is investigated in crinoid and asteroid hosts. Then, the host separation on various symbiotic decapods is monitored to characterize the syndrome in several species and to understand its impact at the behavioral, anatomical and physiological levels.