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Abstract
Marine organisms have developed a high diversity of chemical defences in order to avoid predators and parasites. In sea cucumbers, saponins function as repellents and many species produce these cytotoxic secondary metabolites. Nonetheless, they are colonized by numerous symbiotic organisms amongst which the Harlequin crab, Lissocarcinus orbicularis, is one of the most familiar in the Indo-Pacific Ocean. We here identify for the first time the nature of the molecules secreted by sea cucumbers and attracting the symbionts: saponins are the kairomones recognized by the crabs and insuring the symbiosis. The success of this symbiosis would be due to the ability that crabs showed during evolution to bypass the sea cucumber chemical defences, their repellents becoming powerful attractants. This study therefore highlights the complexity of chemical communication in the marine environment.
Figure 1. Harlequin crabs Lissocarcinus orbicularis on their sea cucumber host (Bohadschia vitiensis).
Figure 2. Chemical structure of saponins found in the saponin cocktails of H. forskali, B. subrubra, B. vitiensis, H. scabra and H. lessoni.For each saponin congener, the structure can be established using the different letters shown in Table 3.
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