More than 100 triterpene glycosides (saponins) have been characterized in holothuroids in the past several decades. In particular, Holothuria forskali contains 26 saponins in its Cuvierian tubules and 12 in its body wall. This high diversity could be linked to a chemical defense mechanism, the most commonly accepted biological role for these secondary metabolites. We performed an integrated study of the body-wall saponins of H. forskali. The saponins are mainly localized in the epidermis and in the mesothelium of the body wall and appear to be released when the holothuroid is stressed. Among the saponins present in the epidermis, one (holothurinoside G) was detected in the seawater surrounding non-stressed holothuroids and three others (holohurinosides C and F, and desholothurin A) were secreted when the animals were stressed. In addition, two new congeners (detected at m/z 1301 and 1317) were also present in the immediate surroundings of stressed holothuroids. These new saponins do not originate from the epidermis and could come from an internal organ. Quantities of secreted saponins were very low compared with the body wall and Cuvierian tubules concentrations. At natural concentrations, saponins do not represent a threat to the health of predatory fish. The deterrent effect of saponins seems therefore to act as an aposematic signal, warning potential predators of the unpalatability of the holothuroid tissues.
P.G. thanks the FRS-FNRS for financial support in the acquisition of the Waters Q-ToF Premier Mass Spectrometer and for continuing support. I.F. and M.W. acknowledge funding from Centre National de la Recherche Scientifique (CNRS), Ministère de l'Education Nationale, de l'Enseignement Supérieur et de la Recherche, Agence Nationale de la Recherche (ANR PCV to I.F.), Institut du Cancer (INCA to I.F.). Work supported by a FRFC Grant no. 2.4535.10. This study is a contribution of the Centre Interuniversitaire de Biologie Marine (CIBIM).
- © 2011.