Sea cucumbers might seem to be sitting targets waiting to be attacked, but they are far from defenceless. Equipped with Cuvierian tubules – sticky threads that the animal expels to ensnare hapless predators – and mutable collagenous tissue, which allows them to stiffen or soften their bodies for protection, sea cucumbers are reasonably well prepared for most attacks. But these echinoderms also have another deterrent mechanism: they taste bad. Patrick Flammang from the University of Mons, Belgium, explains that sea cucumbers produce unpleasant tasting detergent molecules, called saponins. Knowing that saponins can prove fatal for fish in high doses, Flammang and his colleagues decided to find out how sea cucumbers use these noxious chemicals to defend themselves (p. 1347).
Travelling to the south of France, Flammang's students, Séverine Van Dyck and Maïté Todesco, went diving in the Mediterranean to collect Holothuria forskali sea cucumbers. Returning to Belgium, Van Dyck took samples of the sea cucumber's body wall and extracted and purified their saponins. Then she and Pascal Gerbaux analysed the saponins with mass spectrometry and found eight in the animal's body wall, the so-called holothurinosides (A, C, E, F, G, H and I) and desholothurin A. But how were these compounds distributed through the animal's body wall?
Teaming up with Isabelle Fournier and Maxence Wisztorski, mass spectrometrists at the University of Lille, France, Van Dyck and Flammang used an innovative mass spectrometry imaging technique to identify the location of each saponin in the sea cucumber's body wall. Together they found six of the saponins (holothurinosides C, F, G, H and I and desholothurin A) in the body wall's outer layer (epidermis) and two (holothurinosides A and E) in the inner layer (mesothelium).
The team also analysed the saponin distribution in the body walls of mildly stressed sea cucumbers after they had been poked and found that it had changed. Flammang suspects that the smaller saponins in the epidermis of the relaxed sea cucumbers are converted into the larger, more soluble, saponins for release into the water when the animals are stressed.
Next, the team looked at the animal's body wall structure with transmission electron microscopy and saw that cells that had been packed with material in the relaxed sea cucumbers were empty in the stressed animals. However, Flammang says, ‘We cannot be sure where the saponins are located so this difference could come from other phenomena, but it is an indication that something is changing in the epidermis during stress’.
Having analysed the sea cucumber's saponin distribution, Flammang decided to find out how the animals use the compounds for defence. Collecting seawater surrounding relaxed and stressed sea cucumbers, Guillaume Caulier extracted saponins from the water. He found that the relaxed animals released one saponin, while the stressed sea cucumbers released six, two of which were new and could not have come from the sea cucumber's body wall.
Finally, Flammang decided to find out what effects the saponins have on fish. Adding small quantities of the compounds – similar to the amounts released by the sea cucumbers – to aquaria housing Mediterranean rainbow wrasse and ocellated wrasse, Todesco saw that the fish began breathing heavily and raced around the tank; however, they soon settled and none died. Flammang suspects that fish can smell the unpleasant tasting compounds and give sea cucumbers a wide berth to avoid getting a bad taste in the mouth.